The Sydney-AAO Multi-object Integral field spectrograph (SAMI)

24 Pages·2011·1.42 MB·English

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed16December 2011 (MNLATEXstylefilev2.2)
The Sydney-AAO Multi-object Integral field spectrograph
(SAMI)
Scott M. Croom1,2⋆, Jon S. Lawrence3,4, Joss Bland-Hawthorn1, Julia J. Bryant1,
1 Lisa Fogarty1, Samuel Richards1, Michael Goodwin3, Tony Farrell3,
1
0 Stan Miziarski3, Ron Heald3, D. Heath Jones5, Steve Lee3, Matthew Colless3,2,
2 Sarah Brough3, Andrew M. Hopkins3,2, Amanda E. Bauer3, Michael N. Birchall3,

c
e Simon Ellis3, Anthony Horton3, Sergio Leon-Saval1, Geraint Lewis1,
D
A´. R. L´opez-S´anchez3,4, Seong-Sik Min1, Christopher Trinh1, Holly Trowland1

4 1 Sydney Institute for Astronomy (SIfA), School of Physics, Universityof Sydney, NSW 2006, Australia
1 2 ARCCentre of Excellence for All-skyAstrophysics (CAASTRO)
3 Australian Astronomical Observatory, PO Box 296, Epping, NSW1710, Australia
O] 4 Department of Physics and Astronomy, Macquarie University,NSW 2109, Australia
5 School of Physics, Monash University,Clayton, VIC 3800, Australia
C
.
h
p 16December 2011
-
o
r ABSTRACT
t
s Wedemonstrateanoveltechnologythatcombinesthe powerofthe multi-objectspec-
a trographwith the spatialmultiplex advantageof anintegralfieldspectrograph(IFS).
[
The Sydney-AAO Multi-object IFS (SAMI) is a prototype wide-field system at the

1 Anglo-Australian Telescope (AAT) that allows 13 imaging fibre bundles (“hexabun-
v dles”) to be deployed over a 1–degree diameter field of view. Each hexabundle com-
7 prises61lightly–fusedmultimodefibreswithreducedcladdingandyieldsa75percent
6 filling factor. Each fibre core diameter subtends 1.6 arcseconds on the sky and each
3 hexabundlehasafieldofviewof15arcsecondsdiameter.Thefibresarefedtotheflex-
3
ible AAOmega double–beam spectrograph, which can be used at a range of spectral
2. resolutions (R = λ/δλ ≈ 1700–13000) over the optical spectrum (3700–9500˚A). We
1 present the first spectroscopic results obtained with SAMI for a sample of galaxiesat
1 z ≈0.05.Wediscusstheprospectsofimplementinghexabundlesatamuchhighermul-
1 tiplex over wider fields of view in order to carry out spatially–resolved spectroscopic
v: surveys of 104−105 galaxies.
i
X Key words: instrumentation: spectrographs – techniques: imaging spectroscopy –
surveys – galaxies: general – galaxies: kinematics and dynamics
r
a
1 INTRODUCTION Experimental efforts to address galaxy formation have
generally taken two directions. First, galaxy imaging and
Galaxiesareintrinsicallycomplexwithmultiplecomponents
spectroscopic surveys have progressively moved to higher
and varied formation histories. This complexity is the pri-
redshift, in an attempt to directly observe galaxy evolution
mary reason that unravelling the physics of galaxy forma-
and formation. This approach has had much success, plac-
tion and evolution is so challenging. Galaxies are made up
ing strong constraints on the evolution of the global star
of baryons confined to dark matter haloes, and often have
formationrate(e.g.Hopkins& Beacom2006),unveilingthe
multiple distinct kinematic components (e.g. bulge and/or
strongevolutionofblackholeaccretionovermostofcosmic
disc).Therearecomplexinteractionsbetweenthestars,gas,
time (e.g. Croom et al. 2004, 2009; Richards et al. 2006),
dust,darkmatterandsuper-massiveblackholes.Thesecan
tracing the evolution of galaxy size and morphology (e.g.
leadtobothpositiveandnegativefeedbackontheformation
Dressler et al. 1997) and much more.
rate of stars.
The second approach has been to expand our view in
wavelengthratherthancosmictime.Thephysicalprocesses
⋆ [email protected] occurringingalaxiescauseemissionovertheentirerangeof
(cid:13)c 0000RAS
2 Croom et al.,
the electromagnetic spectrum. In order to have a full pic- cladding has been stripped from each fibre to a minimum
ture of galaxy formation, a multi-wavelength approach is over a short length (∼30mm) and the fibres then gently
vital.This hasbeen mademoreachievablewith recent gen- fused together at the input end to provide an IFU (∼1mm
erations of satellites covering the X-ray, ultra-violet, mid- aperture)withhighfillingfactor.Thesecanthenbeusedin
and far-infrared. While the spectral energy distributions of conventionalmulti-fibrespectrographs.
stars tend to peak in the optical or near-infrared, obscu- In this paper, we report on the Sydney-AAO Multi-
ration and reprocessing by dust generates strong mid- and object Integral field spectrograph (SAMI), the first fully
far-infrared emission. Young stars (when not obscured) are operational demonstrator instrument to use hexabundles.
most directly traced in the ultraviolet, while black hole ac- SAMI has 13 IFUs that can be positioned anywhere over a
cretion can generate radiation from the radio to X-ray and 1 degree diameter field of view. In Section 2, we discuss in
gamma-ray bands. Surveys such as the Galaxy And Mass detail the scientific rationale for such an instrument, along
Assembly(GAMA)survey(Driver et al.2011)andtheCos- with some practical considerations regarding sensitivity. In
micEvolutionSurvey(COSMOS;Scoville et al.2007)show Section 3, we describe the SAMI instrument in detail. In
thevalue of this multi-wavelength approach. Section4,weoutlinetheobservationscarriedoutduringthe
The third route, and the one that we address in this commissioningoftheinstrument,theresultsfromwhichare
paper,istofocusonspatially resolvinggalaxies; inparticu- discussed in Section 5. Our conclusions, and goals for the
lar, obtaining spatially resolved spectroscopy. Optical spec- future,are laid out in Section 6.
troscopy allows us to measure a wide range of parameters
including current star formation rates (e.g. via Hα), gas
phase metallicities, stellar ages, stellar metallicities, black
holeaccretionrates,ionizationstructureandextinctiondue
2 SCIENTIFIC RATIONALE
todust(e.g.viatheBalmerdecrement).Themajorspectro-
scopicsurveystodatehaveusedasinglefibre(Colless et al. In this section we outline the key scientific drivers for an
2001; Abazajian et al. 2003) or single slit (LeF`evre et al. instrumentsuchasSAMI.Thefundamentalquestionatthe
2005; Davis et al. 2007) on each object, and so obtain just heart of this work is: how did the galaxy population we see
onemeasurementoftheseparametersforeachgalaxy.More- around us today come about? This requires us to under-
over, these measurements may not be representative of the standthephysicalprocessesthatoccurasgalaxiesformand
galaxyasawhole,butbiased,dependingonwheretheaper- evolve. The galaxy population we see today has some very
tureisplaced.Thisfundamentalproblemisaddressedbyin- distinctivefeatures that need tobe explained.
tegralfieldspectrographs(IFS).Inthelastdecade,projects Oneofthemostfundamentalistheseparationofgalax-
such as SAURON (Bacon et al. 2001) have demonstrated ies into a bimodal distribution according to colour (e.g.
