Ionized gas in the starburst core and halo of NGC 1140

1.2 MB·English

by  M. S. Westmoquette

Mon.Not.R.Astron.Soc.000,1–9(2009) Printed5January2010 (MNLATEXstylefilev2.2)
⋆
Ionized gas in the starburst core and halo of NGC 1140
M. S. Westmoquette1†, J. S. Gallagher III2, L. de Poitiers1
1Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT
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2Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter St., Madison WI 53706, USA
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Accepted2009December17.Received2009December3;inoriginalform2009October15
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ABSTRACT

We present deep WIYN Hα SparsePak and DensePak spatially-resolved optical spec-
]
O troscopyofthedwarfirregularstarburstgalaxyNGC1140.Thedifferentspatialreso-
lutions and coverage of the two sets of observations have allowed us to investigate the
C
properties and kinematics of the warm ionized gas within both the central regions of
.
h the galaxy and the inner halo. We find that the position angle of the Hα rotation axis
p for the main body of the galaxy is consistent with the Hi rotation axis at PA = 39◦,
- but that the ionized gas in the central 20×20 arcsecs (∼2×2 kpc) is kinematically
o
decoupled from the rest of the system, and rotates at a PA approximately perpendic-
r
t ular to that of the main body of the galaxy at +40◦. We find no evidence of coherent
s
a large-scale galactic outflows. Instead multiple narrow emission line components seen
[ within a radius of ∼1–1.5 kpc, and high [Sii]/Hα ratios found beyond ∼2 kpc im-

plying a strong contribution from shocks, suggest that the intense star formation is
1
driving material outwards from the main star forming zone in the form of a series of
v
interacting superbubbles/shells.
8
9 A broad component (100.FWHM.230 kms−1) to the Hα line is identified
6 throughout galaxy disk out to >2 kpc. Based on recent work looking at the origins of
0 thiscomponent,weconcludethatitisproducedinturbulentmixinglayersonthesur-
. facesofcoolgasknotsembeddedwithintheISM,setupbytheimpactoftheionizing
1
radiation and fast-flowing winds from young massive star clusters. Our data suggest
0
0 a physical limit to the radius where the broad emission line component is significant,
1 andweproposethatthislimitmarksasignificanttransitionpointinthedevelopment
: of the galactic outflow, where turbulent motion becomes less dominant. This mirrors
v
whathasrecentlybeenfoundinanothersimilarirregularstarburstgalaxyNGC1569.
i
X
Key words: galaxies: individual (NGC 1140) – galaxies: starburst – galaxies: ISM –
r
a ISM: kinematics and dynamics – ISM: jets and outflows.
1 INTRODUCTION (note the reverse may be true for those with more irreg-
ular morphologies; Silich & Tenorio-Tagle 2001). Although
Low-metallicity, starbursting, dwarf galaxies are a partic-
theejectionoftheISM(includingthefreshlychemicallyen-
ularly interesting class of galaxy for a number of reasons:
richedmaterial)hasclearconsequencesforthefutureevolu-
(1)theirlowmetallicitiesmakethemexcellentanaloguesto
tion of the system in terms of its dynamics and star for-
intenselystarforminggalaxiesathigh-redshift,whichinhi-
mation rate (Dekel & Silk 1986), and for the enrichment
erarchical galaxy formation models are thought to be the
of the intergalactic medium (Aguirre et al. 2001; Cen et al.
buildingblocksofpresent-daymassivegalaxies,and(2)the
2005),somestudiessuggestthatejectionofhotgasthrough
starbursteventcanhavemuchmoreofadestructiveeffecton
bubble blow-out may not be as efficient as first thought
thegalaxythanformoremassivesystems;theirlowgravita-
(De Young & Heckman 1994; Martin 1998). It is therefore
tionalpotentialsmayalsomeanthatforthosewithflattened
important to study such systems to understand how gas is
morphologies, outflowing material is much more likely to
removed and what effects this has in the evolution of the
escape altogether (Larson 1974; Mac Low & Ferrara 1999)
galaxy.
