Extragalactic Fields Optimized for Adaptive Optics

1.5 MB·English

by  Ivana Damjanov

Extragalactic Fields Optimized for Adaptive Optics
Ivana Damjanov1, Roberto G. Abraham1, Karl Glazebrook2, Peter McGregor3, Francois
Rigaut4, Patrick J. McCarthy5, Jarle Brinchmann6,7, Jean-Charles Cuillandre8, Yannick
1
1 Mellier9, Henry Joy McCracken9, Patrick Hudelot9, David Monet10
0
2

n
a
J ABSTRACT

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In this paper we present the coordinates of 67 55′×55′ patches of sky which have the rare

] combination of both high stellar surface density (> 0.5 arcmin−2 with 13< R< 16.5 mag) and
O low extinction (E(B −V) 6 0.1). These fields are ideal for adaptive-optics based follow-up of
C extragalactic targets. One region of sky, situated near Baade’s Window, contains most of the
. patches we have identified. Our optimal field, centered at RA: 7h24m3s, Dec: −1◦27′15′′, has an
h
additionaladvantageofbeingaccessiblefrombothhemispheres. Weproposeafigureofmeritfor
p
- quantifying real-world adaptive optics performance, and use this to analyze the performance of
o
multi-conjugate adaptive optics in these fields. We also compare our results to those that would
r
t be obtainedin existing deep fields. Insome cases adaptiveoptics observationsundertakenin the
s
a fieldsgiveninthispaperwouldbeordersofmagnitudemoreefficientthanequivalentobservations
[ undertaken in existing deep fields.

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v
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8 Subjectheadings: AstronomicalTechniques,AstronomicalInstrumentation,AstrophysicalData,Galaxies
0
. 1. Introduction
1
0 1Department ofAstronomy & Astrophysics, University
of Toronto, 50 St. George Street, Toronto, ON, M5S 3H4 Our understanding of the high-redshift uni-
1
Canada verse has been revolutionized by deep fields, sev-
1
: 2Centre for Astrophysics and Supercomputing, Swin- eral of which have been extensively surveyed at
v burne University of Technology, 1 Alfred St, Hawthorn,
all accessible wavelengths. Figure 1 shows an
i Victoria3122, Australia
X 3Research School of Astronomy and Astrophysics, In- up-to-date summary of the locations of all ex-
r stituteofAdvancedStudies,TheAustralianNationalUni- isting deep fields (red circles). These fields have
a
versity,CanberraA.C.T.,Australia beenprimarilyusedtostudygalaxyformationand
4GeminiObservatory,SouthernOperationsCenter,c/o evolution out to very high redshifts (Cowie et al.
AURA,Casilla603,LaSerena,Chile 1995;Yee et al.2000;Labb´e et al.2003;Bell et al.
5Observatories of the Carnegie Institution of Washing-
2004; Egami et al. 2004; van Dokkum et al. 2004;
ton,813SantaBarbaraStreet,Pasadena, CA91101
Arnouts et al.2007;Davis et al.2007;Scoville et al.
6Sterrewacht Leiden, P.O. Box 9513 NL-2300 RA Lei-
2007; Bouwens et al. 2010; Ono et al. 2010). Be-
den,TheNetherlands
7CentrodeAstrof´ısica,UniversidadedoPorto,Ruadas cause galaxies at such high redshifts are typi-
Estrelas,4150-762Porto,Portugal cally < 1 arcsec in size, kinematical investiga-
8Canada-France-HawaiiTelescopeCorporation,65-1238 tions of galaxies in these fields require adap-
MamalahoaHighway,Kamuela,Hawaii96743,USA tive optics (AO) spectroscopy (Law et al. 2009;
9Institut d’Astrophysique de Paris, UMR7095 CNRS,
F¨orster-Schreiber et al. 2009). The promise of
Universit´ePierreet MarieCurie, 98 bis Boulevard Arago,
75014Paris,France
10The United States Naval Observatory, 3450 Mas- sachusetts Ave,NW,Washington, DC20392-5420
1
Fig. 1.— The galactocentric coordinates of existing deep fields (red circles) and the locations of the fields
better suitedfor AO observationspresentedinTable 1 ofthis paper(blue squares,see Section3 for details).