the power of integral field spectroscopy to capture a range Stratevaet al.2001;Baldry et al.2004).Agalaxy’scolouris
of key observables that are simply not available to single- primarily related tothepresenceofongoing starformation.
aperture surveys. As well as studying the properties listed The second key feature differentiating galaxies is morphol-
aboveinaspatially-resolvedcontext,obtaininggasandstel- ogy. There is a strong correlation between colour and mor-
lar kinematics over an entire galaxy enables us to separate phology, with galaxies lying along the ‘red sequence’ being
dynamical components, measure dynamical mass, examine mostly passive systems with elliptical/spheroidal morphol-
theimpactofwindsandoutflows,anddiscovermergingsys- ogy, while galaxies inhabiting the ‘blue cloud’ are mostly
tems via dynamical disturbances. dominated bydiscs, although this is not always strictly the
Integral field spectroscopy has almost exclusively been case(e.g.Masters et al.2010;Schawinskiet al.2009).While
limited to single-object instruments, meaning that it theyarerelated,thereisnotastrictone-to-onerelationship
is time-consuming to build large samples. The largest between morphology and colour. A clearer understanding
current data set, using the SAURON system on the of galaxies can be obtained if they are considered as be-
William Herschel Telescope, contain ∼ 260 objects ing made up of distinct morphological components (discs,
(ATLAS-3D;Cappellari et al.2011a).TheCALIFAproject bulges and pseudo-bulges) that result from different for-
(S´anchezet al. 2011) aims to target 600 objects with the mation processes and evolutionary histories (Driver et al.
PMASintegralfieldunit(IFU)ontheCalarAltoTelescope 2007). The intrinsic properties of these structural compo-
using ∼200 nights of telescope time. The only multi-object nents are more uniform than those of the galaxies they
integral field spectrograph currently available is that on compose. Their formation pathways are also quite differ-
theVLTFLAMESinstrument(Pasquini et al.2002),which ent, with true bulges built up by violent mergers, discs
contains 15IFUs, each with 20 spatial elements of size 0.52 fromgasaccretion,andpseudo-bulgesfromsecularevolution
arcsecandatotalfieldofview(FoV)of2×3arcsec.Thishas (Kormendy& Kennicutt2004).Disentanglingthesevarious
enabled measurements of the Tully-Fisher relation at ∼0.6 modes is complex, but can be materially aided by the fact
(Yanget al. 2008) as well as a number of other projects. that thestructuresformed havedifferent kinematic proper-
HoweverthesmallFoVofeachIFU,combinedwiththehigh ties as well as different star formation histories.
spectral resolution (≥9000) and associated narrow wave- In order to ascertain the physics involved, we need to
length range limits its applicability for large–scale surveys. determinetheanswerstoanumberofquestionsthatbroadly
Astrophotonic technology (Bland-Hawthorn & Kern fallintofourcategories:(i)howdoesgalaxymassandangu-
2009) is now opening the way to new instrumentation larmomentumbuildup?(ii)when,whereandwhydoesstar
that can address the need for highly multiplexed inte- formation occur?(iii) when,whereandwhydoesblackhole
gralfieldspectroscopy.Hexabundles(Bland-Hawthorn et al. accretion occur? (iv) how are galaxies fuelled and what is
2011;Bryant et al.2011)areopticalfibrebundleswherethe therole of feedback? We will discuss each of these issues in
(cid:13)c 0000RAS,MNRAS000,000–000
The Sydney-AAO Multi-object IFS 3
turn,althoughthereisofcoursesignificantoverlapbetween kinematics that describe the observed projected stellar an-
them. gularmomentumperunitmassofgalaxies,notpossiblewith
othertechniques(Emsellem et al. 2007, 2011;Brough et al.
2011). This measurement enables the separation of early-
2.1 The build up of mass and angular momentum
typegalaxies into fast and slow rotators which are thought
Thestandardpictureofgalaxyformationhasgascoolingto to have very different formation paths. A great success of
form a rotationally supported disc within a cold dark mat- the SAURON and ATLAS-3D projects (Bacon et al. 2001;
terhalo(White& Rees1978).Whilethispictureisbroadly Cappellari et al. 2011a) has been the discovery that most
accepted, feedback and interactions provide major compli- earlytypegalaxieshavesignificantrotation,withonly≃14
cations which are not yet fully understood. percent (predominantly at high mass) being slow rotators.
The scaling relation between circular velocity and stel- Cappellari et al. (2011b) have used ATLAS-3D to demon-
lar mass (The Tully-Fisher Relation, TFR; Tully & Fisher strateakinematicmorphology-densityrelation,whichshows
1977)fordiscgalaxiesprovidesatightconstraintforgalaxy asmoothtransitionofspiralstoearlytypefastrotatorswith
formation models. The circular velocity depends on the ra- increaseddensity,andmassiveslowrotatorsonlyinhabiting
tio of disc mass to halo mass, the dark matter halo profile thehighest density regions. SAMIwould allow such studies
andthedimensionlessspinparameter,λ(Peebles1969).The tobe expandedto probe greater dynamicrange in environ-
largest TFR samples (e.g. Springob et al. 2007) currently ment and examine a such relations as a function of stellar
contain ≃ 5000 galaxies with either long-slit spectroscopy mass.
or spatially unresolved H I velocities. If IFU observations Therateofdarkmatterhalomergingcanbeaccurately
can be made out to large enough radii (typically ∼2.2 disc estimated from simulations (Fakhouri& Ma 2008). Kine-
scale lengths), then they provide substantial advantages in matic information from integral field spectroscopy can be
allowing a clearer picture of distorted kinematics and incli- used to differentiate between quiescent galaxies and those
nation.Circularvelocitiescanbecomparedtotheresultsof undergoingamerger(e.g.Shapiro et al.2008),usingproce-
galaxylensingtoconstrainthedarkmatterhaloprofileand duressuchaskinemetry(Krajnovi´c et al.2006).Untilnow,
examine evidence of contraction of the halo in response to this type of analysis has largely been limited to high red-
thebaryons (Duttonet al. 2010; Reyeset al. 2011). shift (where the merger rate is expected to be higher), but
ThestellarandemissionlinekinematicdatathatSAMI with samples of 103 or more galaxies, statistically mean-
canprovidewillallowdynamicalmassestimateswithinmax- ingful estimates of merger rates can be made at low red-
imum radius probed, using techniques such as anisotropic shift. Fakhouri& Ma (2008) predict that the halo merger
Jeansmodelling(Cappellari2008).Ingeneralitisnotpossi- rate at z ∼ 0 should be ∼ 0.05halo−1Gyr−1 for major
bletodeterminetotalmassbecauseoftheuncertaintyofthe mergers (with mass ratios < 3/1). Integral field observa-
darkmatterhaloparameters.EvenfortheMilkyWay,where tions provide a complementary approach to studies which
manyhalostarscanbeusedtoprobetheouterhalo,theto- focus on thenumberof close pairs toestimate merger rates
talgalaxy mass isuncertain toafactor of two(Smith et al. (e.g. De Propris et al. 2007), as they probe very different
2007). Detailed dynamical techniques are in stark contrast phases of the merger process. It is also be possible to look
to estimates of dynamical mass which simply take a single for weaker dynamical disturbances in discs dueto repeated
velocity dispersion of the galaxy (e.g. from a single fibre minor mergers/interactions (Zaritsky & Rix 1997). In this
observation), (e.g. Taylor et al. 2010). case, it is very advantageous to extend the study to large
It is relatively easy to extract a rotation curve v(r) radius as the effect of tidal perturbations scales as ∼r3. In
from the observed data if the kinematics are fairly well or- this case the challenge is to achieve sufficient sensitivity at
dered (Staveley-Smithet al. 1990). This can then be used large radius (e.g. >∼1.5−2 scale lengths).