NGC 1140 is a nearby (20 Mpc; Moll et al. 2007),
⋆ Based on observations made with the WIYN SparsePak and low metallicity (LMC-like, 0.4 Z⊙; Izotov & Thuan 2004;
DensePakinstruments. Nagao, Maiolino & Marconi 2006; Moll et al. 2007) blue
† E-mail:[email protected] compact dwarf (BCD) galaxy with a total star formation
2 M.S. Westmoquette et al.
rate of 0.7±0.3 M⊙ yr−1 (Hunter, van Woerden & Gal- 2 OBSERVATIONS AND DATA REDUCTION
lagher 1994, hereafter H94b; Moll et al. 2007). The cur-
WeobservedNGC1140ontwoseparateoccasionswiththe
rent star formation is dominated by an extremely Hα-
WIYN1 SparsePak and DensePak instruments. These two
luminouscentralconcentrationthatisknowntohostanum-
data sets, providing observations at complimentary spatial
ber of young massive clusters (YMCs) with ages <5 Myr
resolutions, have allowed us to examine the kinematics and
andmasses>105M⊙(Hunter, O’Connell & Gallagher1994;
nebular properties of the galaxy’s disk, halo and nuclear
Moll et al. 2007); this extreme youth is supported by the
regions in some detail.
detection of strong WR star signatures (Moll et al. 2007).
It is thought that these clusters are the most recent
products of a galaxy-wide starburst induced by a merger 2.1 SparsePak
or interaction with a low luminosity, gas-rich companion
(H94b):theopticalcolours(H94b),broad-bandphotometry Observations of NGC 1140 were obtained with the
(Cid Fernandes et al. 2003), and photometric star cluster SparsePakinstrument(Bershady et al.2004)ontheWIYN
age-dating (de Grijs et al. 2004) all suggest extensive star 3.5-m telescope. SparsePak is a “formatted field unit” sim-
formationthroughoutthegalaxyinthelast1Gyr,andpar- ilar in design to a traditional IFU, except that its 82
ticularly in the last 20 Myr. fibers are arranged in a sparsely packed grid, with a small,
nearly-integral core (Bershady et al. 2004). SparsePak was
The misalignment of the galaxy’s structural compo- designedtomaximisethroughputandspectralresolutionat
nents are consistent with the merger scenario. On large theexpenseofspatialcoverage/resolution,andassuchhasa
scales (.40 kpc), the Hi gas is elongated along a position totallightthroughputof∼90percentlongwardsof500nm.
angle(PA)of−51◦(althoughstronglywarpedonthesouth- Eachfibrehasadiameterof500µm,correspondingto4′.′69
easternside).Intheinnerregions(central∼10kpc),theHi onthesky;theformattedfieldhasapproximatedimensions
gasisconcentratedalonganapproximatelyperpendicularly of72×71.3arcsecs,includingsevenskyfibreslocatedonthe
oriented ridge (PA ≈ +22◦). To first order this neutral north and west side of the main array separated by ∼25′′.
gas all rotates in almost solid-body rotation, with Themappingorderoffibersbetweentelescopeandspectro-
the line of nodes at the PA of −51◦ (i.e. Hi rotation graphfocalplaneswaspurposefullydesignedinafairlyran-
axis PA = +39◦), and exhibits a high velocity dispersion domisedfashioninordertodistributeskyfibresevenlyalong
indicating that the system is not relaxed (H94b). In the the slit and minimise the effects of spectrograph vignetting
V-bandhowever,themorphologyofthisinnerregionissig- on the summed sky spectrum. SparsePak is connected to
nificantlydifferent,beingroughlyrectangularinshapewith the Hydra bench-mounted echelle spectrograph which uses
a PA of 0◦ (H94b). At the south-west end, at a distance a T2KC 2048×2048 CCD detector.
of ∼2.5 kpc, the rectangle has a hook that extends towards We obtained observations of NGC 1140 centred on the
thewest,tracedbyachainofHiiregions(seeFig.1).This Hα peak at α=2h54m33s.43, δ=−10◦01′39′.′3 (J2000) during
chain is aligned with the PA ≈ +22◦ Hi ridge, and it is the period 13–16th December 2004 with a total exposure
these Hii regions that are coincident with the peak of the time of 3×1800 secs. The position of the SparsePak foot-
Hi emission (H94b), not the central Hα concentration as print is shown in Fig. 1, overlaid on an HST/ACS-WFC
might be expected. Further out at fainter V-band surface continuum subtracted F658N image (Prop ID: 9892, PI:
brightness levels, the galaxy appears elongated towards the Jansen). Using an order 8 grating with an angle of 63.25◦
north-west and south-east along a PA ≈ −15◦. Further out (giving a spectral coverage of 6450–6865 ˚A and dispersion
still, a number of low surface brightness shells are observed of 0.20 ˚Apix−1), we were able to cover the nebular emis-
thatmaybelinkedwitheitherpaststar-formationeventsor sion lines of Hα, [Nii]λλ6548,6583, and [Sii]λλ6716,6731.
the merger/interaction (H94b). A number of bias frames, flat-fields and arc calibration ex-
posures were also taken together with the science frames.