The location of the fields has been overplotted on the dust emission map from the Schlegel, Finkbeiner &
Davis(1998)study. Labeledexistingdeep fields areindicatedwith filledcircles. The greensquaredenotesa
1squaredegreeregionwithintheCFHTLSW2field(Section5.2). Thefieldlabeled‘AODF’isoursuggested
optimal field whose properties are studied in detail in Sections 4 and 5.
suchobservationshasbeenheldoutasanexciting rotational gradients about a preferred kinematic
nextstepforoveradecade(e.g.,Ellis et al.1997). axis (Law et al. 2009). F¨orster-Schreiber et al.
Unfortunately, it is now clear that only very lim- (2009) found similarly high velocity dispersions
ited AO observations are going to be undertaken but a much greater incidence of disk rotation in a
in any existing deep fields. predominantly non-AO dataset of predominantly
Coupling integral-field spectroscopy to AO is near-IR selected galaxies. It remains inconclusive
crucialforunderstandingtheformationofmassive whether such differences arise from a difference in
galaxies, particularly disks, since at high-redshift sample (massive vs low mass galaxies) or the fact
it has proven difficult for slit spectroscopy to re- that the non-AO data has seven times poorer res-
liably identify kinematic disks as kinematic and olution on average in natural seeing. This is an
morphological axes are not necessarily correlated important question: physical differences in kine-
(Erb et al. 2006). Even at intermediate redshift matics at high-redshift may diagnose the preva-
(z ∼ 0.6), it has been demonstrated that galax- lence of fast gas accretion along cold flows in the
ies are already kinematically complex and that early Universe (e.g., Bournaud et al. 2007), but
3D integral field spectroscopy (IFS) is essential theymayalsoarisefromsampleselectioneffectsor
to physical understanding and kinematic model- observationallimitations(forexampleGreen et al.
ing (Rix et al 1997; Flores et al. 2006). At the (2010)suggestitissimplythehigh-star-formation
highest redshifts AO IFS observations by some rates which drive the large velocity dispersions).
groups have given different results compared to Most IFS observations at high-redshift are still
non-AO observations of other groups. For ex- done without AO due to the technical difficulties
ample Laser Guide Star (LGS) AO observations of AO andalso to the practicaldifficulties of find-
with kpc resolution (Law et al. 2009) show that ing enoughtargetsnearsufficiently brightstarsin
z =2−3Lyman-breakselectedgalaxieshavehigh existing deep field samples.
intrinsic velocity dispersions and no significant The next stage in the development of this field
2
is to complement kinematical studies by probing (2007) are accessible to the VLT laser guide star
chemical abundance gradients at a sub-kpc scale system (Bonaccini et al. 1999): the loss of 75%
in star-forming galaxies, and to extend existing of the targets is due to the absence of suitably
kinematicalinvestigationstoencompassmorerep- closenaturalguidingstars(NGS),whileadditional
resentative galaxies. This requires AO systems to 25% are lost after suitable color cuts and elimi-
be operating more efficiently (i.e. without per- nation of systems at redshifts obscured by strong
formance limitation imposed by natural guide- OH features. The situation is similar with Gem-
star availability) and, ultimatelely, to multiplex ini, whose AO system has similar sky coverage
if truly large samples are to be obtained. A step (Ellerbroek & Tyler1998). Evenwiththeupcom-
in this direction is already being taken with the ingGeminiMulti-ConjugateAOsystem(MCAO),
MASSIV survey on the VLT, which targets star- the H-band sky coverage at the galactic poles
forming galaxies in the redshift range 1 < z < 2 will only be around15%(Rigaut et al.2000), and
with SINFONI (Epinat et al. 2009; Queyrel et al. largebenefitsforMCAOemergefromhavingmore
2009). The targets are more representative than than the minimum number ofnaturalguide stars.
those being probed by SINS, with median stellar This is because the geometry of the guide stars
masses of ∼ 1010M⊙ and median star formation on the sky impacts the uniformity of Strehl ra-
rates of ∼ 10M⊙yr−1 (the corresponding values tio (Flicker & Rigaut 2001).