to provide dynamical information about the galaxy, partic- Spin angular momentum from galaxy kinematics can
ularly if baryonic information is brought to bear. However directly probe the formation of the large scale structure of
if the kinematic axes are misaligned with the photomet- theuniverseandgalaxyformation. Thespinofgalaxy discs
ric axes, this is often a signature of streaming motions due provides an approximation of the spin of the galaxy’s dark
to a bar or an oval distortion. A dynamical mass can still matterhalo(Sharma & Steinmetz2005),whichiscoupledto
be derived, although the increased number of free parame- thelargescalestructure.Earlyinadarkmatterhalo’slife,it
ters makesthis moreuncertain (Staveley-Smithet al. 1990; experiencestorquesfromthesurroundingdensitylandscape.
Quillen, Frogel & Gonzalez 1994). Any deviations from ro- Thespinsof darkmatterhaloes today retain somememory
tational symmetry are important in their own right. It is of that landscape, so spin is intrinsically linked with the
often very difficult to see the presence of bars, particularly largescale structure.Thislinkmaybeexaminedin N-body
in highly-inclineddiscsystems.But barsareoften betrayed simulations and observations by measuring thedistribution
by the inner twists of the isovelocity contours. Warps are of inferred dark matter halo spin magnitude (Berta et al.
more easily detected on large scales and almost always in 2008) and by the relative orientation of galaxy spin direc-
HI kinematics (Briggs 1990); however, the same effects can tions with each otherand with thelarge scale structure.
now be seen in deep observations of the diffuse ionized gas N-body simulations do not predict a strong align-
in the outer disc (Christlein, Zaritsky & Bland-Hawthorn ment between the spins of neighbouring haloes, although
2010). The physical cause of kinematic distortions can be an alignment between galaxies has been detected in the
examined by large IFU surveys which probe a variety of Tully catalogue of 12,122 nearby spirals (Pen,Lee & Seljak
galaxy parameters. Forexample,aredistortionsmorelikely 2000). Simulations and theory predict an alignment be-
in high density regions dueto dynamical interaction. tween halo spin and the tidal field (Lee & Pen 2000),
Integralfieldspectroscopyalsoenablesstudiesofstellar and an alignment with features in the tidal field like fil-
(cid:13)c 0000RAS,MNRAS000,000–000
4 Croom et al.,
aments (Zhanget al. 2009), sheets (Lee 2004) and voids tailedmulti-wavelengthobservationsareabletorevealthese
(Brunino et al.2007).Anykindofalignmentispredictedto processes (L´opez-Sa´nchez2010; Lopez-Sanchezet al. 2011)
be very weak, however, so could only be seen in large scale However,environmentisonlyonefactor.Feedbackfrom
galaxy surveys. There have been detections of spin align- starformationandaccretionontosuper-massiveblackholes
mentsin thelarge scale structurereconstructed from imag- provides an internal mechanism for transformation. This
ingsurveys(Lee & Erdogdu2007;Paz, Stasyszyn & Padilla feedbackprovidesasolutiontothemismatchofthetheoret-
2008), using inferred galaxy spins from disc shapes. A ical dark matter halo mass function and the observed stel-
tentative detection of spin alignment with filaments was lar mass function (e.g. Baldry, Glazebrook & Driver 2008)
found using the inferred spin of only 201 galaxies around by heating and/or expelling gas in both low mass (via
voids (Trujillo, Carretero & Patiri 2006) and 70 galaxies in star formation) and high mass (via black hole accretion)
filaments (Jones, van deWeygaert & Arag´on-Calvo 2010), haloes (Cattaneo et al. 2006; Baldry, Glazebrook & Driver
pickedfromthelargescalestructureofSDSS.Discrepancies 2008). Extreme outbursts of star formation or black hole
betweentheresultsfoundfromdarkmattersimulationsand accretion may be triggered by mergers or interactions (e.g.
observations indicate a difference in the way that galaxies Hopkinset al.2008),makingalinkbetweeninternalanden-
and dark matter haloes obtain and keep their spin. A large vironmental effects. Oncetheburst is over, anothermecha-
survey of direct spin measurements could reveal whether nism is needed to suppress continued star formation. The
galaxies exhibit the same spin behaviour as dark matter best suggestion for this is mechanical feedback from jets
haloes,andshowhowgalaxyspinislinkedtothelargescale emitted by super-massive black holes (e.g. Croton et al.
structure. 2006),butthisonlyappearstobeefficientinmassivegalax-
ies.
As well as these active processes, the environment has
2.2 When, where and why does star formation
anindirectinfluenceviaformationage.Galaxiesinhighden-
occur?
sityregionsformearlierandsohavehadmoretimetoevolve
Muchrecentobservationalandtheoreticalworkhasfocussed (e.g.Kaiser1984;Bardeen et al.1986;Thomas et al.2005).
onhowbluegalaxiescanhavetheirstarformationquenched, Intheabsenceof othereffects,we would thenexpect tosee
moving them onto the red sequence. Red sequence galax- galaxies in high densityregions havingolderstellar popula-
ies are preferentially found in denser environments (e.g. tions.
Blanton et al. 2005), and star formation is also clearly sup- Disentangling these varied influences on galaxy forma-
pressed at high density (e.g. Lewis et al. 2002). This im- tion is far from trivial. However, studies of the spatial dis-
mediatelysuggestsenvironmentalfactorsplayanimportant tribution of instantaneous star formation rates, integrated
role. When a galaxy falls into a cluster, the ram pressure star formation (via stellar population ages) and metallicity
from the dense intergalactic medium (Gunn & Gott 1972) (both gas and stellar) provide considerable insight. Impor-
may expel the gas from the disc, removing the fuel re- tantly, ram-pressure removal of gas implies that the trun-
quiredforfurtherstarformation.Thereareseveralobserved cation of star formation is an outside-in process (e.g. Bekki
examples of this in rich clusters (e.g. Randall et al. 2008; 2009;Kapferer et al.2009).Gasispreferentiallyremovedin
Sun,Donahue& Voit 2007). In moderately dense regions, the outer parts of galaxies, which are less gravitationally
such as galaxy groups, it may be that ram pressure will bound. This may be a short-lived feature of the galaxies
leavethedisc intact,butcan still removegas from thehalo in dense environments if the gas is eventually completely
ofthegalaxy.Thehaloprovidesareservoirofgaswhichcan removed. Alternatively, stripping can occur over the life-
replenish thedisc. time of a galaxy if the gas is puffed up by the internal
Without the halo gas, the star formation will decline starformation;inthiscase,evenararifiedexternalmedium
andthenceaseasthediscgasisconsumed,leadingtoatran- canremovegasfromthegalaxy(Nichols & Bland-Hawthorn
sition to the red sequence, in a process known as strangu- 2011). Globally, the expectation would be that galax-
lation (Larson, Tinsley & Caldwell 1980). Simulations sug- ies form inside-out, and this implies age and metallic-
gest that this process can be efficient at removing halo ity gradients which are observed (e.g. Shaveret al. 1983;
gas, even in small and/or compact groups (Bekki 2009; Vila-Costas & Edmunds 1992; Steinmetz& Mueller 1994;
McCarthy et al.2008),butthereislittledirectexperimental Chiappini, Matteucci & Gratton 1997).