HST imaging(Fig.1)showsthatthecentralHαconcen-
Basic reduction was achieved using the ccdproc task
tration extends into a network of complex shells, filaments
within the noao iraf package. Instrument-specific reduc-
andbubblesthatfilltheISMofthegalaxyouttoadistance
of ∼2 kpc. High [Sii]/Hα line ratios (H94b; Burgh et al. tionwasthenachievedusingthehydratasksalsowithinthe
2006) and strong [Feii] emission (de Grijs et al. 2004) are noaopackage.Thefirststepwastorunapfindontheflat-
fieldexposuretoautomaticallydetecttheindividualspectra
both indicators that shocks play an important role in the
on the CCD frame. The output of this task is an aperture
halo. Thus the presence of multiple YMCs, the high Hα lu-
identificationtablewhichcanbeusedforeachscienceframe
minosity,strongevidenceofshocks,andthis“froth”ofstruc-
to extract out the individual spectra. The task dohydra
ture in the ionized gas all suggest strong starburst-driven
was then used to perform the bias subtraction, flat-fielding
feedback is taking place, although to date no kinematic ev-
and wavelength calibration. The datafile at this stage con-
idence has been found for a coherent, galaxy-wide outflow.
tained82reducedandwavelengthcalibratedspectra.Arep-
Withallthisinmind,wehaveobtaineddeepspatially- resentativeskyspectrumwascreatedbyaveragingtheseven
resolvedopticalspectraofNGC1140fromtwoinstruments sky fibre spectra, and sky-subtraction was achieved by re-
withdifferentspatialresolutionsandcoverage,toinvestigate runningdohydrawiththesky-subtractionoptionswitched
thepropertiesandkinematicsofthewarmionizedgaswithin on.Cosmic-rayswerecleanedfromthedatausinglacosmic
both the central regions of the galaxy and the inner halo.
In this work, we adopt a systemic velocity for NGC 1 The WIYN Observatory is a joint facility of the University of
1140of1475kms−1 givingadistanceof20Mpc(Moll et al. Wisconsin-Madison,IndianaUniversity,YaleUniversity,andthe
2007), meaning 1′′ ∼97 pc. NationalOpticalAstronomyObservatories.
Ionized gas in NGC 1140 3
(van Dokkum2001),beforefinalcombinationoftheindivid- theWIYNtelescope2.DuringourrunDensePakwasinstead
ualframeswasachievedusingimcombine.Thefinaldatafile attachedtothef13.7modifiedCassegrainport,thuschang-
now contained 82 reduced, wavelength calibrated and sky- ingthedetectorplate-scalefromthedocumentedvalue.The
subtracted spectra. An example reduced and labelled spec- array has 91 fibres, each with a diameter of 300 µm (1.3′′
trum is shown in the top panel of Fig. 3. ontheskyattheCassegrainfocus).Thefibre-to-fibrespac-
In order to determine an accurate measurement of the ing is 400 µm making the overall dimensions of the array
instrumental contribution to the spectrum broadening, we 12.4×19.8arcsecs.Fouradditionalfibresareoffsetby∼30′′
selectedhighS/Nspectrallinesfromawavelengthcalibrated fromthearraycentreandserveasdedicatedskyfibres.The
arc exposure that were close to the Hα line in wavelength, arrangementoftheDensePakfibrearrayonthesky,includ-
and sufficiently isolated to avoid blends. After fitting these ing the sky fibres, is shown in Sawyer (1997, note the di-
lineswithGaussiansinall82apertures,wefindtheaverage mensions given in this reference apply to DensePak at the
instrumental width is 0.7±0.02 ˚A = 31.4±0.5 kms−1. Nasmyth focus). At the time of observation, there were 8
damaged and unusable fibres in the main array making a
usabletotalof83.DensePak’sfibrebundlewasreformatted
2.1.1 Emission line fitting
intoa‘pseudo-slit’tofeedtheHydrabench-mountedechelle
The S/N and spectral resolution of the data have allowed spectrograph, just as for SparsePak.