for SINS are ∼ 1010.5M⊙ and ∼ 30M⊙yr−1, re- The issue of guide star rarity in deep fields be-
spectively). However, most of the SINFONI data comesprominentincaseswheretargetsourceden-
acquired during the MASSIV survey are seeing- sity is low. This is often the case for extragalactic
limitedleadingtoafinalmedianspatialresolution programswhich focus onunusual objects. For ex-
of∼0.6−0.7′′withonly25%oftheMASSIVsam- ample, many of the key projects described in the
ple presentlybeing observedwithadaptiveoptics. JWST Design Reference Mission (Gardner et al.
OftheseAOtargetsonlyafew arebeingacquired 2006) rely on either extreme depth or serendipi-
withthesmallestpixelsize(0.05′′). Themainrea- tous lines-of-sight. IfsuchJWST observationsare
sons are (i) the limitations in the availability of to be synergistic with ground-based AO follow-
natural guide stars which precludes usefully ob- up, in particular with next generation telescopes
serving at finer available pixel scale, and (ii) the like TMT or E-ELT, they cannot be undertaken
difficulty to reachthe depth requiredto probe the efficiently in any existing deep field. It would
low-surface brightness component of galaxies in a be disappointing indeed if only 1%–10% of rare
reasonable exposure time with the smallest pixel targets imaged with JWST in a deep field could
size. Thislatterpointleadstoexpectationsofcon- be followed-up with a ground-based integral field
siderable progress in this subject with the advent units (IFU). It is becoming clear that existing
of 30m-class Extremely Large Telescopes (ELTs). and planned AO systems are set to enable trans-
A basic problem with undertaking AO in ex- formative high-redshift science, but they will do
isting deep fields, even with laser guide stars, is so only in the regions of the sky in which they
that one still needs at least one reasonably proxi- are effective. It is arguable that no existing deep
mate naturalguide startosupply the information fieldissuitableforefficientextragalacticAOwork
needed for tip-tilt correction (Rigaut & Gendron (though of course the cost of obtaining ancillary
1992). In contrast, two of the main selection cri- data equivalent to that already obtained in exist-
teria when identifying deep fields have been that ing deep fields may overwhelmthe gains obtained
they contain as few bright stars as possible to from high-efficiency adaptive optics).
avoid light scattering contamination and satura- In this paper we report on the results we
tion in long exposures, and that they lie in re- have obtained in searching for those fields on the
gionsoflowGalacticextinction(e.g.,Alcala´ et al. sky most suitable for high-efficiency extragalactic
2004). Thus all existing deep fields are near the adaptive optics observations. In Section 2 we de-
Galactic poles, where the density of suitable nat- scribe the important characteristics of deep fields
ural guide stars is near a minimum. For ex- in the context of adaptive optics observations,
ample, Davies et al. (2008) report that only 1% such as the acceptable level of dust extinction,
of the Lyman break galaxy sample of Mannucci field size, and magnitude range of natural guide
3
stars. In Section 3 we describe our attempts to mean extinction as a constraint when exploring
identify the mostsuitable areasonthe sky forun- starcountsurfacedensitymapsforsuitablefields.
dertaking extragalactic AO work, which is based We note that E(B −V) = 0.1 mag corresponds
on the strategy of combining information from to A = R ×E(B −V) ∼ 0.3 mag at visible
V V
all-sky stellar density and extinction maps. Our wavelengths, and that this is a factor of three to
preferred ‘AO-friendly’ field and its first imaging tentimeshigherthanthecorrespondingextinction
resultsaredescribedinSection4. Inthefollowing in near-infrared (NIR) passbands used by current
Section 5 we define a figure of merit for adaptive AO systems.
opticsandusethistocomparetheefficiencyofAO
observing in the proposedfields relative to the ef- 2.2. Field size
ficiency in representative deep fields. Our conclu-
The next factor to consider is the required size
sionsaresummarizedinSection6. Allmagnitudes
of the field. For extragalactic fields, the area of
in this paper are based on the Vega system.