evidence that this is the case. Indirect evidence does point Oneapproach that hasbeenexplored in somedetail as
tosomepre-processingofgalaxies ingroupsbeforetheyfall an alternative to spatially-resolved spectroscopy is the so-
intoclusters(Balogh & McGee2010),butthephysicalpro- called ‘pixel-z’ technique (Conti et al. 2003; Welikala et al.
cess driving this is not clear. Direct galaxy-galaxy interac- 2009, 2008). This approach, analogous to photometric red-
tions are also expected to play a critical role, with major shifts (‘photo-z’),usesa library of templatespectral energy
galaxy mergers triggering star formation (e.g. Ellison et al. distributions (SEDs) to fit the observed optical and near-
2008) and transforming the morphology of galaxies, al- infrared colours of individual pixels within resolved galaxy
though the fraction of galaxies undergoing major merg- images. This technique has been used with some success to
ers (i.e. those with mass ratios of 3:1 or less) in the lo- explore the environmental dependence of star formation in
cal Universe is small (e.g. Patton & Atfield 2008). On the galaxies. Welikala et al. (2008) find that, globally, the sup-
other hand, dwarf star-forming galaxies in the local Uni- pressionofstarformationinhighdensityenvironments(e.g.,
verse are often found interacting with low-luminosity ob- Lewis et al. 2002; G´omez et al. 2003) seems to occur pri-
jects or diffuse H I clouds,. This appears to be the trig- marily in the most strongly star-forming population, and
gering mechanism of their intense star-formation activity to be evidenced by a suppression in the inner regions of
(L´opez-Sa´nchez& Esteban 2008, 2009), although only de- galaxies. Welikala et al. (2009) demonstrate that this effect
(cid:13)c 0000RAS,MNRAS000,000–000
The Sydney-AAO Multi-object IFS 5
seems to hold independently for both early- and late-type downsizing appears qualitatively similar tothat seen in the
galaxy populations,andthatthesuppression instarforma- formation of galaxies (Cowie et al. 1996). In the local Uni-
tion cannot be explained solely by the well-known density- verse, accretion onto black holes is dominated by systems
morphology relation (e.g., Dressler 1980). There are signif- with low black hole masses, <108M⊙, but which are typi-
icant limitations, however, to the ‘pixel-z’ approach. These callyaccretingwithinanorderofmagnitudeoftheEdding-
are related to implicit assumptions made by the technique, ton limit (Heckman et al. 2004). In contrast, higher mass
such as each pixel being represented by an isolated single black holes in the local Universe are accreting with a char-
stellar population with a simple exponential star-formation acteristictimescalesubstantiallylongerthanaHubbletime.
history(Conti et al.2003;Welikala et al.2008).Asaresult, Luminous AGN (as measured by their [O III]λ5007
the method cannot effectively measure the instantaneous luminosity) are found to have younger stellar populations
star formation rate, which can be traced spectroscopically (Kauffmannet al.2003),asmeasuredbytheirDn(4000)and
(e.g. byHα emission). Hδ line indices from SDSS spectra. It is not clear whether
IFU spectroscopy allows us access to both current star thisyoungerstellar population is centrally concentrated, or
formation(viaemissionlines)andintegratedstarformation is distributed more widely across the host galaxy, as the
history (via stellar age and metallicity). Examining the ra- SDSS fibres subtend a physical scale of ∼5 kpc at z =0.1.
dialdependenceofthemeanstellarageandmetallicitygra- IFS data has the ability to explicitly examine the distri-
dients tells us when and where the stars formed in these bution of star formation and stellar population ages across
galaxies, along with a fossil record of the galaxy merger galaxies and investigate whether these are related to accre-
history. The mean stellar age is effectively a luminosity- tion rate.
weighted integral of the star formation history whilst the Lower luminosity AGN tend to have spectra typical
stellar metallicity gradient provides an indication of its of Low-Ionization Nuclear Emission-line Regions (LINERS;
merging history (e.g. Kobayashi 2004). If environmental ef- Heckman 1980). While there is evidence that these form
fectsareresponsibleforthecessationofstarformation,then a continuous sequence with Seyferts (Kewley et al. 2006),
wewouldexpectredsequencegalaxiestohaveyoungercen- there substantial evidence that LINER like emission is ex-
tral ages with past major mergers sign-posted by shallow tended and may be powered by ionization from AGB stars
negative metallicity gradients (i.e. lower metallicities in the (Yan& Blanton2011).Alargegalaxysurveywithspatially
outskirts;Kobayashi2004;Brough et al.2007;Spolaor et al. resolved spectroscopy could resolve this issue. If LINER
2009). emission is not due to a central AGN in most cases this
would require substantial re-interpretation of recent work
on low redshift AGN. At an even more fundamental level,
2.3 When, where and why does black hole
the fraction of galaxies which host AGN is only well de-
accretion occur?
fined locally (i.e. 10s of Mpc; Ho 2008). Approximately 40
A full picture of the physical processes involved in the fu- per cent of these very local galaxies host AGN. At larger
elling of accretion onto super-massive black holes, resulting distances contamination from off nuclear emission increas-
in the phenomenon of an active galactic nucleus (AGN), ingly reduces thesensitivity to weak nuclear emission lines.
is still elusive. It is now known that most galaxies contain A large IFS survey would enable apertures of fixed metric
super-massive black holes, with typical masses a million to size tobedefined,enablingrobust AGN rates as afunction
a billion times that of the Sun (e.g. Gebhardt et al. 2000; of redshift to be determined.
Ferrarese & Merritt2000).Themassoftheblackholecorre- Whilethereisagoodtheoreticalbasisforgalaxymerg-
lateswellwiththemass(orvelocitydispersion)ofthebulge ers triggering luminous AGN (e.g. Hopkins& Hernquist
orspheroidalcomponent(Tremaine et al.2002)inagalaxy. 2009), the evidence for this is mixed. An alternative path-
Thisimpliesanintimateconnectionbetweenthebuildupof way is via violent disc instability in self gravitating discs
stellarmassingalaxiesandtheirsuper-massiveblackholes. (e.g. Bower et al. 2006; Bournaud et al. 2011). Disc insta-
The nature of the connection between star forma- bility is at least likely to play a role at high redshift where
tion and AGN has long been debated (e.g. Sanders et al. cold streams can dominate the mass accretion onto discs
1988), although a resolution remains elusive. The most (Dekelet al.2009).Indeed,alargefraction ofhigh–redshift
luminous AGN (i.e. bright quasars) require >∼109 solar star forming galaxies appear to be discs (e.g. Genzel et al.
