The emission-line regions in the nucleus of NGC 1313 probed with GMOS-IFU: a supergiant/hypergiant candidate and a kinematically cold nucleus

1.1 MB·

by  R. B. Menezes

Mon.Not.R.Astron.Soc.000,1–??(2017) Printed20January2017 (MNLATEXstylefilev2.2)
The emission-line regions in the nucleus of NGC 1313
probed with GMOS-IFU: a supergiant/hypergiant
candidate and a kinematically cold nucleus
7
1
0 R. B. Menezes1,2⋆ and J. E. Steiner1
2 1Instituto de Astronomia Geof´ısica e Ciˆencias Atmosf´ericas, Universidade de S˜ao Paulo, Rua do Mat˜ao 1226,

n Cidade Universita´ria, S˜ao Paulo, SP CEP 05508-090, Brazil
a 2IP&D, Universidade do Vale do Para´ıba, Av. Shishima Hifumi, 2911, S˜ao Jos´e dos Campos, SP CEP 12244-000, Brasil
J

8
1 Submitted2016February27

]
A
ABSTRACT
G
. NGC 1313 is a bulgeless nearby galaxy, classified as SB(s)d. Its proximity allows
h
high spatial resolution observations. We performed the first detailed analysis of the
p
emission-line properties in the nuclear region of NGC 1313, using an optical data
-
o cube obtained with the Gemini Multi-object Spectrograph. We detected four main
r emitting areas, three of them (regions 1, 2 and 3) having spectra typical of H II
t
s regions.Region1 is locatedvery close to the stellar nucleus andshows broadspectral
a
featurescharacteristicofWolf-Rayetstars.Ouranalysisrevealedthepresenceofoneor
[
twoWC4-5starsinthis region,whichiscompatiblewith resultsobtainedbyprevious