ustoaccuratelyquantifythelineprofileshapesoftheemis- On 30th September 2003, we observed the nuclear re-
sion lines. In the central regions where the S/N is highest, gions of NGC 1140 with DensePak by centring the array
we find the emission lines to be composed of two bright, on the coordinates 02h54m33s.43, −10◦01′40′.′6 with a PA
narrow components (FWHM ∼ 20–100 kms−1; corrected of +90◦. The total exposure time was 1800 secs. Fig. 2
forinstrumentalbroadening)overlaidonafainterbroad shows the footprint of the array on the HST/ACS F658N
component(FWHM∼100–230kms−1;alsocorrectedfor image. The 600 line mm−1 grating at an angle of 26.82◦
instrumental effects).Insomespaxelsfurtherout,wesee gave a wavelength range of 5260–8110 ˚A with a dispersion
only the two narrow components. Two split narrow compo- of 1.40 ˚Apix−1, allowing us access to a number of optical
nents are indicative of expanding gas motions. nebular lines. Contemporaneous bias frames, flat-fields and
To quantify the emission line profile shapes, we fitted arc calibration exposures were also observed.
modelGaussianprofilestoeachlinedetectedineachspaxel
of the SparsePakfield using a customised IDL-based χ2 fit-
tingroutinecalledpan(PeakANalysis;Dimeo2005).Ade- 2.2.1 Reduction
taileddescriptionoftheprogramandthecustomisationswe
Reductionwasachievedusingthesamemethodologyasout-
have made to it are given in Westmoquette et al. (2007a). lined for SparsePak, using the ccdproc and hydra tasks
As mentioned above, in many cases we found that addi- withinthenoaoirafpackage.Tracesoftheindividualspec-
tional Gaussian components were needed to satisfactorily
trawereidentified fromthemaster flat-field frame, andthe
fit the integrated line profile; the number of components
resulting aperture identification table was used to extract
fitted to each line was determined using a combination of
theobjectspectrafromthescienceframes.Afterflat-fielding
visualinspectionandtheχ2 fitstatisticoutputbypan.For
and wavelength calibrating the dataset, we formed a repre-
the multi-component cases, we specified a number of rules
sentative sky spectrum by averaging the data from all four
to identify the different line components. This consistent
sky fibres from all three exposures. This sky spectrum was
approach helped both in the minimisation of the fit and in
thensubtractedfromeachoftheobjectspectra.Finalcom-
thesubsequentanalysis.Fordouble-components,thesecond bination of the individual frames was done using imcom-
component (hereafter C2) was sometimes required to fit a bine after cosmic-rays were removed with lacosmic. The
broadunderlyingprofile,andinotherstofitthefaintercom-
final datafiles contained 83 reduced and sky-subtracted ob-
ponent of a split line. Where three components were iden-
ject spectra.
tified, the broadest component was assigned to component
In order to determine an accurate measurement of the
2 (C2), and after that, the brightest to component 1 (C1)
instrumental broadening, we fitted a single Gaussian to a
andthetertiarycomponenttoC3.Wenoteherethatinthe
number of high S/N, isolated arc lines for all 83 aper-
double-component cases where a broad component was not
tures and took the average: the resulting measurement was
detected, the secondary narrow component can be thought 165±10 kms−1. Note that this is considerably higher than
ofas equivalent to C3inthetriple-component cases (where
thatoftheSparsePakdata,andthat,forthemostpart,the
C2representsthebroadcomponent).Inallcasestheindi-
emission line profiles are unresolved.
vidual Gaussian models were constrained to have a
Fig.2showsthepositionoftheDensePakfootprintona
widthgreaterthantheaforementionedinstrumental cut-outoftheHST F658NimagewiththeSparsePakfibres
width.
from Fig. 1. An example reduced spectrum is shown in the
AnumberofexampleHαlineprofilesfromvariousspax-
bottom panel of Fig. 3. A number of residuals remain (e.g.
els are shown in Fig. 4, together with the individual Gaus- around [Oi]λ5577 and Nai λλ5890,5896) as a result of an
sianfitsrequiredtomodeltheintegratedline-shapes.These
imperfect sky subtraction.
werecompiledtoshowthevariationinprofileshapesfound.
Given the spectral resolution of these data, we fitted
the emission lines of Hα, [Nii]λ6583 and [Sii]λλ6717,6731
2.2 DensePak in each spectrum with only a single Gaussian component
DensePak(Barden et al.1998)wasasmallfibre-fedintegral
fieldarraythatwasusuallyattachedattheNasmythfocusof 2 DensePakwasdecommissionedin2008.