masses of gas deposited in their central regions on time- 2008; Wisnioski et al. 2011), rather than mergers. In low
scales of ∼107–108 years (e.g. Croom et al. 2005), requir- redshift samples AGN activity isnot enhanced bythepres-
ingmajor galaxy-wideperturbations(Hopkins& Hernquist enceofanearbycompanion(Li et al.2008b),whilestarfor-
2009), and no doubt triggering substantial star formation. mation is (Li et al. 2008a). This is a somewhat surprising
In contrast, low-luminosity AGN require relatively small result, and may point to a difference in timescale between
amounts of gas, which can be supplied by internal stochas- theonsetofstarformationandtheAGN,withtheAGNoc-
ticprocesses, suchas theaccretion ofcold molecular clouds curring later, after the merger has taken place. Indeed,IFS
(Hopkins& Hernquist 2009). observationsoflocalgalaxieswithobservedoutflowsdemon-
The AGN population evolves strongly with cosmic strates that the AGN timescale is significantly longer than
time. This is most clearly seen in evolution of luminous the starburst timescale (Sharp & Bland-Hawthorn 2010).
quasars from z = 0 to z ≃ 6 (Richards et al. 2006; Large scale IFS observations can directly address the issue
Croom et al.2009),which haveastrong peakin spaceden- of AGN fuelling by examining the kinematic properties of
sity at z ≃ 2−3. Lower luminosity AGN show less severe AGN hosts, and searching for evidence of disc instability
evolution, and peak in space density at lower redshift (e.g. and/ormerging.Inthisregarditwillbeparticularlyimpor-
Hasinger, Miyaji & Schmidt2005;Croom et al.2009).This tant to span a range of accretion luminosities in order to
(cid:13)c 0000RAS,MNRAS000,000–000
6 Croom et al.,
examinewhetherthereisachangefromsecularevolutionat pressure on dust grains in the disc). The gas may arise
low accretion rate to mergers at high accretion rate. from some kind of disc-wide interaction between the disc
and the hot halo, presumably driven by processes related
to star formation in the disc (Cox 2005). But there is also
2.4 Feeding and feedback
the prospect that some of this gas is related to warm gas
The accretion of gas onto galaxies remains a largely un- accretion onto the disc (Bland-Hawthorn 2009) or involved
solvedproblem.Whethergasentersthegalaxypotentialina in a large-scale circulation or recycling of gas through the
hotphaseandthencoolsdown(Binney,Nipoti & Fraternali halo (Marinacci et al. 2011).
2009), as a warm rain (Bland-Hawthorn et al. 2007), or as Aswiththestudyofgalaxywinds,alargeSAMIsurvey
an HI complex like the high velocity clouds in the Galactic has the potential to greatly increase the sample of known
halo(Sancisi et al.2008),oralloftheabove,isunclear.Af- galaxies with vertically-extendedwarm discs. Witha larger
ter a review of the evidence, Binney (1992) concluded that sample, itwill bepossible tocorrelate thepresenceofthese
theouterwarpsofHIdiscsweresomeofthebestevidenceof discswith thediscstar formation rate,nuclearactivity and
ongoingdiscaccretion.Oncethegassettlesintothegalaxy, galaxy mass.
theoutstandingissuesarehowthegasfeedsintothenuclear
regions and,in particular, onto a central black hole.
In a survey of 103 − 104 galaxies, SAMI offers
2.5 Limitations of current spectroscopic surveys
the prospect of studying nuclear activity (AGN, star-
burst, LINER) and star formation within the context of Historically, telescopes were used to observe one source
the extended galaxy. The kinematic signatures of outer at a time. But with technical advances in optical fibres,
warps and inner bar streaming are relatively easy to pick it was realized in the early 1980s that many sources
out in HI (Staveley-Smithet al. 1990) or in ionized gas could be observed simultaneously across the telescope
(Christlein, Zaritsky & Bland-Hawthorn 2010). Thus, we focal plane by precisely positioning fibres in the field
cannowdirectlyassociate thisactivitywithlarge-scale disc (Barden, Ramsey & Truax 1981; Gray 1983). This led to
disturbances, assuming these exist. Traditionally, in large anexplosioninwide-fieldspectroscopicsurveys,notablyin-
galaxy surveys, the association of activity and dynamical cluding the 2-degree Field Galaxy Redshift Survey (2dF-
disturbances is made from the proximity of galaxies in po- GRS;Colless et al.2001),2-degreeFieldQSOSurvey(2QZ;
sition and redshift space (e.g. Nikolic, Cullen & Alexander Croom et al. 2004), 6-degree Field Galaxy Survey (6dFGS;
2004; Li et al. 2008a). With full kinematic information a Jones et al. 2009) and Sloan Digital Sky Survey (SDSS;
muchmoredirectdeterminationofthetriggeringofactivity Yorket al. 2000) amongst several others. Between them,
will be possible. such surveys have obtained spectra for approximately 1.5
Analternativeapproachistostudytheimpactofthein- millionextragalactictargets.Newinstrumentsrecentlycom-
nerdisc on theextended properties of galaxies (e.g. Martin missioned(e.g.LAMOST;Su et al.1998)orinconstruction
1998; King & Pounds 2003). Jets carry energy, and winds (e.g.VIRUS;Hill et al.2004)areabletoobservethousands
carry gas and metals, far from the nucleus. In a recent of sources at a time.
integral field study of ten AGN and starburst galaxies, Considerable advanceshavebeen made possible by the
Sharp & Bland-Hawthorn (2010) find that starburst winds 2dFGRS and SDSS, which use a single optical fibre per
are largely shock ionized, while AGN winds show the hall- galaxy.However,withfibrediametersof2and3arcseconds
mark ofphoto-ionization bytheaccretion disc,clearly indi- respectively, these projects sample less than half the light
catingthatthestarburstphenomenonisveryshort.Forone from a galaxy at the median distance of thesurveys. These
oftheobjectsobservedontheSAMIcommissioningrun,we single apertures limit thesurveysin two ways.
see the ionization characteristics typical of nuclear activity First, it is impossible for single-fibre surveys to mea-
for gas off theplaneof an inclined disc(see Section 5.4 and sure spatially varying spectral properties, which prohibits
Fogarty et al., in prep.). A large-scale wind is confirmed the study of crucial observables such as kinematic merger
by the broad emission-line profiles along the minor axis. rates, galaxy rotation and dynamical mass, star formation
This is a remarkable testament to the power of spatially- gradients,metallicitygradients,agegradientsanddetection
resolved kinematic and ionization information. In the full of galaxy winds and/or outflows.
survey, SAMI is likely to uncover hundreds of new outflow The second limitation is that with single-fibre spec-
sourcesconnectedeithertonuclearactivityorinnerdiscstar troscopythemeasuredsignaldependsonmanythings:(i)in-
formation.Thereisanevenrarerclassofgalaxieswithdisc- trinsic properties, like source luminosity, size and distance;
widewindsthatSAMIwillalsoinevitablyaddto(Strickland (ii) atmospheric conditions, particularly seeing; (iii) instru-
2007). mental properties, like fibre aperture size and positioning
A number of edge-on spiral disc galaxies have ver- accuracy,andopticalfocusoverthefield;(iv)telescopeprop-
tically extended ionized gas in their haloes (Rand 1996; erties, such as pointing and guiding precision; and perhaps
Dettmar 1992). The Reynolds Layer in our Galaxy, re- othereffects.Manypublishedpapersmakethemistakeofas-
cently mapped by the WHAM Hα survey telescope suming the surveys provide spectrobolometry (i.e. the spec-
(Madsen, Haffner & Reynolds 2006), is a good example of trum of the total light output by the source) rather than
this phenomenon (see also Gaensler et al. 2008). The ion- the spectrum from an (often ill-defined) spatial sample of
izationcharacteristicsofthisgasdoesnotappeartobecon- the source. The inherent dangers of aperture effects have
sistentwithanyknownmechanism(i.e.UVphoto-ionization long been known in astronomy, but have often been under-
by hot young stars, radiation from old supernova bubbles, appreciated or ignored. Ellis et al. (2005, see their Fig. 8)
shocks from supernovae, cosmic ray heating, or radiation clearly demonstrate that the single-aperture fibre spectra
(cid:13)c 0000RAS,MNRAS000,000–000
The Sydney-AAO Multi-object IFS 7
from atypicalgalaxysurveymaybeonlyweaklycorrelated
with thephotometric classification.