the field is driven by a desire to minimize the im-
pact of cosmic variance, because scale-dependent
2. Desirable characteristics of extragalac-
inhomogeneity is often the dominant source of er-
tic fields optimized for adaptive optics
ror in measurements derived from galaxy popula-
tions within a survey volume. The survey volume
Inthissectionweconsiderthedesirablecharac-
naturally depends on the area on the sky and the
teristicsofextragalacticfieldsoptimizedforadap-
chosenredshiftrange,butforconcretenesswewill
tive optics. The relevant considerations include
assume that most extragalactic work will explore
the maximum acceptable level of extinction from
a range of redshifts from z = 0 to z = 4, which
the intergalactic medium, the minimum useful
encompassesmostofthe star-formationhistoryof
area on the sky, and number density and mag-
theUniverse. Forsuchsurveys,areasontheorder
nitude range of available natural guide stars. We
ofasquaredegreeareneededinordertomaintain
willconsidereachofthesefactorsinturn,anddis-
fractional errors on number counts near the 10%
cuss the importance of each of these factors using
level, and to probe a wide range of cosmic struc-
verygeneralprinciples,inordertolookforconsid-
tures. This isfairlyeasytodemonstrateusingon-
erations that will remain relevant for future AO
linetoolssuchastheCosmicVarianceCalculator1
systems.
describedinTrenti & Stiavelli(2008),butaneven
2.1. Extinction simpler wayto showthis is to use the analytic ex-
pressionsprovidedby Driver & Robotham(2010)
Althoughmostadaptiveopticsisundertakenin toestimateandcomparecosmicvariancefordiffer-
thenear-infraredwhereextinctionislowerthanat ent field sizes. These authors employed counts of
visible wavelengths, it is clear that for any num- galaxies near the characteristic break in the lumi-
berofreasons,includingreliabilityofphotometric nosityfunction(M∗)intheSloanDigitalSkySur-
redshifts and ‘future-proofing’ the fields so as to vey Data Release 7 (SDSS DR7, Abazajian et al.
makethemusefulwhenAOworkmovestoshorter 2009)toderiveanempiricalexpressionconnecting
wavelengths, that the ideal fields will lie in re- cosmic variance and survey volume. Assuming a
gions of low Galactic extinction. As any glance single sight-line and a rectangular geometry, the
at the night sky will attest, patchy extinction can fractional error in the counts of M∗ galaxies is
be ratherhighinregionswithhighstarcounts. It given by:
is therefore important to define an upper limit to
the acceptable extinction in order to exclude un-
suitable fields. A value of E(B −V) ∼ 0.15 mag ζ = (1−0.03(p(A/B)−1)
is a good starting point, because Fukugita et al. × (219.7−52.4(log [A·B·291.0])
10
(2004) and Yasuda et al. (2007) show that galac- 2
+3.21(log [A·B·291.0] ))
tic extinction estimates become fairly unreliable 10
in regions with E(B − V) & 0.15 mag. To err pC/291.0. (1)
on the conservative side, in this paper we will use
an upper limit of E(B − V) = 0.1 mag on the 1 http://casa.colorado.edu/~trenti/CosmicVariance.
html
4
Fig. 2.— Cosmic variance, quantified using Eq. 1, as a function of redshift for four fields covering different
areas on the sky. Redshifts of presented points correspond to median values for redshift ranges indicated
with doted lines. The effect of small-scale inhomogeneity on the field size we propose (∼ 1 square degree,
denoted as AODF) is comparable to the COSMOS field cosmic variance, and much less prominent than in
the other two (smaller) fields, GEMS and GOODS.
where A, B, and C are the median redshift trans- for a contiguous area survey field intended to al-
verse lengths and the radial depth of the sur- lowabroadrangeofinvestigationsusingadaptive
vey, respectively, expressed in units of h−1 Mpc. optics, although another important factor is that
0.7
(Note that the derived cosmic variance is for a survey of this size will contain many thousands
M∗ ± 1 mag population only and will take of strong line emitting objects, which are obvious
higher values for more massive halos, see e.g., targets for present-generationAO systems.