4 M.S. Westmoquette et al.
(constrained to have a width greater than the afore- likelyartificiallyelevatedatthelevelofafewtensofkms−1
mentionedinstrumentalwidth).Twofibrescoveringthe by multiple (unresolved) emission components along the
brightest, central-most regions show evidence of multiple line-of-sight.Aneffectsimilartothisisalsoverylikelytobe
components in the Hα emission line; in both cases they at work here, particularly in the central regions. Here, the
areresolvedintotwocomponentsseparatedby∼100kms−1 presenceofcomplexfilamentaryemissionlinestructuresim-
(theseadditionalcomponentsarenotshownintheemission plies multiple overlapping shells or dynamical components
line maps presented below). on scales less than the DensePak or SparsePak fibre sizes.
The presence of a second narrow component (assigned
to C3 where a broad component is identified and C2 oth-
erwise) is indicative of expanding gas motions. These split
3 EMISSION LINE MAPS
lines are seen within the nuclear regions (central part of
In this section we present and describe the SparsePak and array) and in the chain of Hii regions, where the velocity
DensePak emission line maps created from the line profile differences between the two components are 10–60 kms−1.
fits described above. We begin by looking at the SparsePak These kind of velocity differences are consistent with those
results. expectedfromregionsofintensestarformation,andsuggest
the presence of outflowing gas.
3.1 SparsePak
3.1.2 Nebular diagnostics
3.1.1 Kinematics
The[Sii]λλ6717,6731lineswereusedtoderivetheelectron
The spatial distribution FWHM and radial velocity of each densities of the ionized gas. For the majority of the galaxy
Gaussian component identified are shown in Figs. 5 and 6, where the the [Sii] lines were detected (the central ∼2 kpc
respectively. and along the chain of Hii regions), the derived values fell
Narrow .100 kms−1 Hα components are detected out below the low-density limit at ∼100 cm−3. However, in an
to >40′′ (∼3.8 kpc) from the nucleus (Fig. 5 left panel), areaencompassingthecentralfewspaxelsandthoseextend-
corresponding well to the limit of emission seen in the deep ing ∼20′′ to the south, we find C1 densities rise to a few
HαimagepresentedbyH94b.InC1,theimprintofthelarge- 100 cm−3. In the spaxels located in the central part of the
scale rotation of the galaxy can clearly be seen (Fig. 6 left array(inner∼1kpc)asecond[Sii]componentcouldalsobe
panel),andapolynomialsurfacefittothesedatashowsthat identified. The densities measured in this component were
thepeakrotationamplitude(∼20kms−1kpc−1)isalongPA again mostly at or just above the low-density limit.
= −51◦ (i.e. rotation axis PA = +39◦), in agreement Theseresultsareconsistentwiththoseofpreviousstud-
with the Hi data of (H94b). In C1, the inner ∼20×20′′ ies: H94b find densities of a few 100 cm−3 in the central
of the galaxy shows hints of rotating at a different position 20–40′′ ofthegalaxy,andMoll et al.(2007)useoxygenline
angle.ThisisdiscussedfurtherinlightoftheDensePakdata diagnosticstofindadensityof60±50cm−3 inthenebular
presented in Section 3.2.1. We would also like to note that region directly surrounding starburst knot A (the northern
themovementofHα-emittinggasassociatedwiththechain starclustercomplex).Burgh et al.(2006)findelectronden-
ofHiiregions(Fig1)isconsistentwiththerestofthehalo sitiesofjustover100cm−3 fromthe[Sii]ratiomeasuredin
(and with the findings of H94b). the nuclear regions.
An underlying broad component (C2; 100–230 kms−1) The forbidden/recomination line flux ratio of
can be identified throughout the disk and inner halo, ex- [Sii](λ6717+λ6731)/Hα can be used as an indicator
tending out to a radius of &2 kpc (Fig. 5 central panel). of the number of ionizations per unit volume, and thus
Ingeneral,broaderlinewidthsareseen onthewesternside the ionization parameter, U (Veilleux & Osterbrock 1987;
of the galaxy. This is mirrored in both C1 and C2, where Dopita et al. 2000, 2006). [Sii]/Hα is also particularly
C2 widths increase up to FWHM = 230 kms−1 (Fig. 5; sensitive to shock ionization because relatively high-
see also Fig. 4 top-right panel). No broad Hα emission is density, partially ionized regions form behind shock
associated with the chain of Hii regions towards the south- fronts which emit strongly in [Sii] thus producing an
west. Here only two narrow components are present (note enhancement in [Sii]/Hα (Dopita 1997; Oey et al. 2000;
thatthesecond,faintercomponenthasbeenassignedtoC2 Osterbrock & Ferland2006).Mapsofthisratiointhethree
– our assignment convention is described in Section 2.1.1). line components are shown in Fig. 7. In the scale bar, we
In general, the radial velocities of the broad C2 component have indicated a fiducial value of log([Sii]/Hα) = −0.4
areblueshiftedby∼10–50kms−1 comparedtoC1,suggest- above which non-photoionized emission is thought to play
ing that it is associated with gas expanding out of the star a dominant role (Dopita & Sutherland 1995; Kewley et al.