Aperture biases can manifest themselves in two ways.
The fibres subtend an increasing linear size with increasing
distance of the galaxy, potentially causing spurious evolu-
tionary effects (and spurious luminosity-dependent effects
in a flux-limited sample). Second, important galaxy prop-
erties, such as star formation rate and metallicity can have
substantialgradients,meaningthatobservationsofjust the
central regions are not representative of global values (e.g.
Kewley & Ellison 2008).
2.6 Why multiplexed IFUs?
Integral field spectroscopy allows us to gather data on key
observablesthat aresimply inaccessible tosingle apertures.
IFUs are usually a single monolithic array of lenslets, with
the light fed to a spectrograph via optical fibres (or alter-
Figure1.Thesize–surfacebrightnessdistributionforanr-band
natively an optical image slicer). As a consequence, they
sample of SDSS galaxies limited to rser < 16.5 (extinction cor-
can only target one object at a time and so IFU surveys rected).Sizeisdefinedasthehalf-lightradius,Re,andthesurface
havetypicallyonlytargetedafewtensofgalaxiesinspecific brightness, µe, is also given at this radius. The marginal distri-
classes (e.g. Pracy et al. 2009; Emsellem et al. 2007). butions of size and surface brightness are shown below and to
The process of galaxy formation and evolution is in- the right of the main panel. Red dashed lines show the median
herently complex, with the observed galaxy properties de- ofeach parameter, whilethe dotted redlines show the 10th and
pendingonalargenumberofparameters,suchashosthalo 90th percentiles of the distributions. The blue solid line marks
theradiusofasinglefibrecoreinSAMIandthebluedashedline
mass,stellarmass, mergerhistoryetc.Aswellasthemulti-
istheradiusofthe61-corehexabundle.
dimensional nature of galaxy properties, there is inherent
stochasticity in the process. This is at least in part due to
ourinabilitytoaccuratelytracetheformation historyofin-
First we consider a simple apparent-magnitude-limited
dividualobjects,butalsoderivesfrominherentlynon-linear
physics such as that involved in the collapse of molecular sample with rser < 16.5 (extinction corrected, where rser is
the galaxy SDSS r-band magnitude derived from a Sersic
clouds to form stars.
model fit to the photometry). This limit was chosen as the
Themulti-dimensionalnatureofthegalaxypopulation,
galaxy surface density approximately matches the density
combinedwith thisstochasticity (whichatsome levelcould
beconsideredasextrahiddenparameters),meansthatlarge of IFUs in SAMI. The distribution of half-light radius, Re,
surveys are required to extract the key relations between and surface brightness at Re is shown in Figure 1. For such
physicalproperties.Thesuccessofsurveyssuchas2dFGRS a sample the SAMI hexabundle IFUs reach to 1Re for all
but the largest 10 percent of galaxies (blue dashed line),
andSDSShasinlargepartbeenduetotheirabilityto‘slice
and dice’ the galaxy distribution and still have statistically while 1Re is sampled by at least 3 IFU elements for all but
the smallest 10 percent of galaxies. In other words, for the
meaningful samples in each bin of the parameter space.
central 80 percent of this sample, SAMI can give spatially
The need to extend integral field observations to large
samples is well understood in the above context, and has resolved spectroscopy out to at least 1Re. The median sur-
drivenrecentprojects suchasATLAS-3D(Cappellari et al. face brightness at 1Re is µe ≃22magarcsec2.
We show how size and surface brightness vary with
2011a)andCALIFA(S´anchez et al.2011).Theseimpressive
redshift for this sample in Figures 2a and b. The typical
projects are limited by their use of monolithic integral field
sizes in arcseconds of the galaxies stay relatively constant
units,whichmeansthattargetingmorethanafewhundred
with redshift, largely because the r-band limit selects more
objectsisprohibitivelyexpensiveintermsoftelescopetime.
massive (and therefore larger) galaxies at higher redshifts.
This limitation naturally drives us to multi-object integral
A natural alternative is to select a volume-limited sample,
fieldspectroscopy(i.e.multiplexedIFUs),thesubjectofthis
which is shown in Figures 2c and d. In this case we choose
paper.
Mr <−19.5 and z <0.075, which gives similar numbers of
targets (i.e. similar surface density) to the r-band cut used
above. In this case we see, unsurprisingly, that the galaxies
2.7 Size and surface brightness
aresmallerathighredshift,butthatthemedianRe ismore
Akeychallengeformulti-objectintegralfieldspectroscopyis than twice the radius of a fibre core (and more than three
to obtain sufficient signal-to-noise at low surface brightness fibrecores out to z≃0.06).
levels for all the targets observed simultaneously in a given An apparent-magnitude-limited sample can be consid-
pointing.Inthissection wepresentsomepreliminary inves- ered as a set of volume-limited samples with small redshift
tigations of the properties of potential SAMI targets. We intervalsandabsolute magnitudelimits that varywith red-
will use the recent Sersic fits and bulge-disc decomposition shift.Itishighlylikely thattheoptimalsolution fortarget-
carried out by Simard et al. (2011) on the SDSS photome- inggalaxiesforamulti-objectIFUinstrumentinvolvestak-
try. ing multiple volume-limited samples, allowing optimal use
(cid:13)c 0000RAS,MNRAS000,000–000
8 Croom et al.,
block includes 63 fibres (all the fibres from one hexabun-
dle plus two fibres for sky subtraction). A fusion-spliced
ribbonised fibre cable of length ∼42m joins the two in-
strument ends together. A near real-time data pipeline,
based on the 2dFdr code (Croom, Saunders& Heald 2004;
Sharp & Birchall2010),hasbeenwrittentoreducethedata.
The following subsections describe the instrument re-
quirements, fibre cable, the hexabundles, the prime focus
unit, the SAMI field plates, the AAOmega spectrograph,
theinstrument control and data reduction software.
3.1 Instrument requirements
The SAMI instrument was designed to be a technology
demonstrator and to carry out significant science pro-
Figure2.Sizeandsurfacebrightnessasfunctionsofredshiftfor
grammes. As a result, the final instrument design is influ-
a magnitude-limited galaxy sample with rser < 16.5 (panels a
encedbyamixofscientificandtechnicalconstraints.Akey
and b) and for a volume-limited sample with Mr < −19.5 and
z < 0.075 (panels c and d). The black solid line is the median constraint was to develop the system on a rapid time-scale,
andtheblackdottedlinesarethe10thand90thpercentiles.The which naturally led totheuse of theplug-plate system and
bluesolidlineistheradiusofasinglefibrecoreinSAMIandthe the already available AAOmega spectrograph. The flexibil-
bluedashedlineistheradiusofa61-corehexabundle. ityofAAOmega,witharangeofresolutionsandwavelength
settings also enables a variety of science.