Moustakas & Somervillle 2002). Results com- We have computed the surface density of
puted using this equation are presented in Fig- strong H line emitters (which we define to
α
ure 2, which shows the calculatedcosmic variance be F > 10−16 erg cm2 s−1, corresponding
Hα
for a number of surveys,and comparesthese with to the flux density of bright line emitters in
ourproposedfieldsizeofaroundonesquaredegree F¨orster-Schreiber et al. 2009; Law et al. 2009) on
(actually 55′×55′, for technicalreasonsdescribed thebasisofdirectmeasurement(Villar et al.2008;
below). Shim et al. 2009) as well as using indirect esti-
Figure2 showsthat the calculatedcosmic vari- mates scaled from UV flux (Bouwens et al. 2009)
ance for our proposed field size results in frac- and measurements of [OII] (Cooper et al. 2008).
tional counting errors of around 10 − 15% (per By incorporating all the available information we
unit redshift interval) for counts of M∗ galax- estimatethisvaluetobe2–5H lineemitterswith
α
ies at redshifts between z = 1 and z = 4. flux > 10−16 erg cm2 s−1 ˚A−1 per square arcmin
This is only slightly higher than for the COS- at1<z <1.5,decliningto1–2persquarearcmin
MOS field (Scoville et al. 2007), but quite sig- in the redshift interval 2 < z < 2.5. The deep
nificantly better than for smaller volume sur- fieldsproposedinthispaperwillthushavearound
vey fields, such as GEMS (Rix et al. 2004) and 10,000suitable targets for AO-basedfollow-up. A
GOODS (Dickinson et al. 2003). On this ba- significantfractionofthese willbe lostforvarious
sis alone we would argue that something around reasons (e.g., if H lies on an airglow emission
α
square degree is probably the right minimum size line, Davies et al. 2008), and a small number of
5
remaining objects will still lack suitable guiding site. Wind speeds in the upper atmosphere are
stars (see Section 5). However, thousands of AO- around20m/s,soittypicallytakesaround0.005s
accessible targets will remain, presenting a mul- for a patch of atmosphere to move a coherence
tiple order-of-magnitude change from the current length(Rogemann & Welsh1996). The minimum
situation. frequency of an AO system is therefore around
200Hz,althoughinrealityonewouldwanttoboth
2.3. Guide star limitations Nyquist sample the signal and allow time for ac-
tuator lag in applying a correction, so a realistic
We nowexplorethe brightnessofnaturalguide
minimum is around 1 kHz.
stars needed for effective use of adaptive optics.
How many photons from a natural guide star
Ourfocuswillbeonthethefollowingthreeclasses
of AO systems: are needed in this time depends on the specific
type of correction, but we can bracket our analy-
• Case 1: Laser-assisted adaptive optics sys- sisbyconsideringtwoextremes: (i)tip-tiltcorrec-
tems on 8m-class telescopes, for which nat- tion,forwhichrelativelyfaintstarssuffice,and(ii)
ural guide stars are needed to supply tip- fullcorrectiontoobtaindiffraction-limitedperfor-
tiltcorrections. Suchsystemswilldefinethe mance, for which bright stars are needed.
state of the art for the next few years. A zeroth magnitude star has a R-band flux of
3080Jyatthetopoftheatmosphere2,correspond-
• Case 2: Ground-layer adaptive optics sys- 11 2
ing to 2.02 × 10 photon/m /s (Bessell 1979).
tems for 4m-class telescopes. Such sys-
Thusan8mtelescopecaptures∼600R-bandpho-
tems are now being proposed as a means
tons from an 18th magnitude star in one millisec-
of revitalizing 4m-class facilities (e.g CFHT
ond. In the foreseeable future no AO system will
’IMAKA, Lai et al. 2008). These facilities
have a quantum efficiency approaching unity, but
will also require natural guide stars for tip-
evenwithanend-to-endefficiencyof20%over100
tilt correction.