formingzones.Thesespeedsaremoretypicalforexpanding 2001; Calzetti et al. 2004). In environments such as these,
supershells as compared to the much larger velocities seen the most likely excitation mechanism after photoionization
in galactic winds. is that of shocks. Outside of the central 2 kpc region,
At this point we would like to note that in NGC 1569 the ratios in all three line components rise significantly
we compared the range of emission line radial velocities above this non-photoionization threshold. This suggests
measured in individual spaxels over one 5 × 3.5 arcsecs that shocks dominate the gas excitation in the halo (as
GMOS IFU field-of-view (Westmoquette et al. 2007b) with previouslynotedbyCalzetti et al.2004intheouterregions
the width of lines from SparsePak fibres that covered ap- of other moderate luminosity dwarf starburst galaxies) and
proximatelythesamearea(Westmoquette et al.2008),and thus may play a central role in driving the gas outward.
found that the SparsePak FWHM measurements were very The ratio of [Nii]λ6583/Hα can be used in a sim-
Ionized gas in NGC 1140 5
ilar fashion to [Sii]/Hα as a diagnostic of the gas ex- 4 DISCUSSION AND CONCLUSIONS
citation level and to search for the presence of shocks
In this paper we have presented deep spatially-resolved
(Veilleux & Osterbrock 1987; Dopita & Sutherland 1995).
optical spectra of NGC 1140 obtained with the WIYN
Fig. 8 shows maps of this ratio for all three components;
SparsePak and DensePak instruments. The different spa-
here the marker in the scale bar represents a fiducial non-
photoionized threshold of log([Nii]/Hα) = −0.1. Again we tial resolutions and coverage of the two sets of observations
haveallowedustoinvestigatethepropertiesandkinematics
seethatthelineratiosincreasefromthecentralregionout-
of the warm ionized gas within both the central regions of
wards, but in this case never rise above our adopted non-
the galaxy and the inner halo, and have revealed a num-
photoionization threshold. Here we should note that metal-
licity plays a strong part in setting the Hii region/shock berofinterestingresults.Wewillnowsummarisetheseand
discuss their implications in the context of the galaxy as a
boundary for these diagnostic ratios, in the sense that a
whole.
lowerNabundance(aswouldbeexpectedatthelowmetal-
licityofNGC1140)woulddecreasethenon-photoionization
threshold (Dopita et al. 2006).
4.1 Kinematical structure of the disk and halo
Again our results agree very well with previous stud-
ies: H94b measured log([Sii]/Hα) ∼ −0.7 to −0.3 and WehavemeasuredthepositionangleoftheHαrotationaxis
log([Nii]/Hα) ∼ −1 to −0.7 in the central regions of NGC formainbodyofthegalaxy,andfindittobeconsistentwith
1140, and Burgh et al. (2006) found [Sii]/Hα to vary with the Hi rotation axis at PA = 39◦ (H94b).
radiusfromaminimumoflog([Sii]/Hα)=−0.9atthepeak The peak of the optical emission lies roughly at the
oftheHαemission,tolog([Sii]/Hα)=−0.3attheedgesof centre of the integrated Hi morphology (H94b). The Hi
the galaxy. ridgefeatureisorientedroughlyperpendicularlytothema-
jor axis, and contains the peak of the Hi emission, which
lies to the south-west of the optical peak, coincident with
3.2 DensePak results the chain of Hii regions (H94b). Despite their appearance,
the chain of Hii regions located towards the south-west are
As described in Section 2.2.1, the emission lines in the
notpartofatidaltail(whichmightbeexpectedtobekine-
DensePak spectra were almost all unresolved meaning that
matically distinct), but appear to rotate together with the
a single Gaussian component fit was all that was required.