In order to make a substantial advance over previous
oftheIFUFoVwhilebroadlypopulatingthedistributionof
facilities, the multiplex of the system had to be at least an
galaxystellarmass.Oneissuethatneedstobeconsideredis
order of magnitude better than previous monolithic IFUs
that in an apparent-magnitude-limited sample, objects in-
(i.e. SAMI required at least ≃ 10 IFUs). The median op-
evitably pile up around L∗, so suitable sampling may need
tical seeing at the AAT is ≃ 1.5 arcsec, so the fibre cores
tobeimplementedtoefficientlycoverawiderangeinstellar
were approximately matched to this (1.6 arcsec diameter).
mass.
Althoughundersamplingtheseeing,thisisgenerallyprefer-
Last considerations in target selection include whether
ableto havingsmaller core sizes for anumberof reasons: i)
tosampleallthegalaxiesinagivenvolumeortochoosefield
larger fibre cores provide more independent resolution ele-
locations whichuniformlysamplethedistributionofgalaxy
ments, ii) smaller fibre cores will lead to data being read-
environment.Thechallengewiththelatterapproachisthat
noise limited in the blue, iii) larger fibre cores provide bet-
there are a variety of different local density estimators and
tersurface brightnesssensitivity,iv) larger fibrecoresallow
the relation between them is not trivial (see Brough et al.,
higherfillfactors,givenafixedminimumfibrecladding(i.e.
in preparation). There is also a challenge to balance there-
5µm in ourcase),v) critical sampling oftheseeing can still
quirementsofmaintainingsufficientspatialresolutionbelow
beachieved with dithered exposures.
1Re, as well as reaching > 2Re which is a requirement for Given the above, the final key instrument design deci-
reachingtheturn-overintherotationcurvesofdiskgalaxies
sionwasthenumberoffibresperIFU.Thechoiceof61fibres
(e.g.forTully-Fisheranalysis).Separatesampleswithinthe
per IFU was made largely on the basis of the known capa-
samesurveyareathatcanbeobservedconcurrentlymaybe
bilitytomanufacturesuchbundles.However,thisnumberof
the most natural solution to this problem. Further discus-
fibres also matches other requirements. Larger numbers of
sionofthedetailedsampleselectionforalargeSAMIgalaxy
fibres per bundle would have restricted the multiplex given
survey will bedeferred to a futurepaper.
the fixed AAOmega slit length, and the chosen 15 arcsec
IFU diameter provides a good match to the scale length of
galaxies in local samples chosen to match the surface den-
3 THE SYDNEY-AAO MULTI-OBJECT sity of SAMI IFUs within the 1–degree diameter field of
INTEGRAL FIELD SPECTROGRAPH view (see Section 2.7). If designed for a single experiment,
(SAMI) oneoptionwouldhavebeentohaveavarietyofbundlesizes
to match the specific galaxy size distribution of the target
With a view to providing the first on-telescope demonstra-
sample. However, for ease of manufacture, and to maintain
tion of hexabundle technology, the Australian Astronomi-
multiplex and flexibility, we chose to have all the IFUs the
cal Observatory (AAO) and the University of Sydney have
same size.
collaborated on the development of the SAMI instrument
forthe3.9mAnglo-AustralianTelescope(AAT).SAMIuses
13×61-corehexabundlesthataremountedonaplug-plateat
3.2 Fibre cable
the1-degreeFoVtripletcorrectortop-endfocusoftheAAT.
Atf/3.4,with105µmcorediameterfibres,eachhexabundle SAMI is mounted at prime focus on the AAT and feeds
samples a 14.9arcsec diameter field at 1.6arcsec per fibre theAAOmegaspectrograph located intheCouderoom,re-
core. At the output end, a total of 13 V-groove slit blocks quiring a fibre cable run of ∼42m. Within the fibre bun-
are mounted at the slit of the AAT’s AAOmega spectro- dle a total of 819 fibres are used: 793 for the hexabundles
graph (Sharp et al. (2006); also see Section 3.7). Each slit (13×61 fibres), and 26 for the sky fibres. The hexabundles
(cid:13)c 0000RAS,MNRAS000,000–000
The Sydney-AAO Multi-object IFS 9
wereeachsuppliedwithafibrepigtaillengthof1mencased
in a reinforced furcation tube. Each of the hexabundle and
sky fibres were fusion spliced onto a fibre of length ∼42m
that is terminated in a V-groove block at the spectrograph
entrance slit (see Section 3.7 below). For this fibre run we
choseafibreribboncabletominimisetherequiredassembly
effort. Each 250micron thick ribbon contains 8 fibres, and
8 ribbons are used for each of the 13 units (one hexabun-
dle plus two sky fibres). To minimize losses in the splicing
process, and because of its ready availability, we matched
the ribbonised fibre type to that used in the hexabundles
(ThorLabs AFS105/125Y).
A ‘splice-box’ is mounted on the internal wall of the
telescope top-end barrel that incorporates 13 closed-cell
polyethylene trays to secure and protect the individual
splices of each unit. The inputs to this box are the 26 sky
fibres (each individually sleeved with reinforced PVC fur-
cation tubing and terminated with SMA connectors) and
the13 individualhexabundletubes.Theoutputof thisbox
is the fibre bundle. The bundle is protected during its run
(which goes through the top end telescope ring, beside the Figure3.Animageofahexabundlefrontfacet.Thisshowsthe
primary mirror support, and through the declination drive 61optical fibrecores,whichhave atotal active areaof diameter
axistotheCouderoom)byaninnercoveringofbraidedca- 980µm.Surroundingthefibrecoresisaglassferrule(blackring),
ble sleeving and an outer double-split nylon conduit giving whichinturnis surroundedbythe central steel pinof the SMA
light-weight yet relatively strong protection. connection, whichextends beyondtheedgeoftheimage.
Thoughtheuseofribbonisedfibreworkedwellinterms
of handling and assembly, tests showed that ribbonising
caused a loss due to focal ratio degradation (FRD), as dis- The use of bare fibre hexabundles directly at the tele-
cussedinSection3.4.Thefibretypeusedprovidesrelatively scopefocalplaneisthemostnovelelementofourinstrument
good throughputabove450nm,thoughitsuffershigherab- design. There are several advantages to such an arrange-
sorption losses in theUV than other fibresmore commonly ment as an alternative to fibre-lenslet coupled IFUs that
used in astronomical instruments. The current fibre cable have been used elsewhere (e.g. Kelz & Roth 2006). First,
described in this paper will be replaced in the first half of thehexabundleis fabricated as a one-step process, whereas
2012 to providemuch improved blue throughput. a fibre-lenslet system requires thefibrebundleto be manu-
factured and then bonded to the lenslet array(s). Secondly,
there are no optical elements required in our hexabundle
3.3 Hexabundles
design. Lenslet or microlens array IFUs require at least 2
Bland-Hawthorn et al. (2011) introduced a new imaging fi- surfaces and more often up to 8 surfaces before the fibre
bre bundle optimized for low-light astronomical applica- face, to adjust for plate scale and to preserve telecentricity
tions.Thehexabundleincorporatesseveraltechnologicalin- of the telescope beam into the fibre. These extra surfaces
novations in order to achieve a high fill factor. A primary add to system loss. A trade-off must be made against the
goal of the hexabundle design is that the performance of potentialforextralossesinthehexabundle(e.g.fromFRD-
individual multimode fibres should be as good as the same seebelow)andfromitslowerfillfactor.ForSAMI,asweare
fibrein isolation. Consequently, we were forced to reject an feedingthehexabundleswithafastbeamtheFRDshouldbe
earlier design where the fibre cores were forced into non- minimised(ifusedwiththeappropriatefibretype).Finally,
circular shapes because the added focal ratio degradation the hexabundle solution offers the opportunity of reduced
(numericalapertureup-conversion)wasfoundtoleadtosig- pitch between IFUs relative to a lenslet solution that must
nificant light loss (Bryant et al. 2011). work at a higher magnification.