photons will remain, which is ample for obtaining
a reasonable centroid. Thus, at least in principle,
• Case3: AOsystemson30m-classtelescopes,
anAO system onan8m telescope canuse R=18
some designs for which rely on AO for rou-
mag stars for tip-tilt corrections. Since the num-
tine operation. In this case we mainly seek
ber of photons from a star imaged with a 30m
fields with an abundance of natural guide
telescope is about fourteen times greater than for
stars bright enough to feed laser-unassisted
an 8m telescope, a 30m telescope can do tip-tilt
AO systems. Laser beacons may not be
correctionsonguide starsdownto aroundR=21
available at all times, and the existence of
mag. On the other hand, a 4m telescope needs
extragalacticfields inwhich they arenot es-
stars of about R = 16.5 mag for tip-tilt correc-
sentialmay be extremely attractive for tele-
tions. We emphasize that these numbers are all
scopes that heavily emphasize AO.
for rather idealized AO systems. For example, in
We will begin by first outlining the general the real-world situation of the Gemini Altair AO
problem before focusing on the parameter space system, tip-tilt reference starsof aroundR∼16.5
appropriatetothe specific casesabove. As willbe mag (over a magnitude brighter than the some-
shown below, in practise it is Cases 2 and 3 that what ideal case discussed above) are found to be
drive our chosen magnitude limits. highly desirable for high-performance AO opera-
tion.
In order to function an AO system needs to
capture photons from a star, compute a correc- Much brighter natural guide stars are needed
tion, and apply this correction to an optical sur- forusewithnaturalguideAOstarsystemsthatat-
face. The frequency over which an AO system tempt to achieve diffraction-limited performance.
must operate is set by the velocity of the atmo- Inthiscasethesizeofrelevanceisnotthefullaper-
sphereandtheatmosphericcoherencelength. The
coherencelengthisthelengthscaleoverwhichthe 2For concreteness we consider the brightness of guide stars
index ofrefractionofthe atmosphereis effectively atvisiblewavelengths,thoughtheargumentcanbegener-
alizedtostarsatarbitrarywavelength.
constant, and is typically around10 cm at a good
6
ture of the telescope, but rather the sub-aperture density for UCAC2 stars in the range 13 −
definedbythecoherencelengthoftheatmosphere 16.5magusing the HEALPIXdataanalysispack-
which in turn drives the number of needed actu- age (G´orski et al. 2005) that performs pixeliza-
ators. An R = 13 mag star supplies ∼ 10 pho- tion of the sphere with equal area pixels. Two
tons in 1 ms to a 10 cm diameter sub-aperture. maps have been produced: one with the reso-
The number of photonsper binneeded to reliably lution of 6.′871 to match the resolution of the
compute a wavefrontdepends critically on factors available HEALPIX map of Galactic reddening
such as the read noise of the detector, but ten E(B −V) and the other (see Figure 3) with the
photons per coherence-length sized patch on the coarser sampling of 55′ (the HEALPIX resolution
pupil is a reasonable lower limit. Note that in that is closest to the 1◦ ×1◦ field size, see Sec-
the case of diffraction-limited AO (and unlike the tion2.2). Theresolutionofthe existingE(B−V)
case with tip-tilt correction), having a larger tele- map was degraded to match the 55′ resolution of
scope does not gain one a fainter magnitude limit the star counts surface density map by taking the
for natural guide stars, and in fact AO becomes average extinction value for each cell. The coarse
harder because the system requires more actua- resolutionextinctionandstarcountmapsareboth
tors. Wealsonotethatthepresenceofbrightstars shown as panels at the top-left and top-right of
(R.14 mag) in the field with AO correction can Fig. 3. Note that the UCAC2 catalog has a gap
potentially cause problem for infrared (IR) detec- in coverage at high declination (shown in gray in
tors by leaving long-lasting (up to an hour) resid- the figure), but any AO-optimized fields which
ual flux. Although this would affect imaging in might exist at these very high declinations would
case when AO correction is applied across a wide be generally unsuitable anyway. Any such fields
field of view, the main motives for developing an would be inaccessible from Chile and be at quite
AO-friendly deep field (see Sections 1 and 5) are high airmass most of the time for major north-
high resolution IFU or multi-object spectroscopic ern hemisphere observatories (including those on
surveys, that would not be influenced. Mauna Kea).