rest of the galaxy. This makes sense since we know they
The spatial distribution of Hα flux and radial velocity, and
are coincident with the Hi ridge identified by H94b, and
the [Sii]/Hα and [Nii]/Hα line ratios, resulting from these
the peak of the Hi emission. What do the chain of Hii re-
fitsareshowninFig.9.ThelocationoftheDensePakarray
gions represent? Split Hα lines with velocity separations of
on the HST Hα image is shown in Fig. 2. a few tens of kms−1 are seen at the location of the chain
ofHiiregions indicating that they areyoung,star forming,
and dynamically active. It is possible that we are witness-
3.2.1 Kinematics
ing a sequential progression of star formation through the
Fig. 9b shows the Hα radial velocity map. It is clear that disk(perhapstheHiiregionsrepresentthestubofatidally
the velocity gradient observed does not follow what is seen induced spiral arm), and that in a few Myr the galaxy will
onlargescalesin,forexample,ourSparsePakmaps(Fig.6). lookelongatedinHαalongthe+22◦ ridgedirectionasstar
Itisinsteadoffsetbyapproximately90◦ toaPAof∼+40◦. formation takes hold.
ThiscorroborateswhatwashintedatintheSparsePakdata Ourdataclearlyshowthattheionizedgasinthecentral
(Section3.1.1;andinthelong-slitdataofBurgh et al.2006), 20×20 arcsecs (2×2 kpc) region of the galaxy is kinemat-
and suggests the presence of a counter-rotating core within ically decoupled from the rest of the system, and rotates
the central ∼20×20′′ with a rotation axis aligned roughly at a PA approximately perpendicular to that of the main
perpendiculartothemainbodyrotationaxis.Thiswasalso bodyofthegalaxyat+40◦.Thisfindingofyetanothermis-
found in the long-slit data of Burgh et al. (2006). aligned structural component is not unsurprising, and only
addsweighttotheinteraction/mergerscenarioproposedby
H94b.
3.2.2 Nebular diagnostics
Most DensePak spaxels exhibit [Sii]-derived electron den-
4.2 The starburst environment and the properties
sities below the low-density limit (100 cm−3), although a
of the outflowing gas
smallnumbershowdensities ofafewhundredcm−3.These
resultsareconsistentwithourSparsePakmeasurementsand We see no evidence of coherent large-scale outflows within
with previous studies as described above. NGC 1140. Instead, the Hα morphology (Fig 1), the fact
Inboth[Sii]/Hαand[Nii]/Hα(Fig.9candd)thelow- that we find multiple narrow line components with velocity
est ratios are coincident with the location of the Hα peak separations of 10–60 kms−1 within a radius of ∼1–1.5 kpc,
(Fig. 9a), then increase radially outward (the similarity of andthefactthatbeyond∼2kpcthe[Sii]/Hαratiosrisesig-
the two maps is striking). No evidence for shocked line ra- nificantly implying a strong contribution to the excitation
tios(ineither[Sii]/Hαor[Nii]/Hα)arefoundinthesecen- levels by shocks, all suggest that locally intense star forma-
tral regions. Our measurements are again very consistent tion is driving material outwards in the form of a series of
with those from the SparsePak data (once the finer spatial interactingsuperbubbles/shells(i.e.the“froth”).Thelackof
sampling has been taken into account), and with previous anycoherentwindmaybeduetothedisturbedstructureof
studies (H94b; Burgh et al. 2006). thegalaxydisk,and/orthemodestthermalpressuresinthe
6 M.S. Westmoquette et al.
starburstcore(impliedbythelowelectrondensities)and/or hand, consists of two large bi-polar plumes of outflow-
the low global average star formation rate (<1 M⊙ yr−1; ing gas that are relatively well structured and colli-
H94b; Moll et al. 2007). mated (Shopbell & Bland-Hawthorn 1998; Ohyama et al.