Withineachhexabundle,the61circularfibres(withnu-
merical aperture, NA=0.22) are lightly fused together and
infilled with low-stress glue. The multimodefibres haverel-
3.4 Focal ratio degradation and throughput
atively small core diameters of 105µm. In order to achieve
ahigh fillingfactor (75 percent),it was necessary toreduce Focalratiodegradation(FRD)increasesthesizeofthelight
the cladding to only 5µm. The length of the fused region cone coming out of a fibre compared to that put in. The
is very short (≈30mm) to minimize cross-talk between the worse the FRD, the more light will be lost from the f/3.15
fibrecores.Thefibrebundleisheldwithinareinforcedflex- acceptance cone of the AAOmega spectrograph. However,
ibleplastictubingthatisbothstrongandlight-weight.The FRD can be partially controlled by minimising the stresses
optical head is supported by a stress-relieving sleeve and is on the fibres when installing them in the system. An addi-
inserted into an SMA connector. This is to allow the fibre tional loss comes from the throughput of the fibres, which
bundletobemanually attachedtothefield platewith rela- is primarily dependenton thefibretypeand length.In this
tiveease.Therepeatablepositioningaccuracymadepossible sectionwedeterminetheperformanceofthefibrecableand
bytheSMAconnectorismuchbetterthanafibrecoresize. hexabundlesby analysing FRD and throughput. As will be
(cid:13)c 0000RAS,MNRAS000,000–000
10 Croom et al.,
Table 1. FRD and throughput results for the SAMI fibres and
hexabundles.ThebareandribbonisedfibreusedisthesameAFS
fibreasinthehexabundles.Resultsareshownforthecentralcore
(core 1) of hexabundle number 15 before the hexabundle was
splicedtothe fibrerun.Thenforcore1andfor2other cores (6
and18)inthesamehexabundle,whenthebundlewasfirstspliced
to the 42m of ribbonised cable with slit block attached. Lastly,
for the hexabundle with 42m of ribbonised cable and slitblock,
butmeasuredafterthe firstSAMIcommissioningrun,whenthe
ribbonised cables had been packed into a braided cable sleeve
and outer nylon conduit. Cores 1 and 18 are on the edges of a
ribbonised cable, while core 6 is in the centre of a ribbon. All
measurementsarebasedonanf/3.4inputtothehexabundleand
f/3.15output(acceptedbyAAOmega).TheNAup-conversionis
the difference between the input and output NA at 90 percent
encircled energy and has an error in each case of ±0.009. The
predicted performance is what we expect to achieve when the
fibre run has been replaced with a higher throughput fibre (e.g. Figure 4. Encircled energy (EE) vs numerical aperture (NA)
PolymicroFBP)andtheeffects ofribbonisingareremoved. profilesinB-bandforaninputoff/3.4.LargerFRD(orNAup-
conversion)shiftsthecurvestotheright.Theverticallinemarks
anoutputoff/3.15(intoAAOmega).Theblackcurveistheinput
Fibre Throughput NAup-conversion light curve. The cyan line is the FRD of the hexabundle alone,
Red Blue Red Blue withouttheribbonisedcable.Thegreen,redandbluesolidlines
% % 90%EE 90%EE that are nearly on top of each other, are the curves for cores 1,
6and18oncesplicedtotheribbonised42mcableandattached
Hexabundle15: to the slit block, but before being put into the cable sleeve and
core1alone 96±6 89±5 0.003 0.009 outernylonconduit(errorsare±0.006inNA).Thedashedcurves
core1plusribbon 67±7 41±6 0.028 0.037 (witherrorsof±0.0065)areforfibres6,1and18(samecoloursas
core1plusribbon above),aftertheribbonisedfibrewasputintothecablesleeveand
afterrun 44±7 24±6 0.067 0.073 outer nylon conduit and had been transported to the telescope,
installedandusedforthefirstcommissioningrun.
core18plusribbon 64±8 43±7 0.026 0.034
core18plusribbon
afterrun 37±8 22±7 0.076 0.082
increase in FRD,with theNAup-conversion(at 90 percent
core6plusribbon 65±8 40±7 0.028 0.037 encircled energy) for the hexabundle with ribbonised fibre
core6plusribbon measured to be significantly worse than for thehexabundle
afterrun 56±8 32±7 0.037 0.044
alone. Theend finish on thebundlealone was a cleave, but
Predictedperformance 84 62 themeasurementthroughtheribbonisedfibreandslitblock
withreplacement fibre had the advantage of a polished end finish which should
improvetheFRD,sotheFRDintroducedbytheribbonised
42mbarefibre 78±5 58±5 0.008 0.018
fibremay be a little worse than these numbersindicate.
42mribbonised 73±5 53±5 0.022 0.031
In order to differentiate between the effect of the 42m
of fibre and the ribbonising, we separately compared bare
and ribbonised fibre throughputs (Table 1, lower panel).
seen below, thedominant source of losses is thefibrecable, This was done by measuring the throughput from 10m of
rather than thehexabundles. bare fibre of the same typeas used in the hexabundles and
The FRD and throughput were measured for a SAMI ribbonised fibre (AFS105/125Y). The 10m was then cut,
hexabundle using an LED source that was fed through spliced together and measured to give the splice loss. Then
Bessel B and R filters (centred at approximately 435nm the splice was cut and an additional 42m was spliced in
and 625nm) and re-imaged to form an f/3.4 beam. This place. This was then cut out and 42m of ribbonised cable
was then input into several hexabundle cores in turn. The was spliced in instead. The throughput could then be com-
output fibres were imaged in the far-field using an SBIG paredforthebareandribbonisedcableafteraccountingfor
camera. After flat-fielding the images, the centre of each theinitial10m.Havingtheinitialfibreinplacemeantthat
output spot was fitted. Encircled energy was calculated in the FRD results for the bare and ribbonised fibre were not
concentric circles about the centre position using software affected by coupling into the fibre or end finishing effects
packages within iraf. as these remained the same for both tests. Variations in
Weinitiallycomparedtheperformanceofahexabundle throughputduetothedifferentspliceshavebeentakeninto
alone to the same hexabundle when spliced to the 42m of accountintheerrors.Table1showsthatwhilethebarefibre
ribbonised cable with a slit block attached (hexabundle15, results will include end effects, the ribbonising of the fibres
‘alone’vs‘plusribbon’inTable1).Forthreedifferentcores results in significantly worse FRD. The bulk of the loss in
(numbers1,6and18;seeFigure3),thethroughputdropped throughputisduetothelengthoffibre(78percentthrough-
to≃65percentintheredand41percentinthebluewiththe put in red and 58 percent in blue), however the FRD from
additionoftheribbonisedcableplusslitblock.Figure4and ribbonising results in less of that light coming out within
Table 1 show that this drop is identified with a significant thef/3.15 acceptance cone of thespectrograph.
(cid:13)c 0000RAS,MNRAS000,000–000