On the basis of the considerations just given, Before proceeding with a detailed analysis, it
oursearchforlocationsonthe skysuitable forex- is instructive to note that many positions in the
tragalactic adaptive optics focuses on stars in the sky likely to be suitable for our purposes can be
magnitude range 13 <R < 16.5 mag. The bright identified easily by simply looking for maxima in
end is set by the apparent magnitude of stars a map obtained by multiplying the stellar density
neededtosupplyguidestarsfornaturalguide-star map by the inverse of the extinction map. This is
AO systems (independent of telescope aperture), shownasthelargebottompanelinFigure3. Local
whichisessentiallyCase3above. Thefaintendis maxima in this map do not necessarily define re-
set by the apparent magnitude of stars needed to gionssuitableforAO,becausesomelocalmaxima
supplytip-tiltreferencestarsforreal-worldopera- correspondtoregionswithlowstarcountsbutex-
tionofexistingAOsystemson8m-classtelescopes tremely low extinction. However, this figure acts
(Case 1) and for ideal-case laser-based ground- asanaturalstartingpointforthenextstepinour
layerAOwith4m-classtelescopes(Case2above). analysis.
Having identified candidate fields using the
3. Identification of the suitable fields for analysis just described, we then looked at all
Adaptive Optics the candidate fields individually to try to bet-
ter understand their characteristics. To be ex-
Inorderto find the regionsonthe sky with the
plicit, we firstidentified allHEALPIXcells whose
properties we have just described, we rely on full
13 < R < 16.5 mag stellar density Σsc was
sky reprocessedcomposites of the COBE/DIRBE Σsc > 0.5 arcmin−2 and whose extinction was
and IRAC/ISSA dust maps (Schlegel et al. 1998)
E(B−V)60.1. We found 442 one squaredegree
and the UCAC2 astrometric catalog of ∼ 5 ×
cells met these criteria, and these were then ex-
107 stars with declination in the [−90deg,+(40−
aminedfurther. Thedistributionofstellardensity
52)deg] range (Zacharias et al. 2004). We con-
and extinction for these cells is shown in Fig. 4,
structed a full sky map of star count surface
color-coded by right ascension. In order to cull
7
Fig. 3.— (Top left:) An all sky map of extinction, scaled logarithmically. The solid line grid corresponds to
the celestial coordinate system with RA in degrees increasing to the left. Zero degrees lies at the center of
the figure. A Galactic coordinate system is over-plotted with dashed lines. (Top right:) The corresponding
map of star count surface density for stars in the 13−16.5 magnitude range. The region shown in gray
corresponds to a high declination gap in coverage in the UCAC2 stellar catalog. As noted in the text,
any AO-friendly fields which might exist at very high declination would be unsuitable for other reasons.
(Bottom:) A map constructed by multiplying the map at the top left by the inverse of the map at the top
right. Maxima in this figure correspond to potentially interesting locations for undertaking extragalactic
adaptive optics observations. Red circles present the positions of 67 fields well-suited to extragalactic AO.
See text for details.
8
Fig. 4.—ExtinctionE(B−V)asafunctionofthestarcountssurfacedensityΣsc for44255′×55′ fieldswith
Σsc > 0.5 arcmin−2 and E(B−V)6 0.1. The fields are color-coded based on their equatorial coordinates.
The dashed line encloses 67 fields with Σsc >0.65 arcmin−2 and 0.05.E(B−V)[mag].0.087. The fields
flagged with open circles have the highest star counts surface density or the lowest mean extinction or its
standard deviation. Colored arrows point at the representative fields for each of the three sightlines (see
Appendix A for details). The proposed ‘optimal’ field described in Section 4 is labeled ‘AODF’ and flagged
with an open box.
9
0.02
1
1
0.015
0
0.01 0
-1
0.005
5
1
4
0.2
3
0 2
0.15
1
0
0.1 -1
-1
00..88 1 1.2 1.4 00..88 1 1.2 1.4 00..88 1 1.2 1.4
Fig. 5.— The three higher-order moments of the extinction and star count surface density distributions as
functions of the mean star count surface density for 67 fields from Table 1. The fields are color-codedbased
on their equatorial coordinates, as given in Figure 4. The fields flagged with open circles or with colored
arrowscorrespondto the flaggedfields in Figure 4. Our optimal AO-friendly field is labeled as in Figure 4.
10