Abroadcomponent(100.FWHM.230kms−1)tothe 2002; Westmoquette et al. 2009a). The halo of NGC 1140,
Hα line is seen throughout the galaxy disk out to >2 kpc. with its complex, frothy morphology, seems mid-way be-
Broad emission line components have been observed before tween these two examples.
intheHβlineprofilesextractedfromthenebularegionssur- Firstly, NGC 1140 is almost three orders of magnitude
roundingtheSSCknotsAandBbyMoll et al.(2007),and more massive than NGC 1569 and has a rotational velocity
in many other nearby starburst galaxies (e.g. Izotov et al. vrot ∼100kms−1 (H94b)comparedto∼35kms−1 inNGC
1996; Homeier & Gallagher 1999; Marlowe et al. 1995; 1569(Stil & Israel2002).Sincetheescapespeed,vesc,scales
Mendez & Esteban 1997; Sidoli, Smith & Crowther 2006). as v2R, vesc in NGC 1140 is >3 times greater than that
For many years the nature of the energy source for these in NGC 1569. The two galaxies have similar star formation
broad lines has been contested, but recent detailed IFU ratesperunitareasoitmaynotbesurprisingthattheburst
studies of the ionized ISM in the starburst galaxies NGC in NGC 1140 is producing less in the way of an outflow.
1569andM82(Westmoquette et al.2007a,b,2009a,b)have NGC 1140 and M82 have similar vrot and sizes, so
allowed us to address this problem. Accurately decompos- should have roughly similar values of vesc. However, both
ing the emission line profiles and mapping out their prop- thethermalandturbulentpressuresatthebaseoftheM82
erties led us to the conclusion that the broad underlying wind are >10 times that in NGC 1140 meaning that M82
component is produced in turbulent mixing layers (TMLs; must have a significantly higher energy density in its ISM
Slavin et al.1993;Binette et al.2009)onthesurfacesofcool and is therefore able to drive a large-scale superwind in a
gasknots,setupbytheimpactoftheionizingradiationand way NGC 1140 cannot. The presence of a broad emission
fast-flowing winds from the YMCs (Pittard et al. 2005). In line component in both cases suggests that despite this dif-
both these galaxies, we found evidence for a highly frag- ference, the underlying structure of cloud boundaries and
mented ISM that provides copious cloud surfaces on which turbulent mixing layers are similar, and that these features
TMLs can form thus explaining the pervasiveness of this are not directly linked to the magnitude of the winds.
broad component. The existence of multiple YMCs in the
centralregion,thecomplex,“frothy”Hαmorphologyofthe
diskandhalo(Fig.1),andthesimilarityinwidthandmor-
ACKNOWLEDGMENTS
phologyofthebroadcomponentobservedhere,stronglysug-
gest a similar origin for this component. The same conclu- We thank the anonymous referee for comments that led to
sion was reached by Moll et al. (2007). an improvement of the clarity of this paper. MSW would
Can we determine if the radius at which we stop see- like to thank Daniel Harbeck and the staff at the WIYN
ing the broad component is physical or simply an effect of Observatory for their support during the observing runs,
thedata?ExaminationofanumberofHαlineprofileswith and Phillip Cigan for reducing the DensePak observations.
similar S/N levels from spaxels near this boundary reveals MSW also thanks the University of Wisconsin–Madison for
slight evidence for the former: spaxels 82 (coordinates +20, the warm hospitality received in support of this project.
0)and37(−25,−8)bothexhibitabroadHαcomponentand JSG’sresearchwaspartiallyfundedbytheNationalScience
haveintegratedHαS/Nratiosof∼55and∼37,respectively, Foundation through grant AST-0708967 to the University
whereas spaxels 28 (−20, 0) and 63 (15,8), with integrated of Wisconsin-Madison.
HαS/Nratiosof∼85and∼45,respectively,donot.Finding
aphysicallimittothebroadcomponentregionechoeswhat
we found in NGC 1569 (Westmoquette et al. 2008). Here
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Figure 1.HST/ACS-WFCcontinuumsubtractedHαimageofNGC1140(inverselogscaled)withSparsePakfootprintoverlaid.
Ionized gas in NGC 1140 9
Figure 2. HST/ACS continuum subtracted Hα image of the
central regions of NGC 1140 (inverse log scaled) with DensePak
footprintoverlaid(bold)ontheSparsePakfootprintfromFig.1.
Figure3.ExampleSparsePakandDensePakspectrashowingthe
full spectral range of each observation (in arbitrary flux units).
The identified nebular lines are labelled. A number of residuals
remainintheDensePakspectrumasaresultofanimperfectsky
subtraction.
10 M.S. Westmoquette et al.
Figure4.ExampleHαlineprofileschosentorepresentthemaintypesofprofileshapesobservedoverthefield.Observeddataisshown
byasolidblackline,individualGaussianfitsbydashedredlines(includingthestraight-linecontinuumlevelfit),andthesummedmodel
profileinsolidred.Fluxunitsarearbitrarybutonthesamescale.Beloweachspectrumistheresidualplot.