1 studies. Region 4 shows spectral features (as a strong Hα emission line, with a broad
v component) typical of a massive emission-line star, such as a luminous blue variable,
5 a B[e] supergiantor a B hypergiant.The radialvelocity map of the ionized gasshows
4 a pattern consistentwith rotation.A significant dropin the values of the gas velocity
2
dispersion was detected very close to region 1, which suggests that the young stars
5
therewereformedfromthiscoldgas,possiblykeepinglowvaluesofvelocitydispersion.
0
Therefore,althoughdetailedmeasurementsofthe stellarkinematicswerenotpossible
.
1 (due to the weak stellar absorption spectrum of this galaxy), we predict that NGC
0 1313mayalsoshowadropinthevaluesofthestellarvelocitydispersioninitsnuclear
7
region.
1
: Keywords: techniques:spectroscopic–stars:emission-line,Be–stars:Wolf-Rayet–
v
galaxies:individual:NGC 1313– galaxies:kinematics anddynamics– galaxies:nuclei
i
X
r
a
1 INTRODUCTION thoughthenorthernarmisalittlemoredevelopedthanthe
southern one. Being located at a distance of only 4.5 Mpc,
Late-type galaxies usually have low mass, a large amount
this is an ideal target for morphological and spectroscopic
of gas and dust and show significant star formation. Their
studies of bulgeless galaxies with large telescopes.
spheroidal component, the bulge, is much less prominent
than in the case of early-type galaxies. An extreme class Using a ground based Hα image, Marcelin & Gondoin
of these objects is composed of the bulgeless galaxies (with (1983) elaborated a catalogue of 375 H II regions in NGC
morphological types from Scd to Sm), which have essen- 1313, some of them very close to the nucleus. The authors
tially no bulge (B¨okeret al. 2002). Many of the processes concludedthattheobservedHIIregionshaveanaveragedi-
related to the formation and evolution of these objects ameterof18.4pc,thebrightestonesbeinglocatedalongthe
are not completely understood. NGC 1313 is a member of spiralarms.Pagel, Edmunds& Smith(1980)analysedspec-
this extreme class of late-type galaxies, classified as SB(s)d trophotometricdataofsixHIIregionsinNGC1313andver-
by de Vaucouleurs (1963), who mentioned that this object ifiedthattheabundancedistributionisveryuniformandcan
seems to be a transition between the morphological types begivenby12+log(O/H)=8.26±0.07.Suchabundanceis
SBc and SBm. It has almost symmetric spiral arms, al- similar to that of the Large Magellanic Cloud (8.37±0.22;
Russell & Dopita 1990). In a latter study, Walsh & Roy
(1997) presented multi-fibre spectrophotometry of 33 H II
⋆ E-mail:[email protected] regionsinthisgalaxyandobtainedasimilarabundancedis-
(cid:13)c 2017RAS
2 Menezes & Steiner
Figure1.Left:HSTimageofNGC1313,obtainedwithWFPC2intheF606Wfilter.Right:magnificationofthehighlightedsectionof
theimageshownatleft,withourGMOS/IFUpointingindicated.
tribution for the disc, given by 12+log(O/H)= 8.4±0.1, in the Southern hemisphere brighter than B = 12.0. The
the bar possibly having a higher abundance than the disc principal investigator of this project is JES. The obser-
by a factor of 0.2 dex. vations are being taken, in the optical, with the integral
Hadfield & Crowther (2007) performed a survey of field unit (IFU) of the Gemini Multi-object Spectrograph
Wolf-Rayet stars (WRs) in NGC 1313, using optical data (GMOS), mounted at the Gemini-south telescope. NGC
obtained with the Very Large Telescope, and detected 70 1313 isoneofthebrightestbulgeless galaxies inoursample
objects with spectral features characteristic of WRs, one of and, therefore, deserves special attention. In this paper,
them nearly coincident with the nucleus of the galaxy. The we present the first detailed study of the emission-line
observed nebular properties resulted in an abundance dis- properties in the nuclear region of NGC 1313, based on an
tributionof12+log(O/H)=8.23±0.06. Inaddition,using optical data cube obtained with the GMOS-south/IFU, as
continuum-subtractedHαimagesofthisgalaxy,theauthors partoftheDIVING3D survey.InSection2wedescribethe
estimated a global star formation rate of 0.6 M⊙ yr−1. observations, the data reduction and the data treatment.
The results of our analysis are presented in Section 3. In
The first kinematic study of NGC 1313 was made
Section 4 we discussand compare our results with previous
by Carranza & Agu¨ero (1974), who analysed the kine-
studies. Finally, we summarize our work and present our
matics of the Hα emission line and verified that the
conclusions in Section 5.
rotation centre does not coincide with the nucleus.
Marcelin & Athanassoula (1982) mapped the velocity field
of the ionized gas in this galaxy, using interferometric data
of Hα, and concluded that the rotation centre is located 2 OBSERVATIONS AND DATA REDUCTION
outside the bar, at a distance of 1.5 kpc from the nucleus.
The observations of the nuclear region of NGC 1313, with
However, a detailed analysis of the H I flux and kinemat- coordinates RA = 03h 18m 16.0s and Dec. = -66◦ 29′ 54′.′0,
icsbyPeters et al.(1994)revealedthatmostofthevelocity
weretakenon2012December4.Three590sexposures,with
fieldcanbeinterpretedasasimplecircularrotation around
spatialditheringof0.2arcsec,wereobtained.Asweusedthe
the nucleus, with no displacement of the rotation centre.
IFUintheone-slitmode,thesciencefieldofview(FOV)of
Peters et al. (1994) also detected a tidal interaction with a
theobservations (sampled by 500 fibres) has 5 arcsec × 3.5
possible satellite galaxy, which was recently disrupted.
arcsec and the sky FOV (located at a distance of 1 arcmin
Silva-Villa & Larsen(2011)analysedthestarformation from the science FOV and sampled by 250 fibres) has 5
historyofthisgalaxy,usinghigh-resolutionimages,obtained arcsec × 1.75 arcsec. The observations were taken with the
with the Hubble Space Telescope (HST), and determined B600+G5323 grating, which resulted in a spectral coverage
star formation rate densities of (14.9 −30.9) ×10−3 M⊙ of 4500 − 7350˚A and a spectral resolution of R ∼ 3085.
yr−1 kpc−2 for the whole galaxy. In addition, the authors Basedonacquisitionimages,weestimatedaseeingof∼0.64
detected evidenceof a recent starburst south-west from the arcsec. Fig. 1 shows an HST image of NGC 1313, obtained
nucleus, which is compatible with the hypothesis of a tidal with the Wide-field Planetary Camera 2 (WFPC2) in the
interaction with a disrupted satellite galaxy. F606W filter, with our IFU pointing indicated.
Besides the presence of H II re- The required calibration images (bias, GCAL flat, twi-
gions (Marcelin & Gondoin 1983) and WRs light flat and CuAr lamp) were obtained during the obser-
(Hadfield & Crowther 2007), there is not much infor- vations.Inordertoperformfluxcalibrationofthedata,the
mation in the literature about the nuclear region of NGC standardstarLTT1020wasalsoobserved,on2012Novem-
1313. Our research group is currently conducting the Deep ber28.ThedatareductionwasperformedusingtheGemini
IFS View of Nuclei of Galaxies (DIVING3D) survey, IRAFpackage.Thefirstpartofthedatareductionconsisted
whose goal is to observe the nuclear regions of all galaxies of the trim and bias subtraction of the data. After that,
(cid:13)c 2017RAS,MNRAS000,1–??
The emitting areas in the nucleus of NGC 1313 3
Figure 2.Average spectrum and image of the collapsed data cube of NGC 1313, after the data treatment. Magnifications of different
wavelength rangesoftheaveragespectrum areshownatthebottom.
the cosmic rays were removed with theL.A.Cosmic routine of the data cubes, using the Butterworth spatial filtering
(van Dokkum2001).Then,thespectrawereextracted,cor- (Gonzalez & Woods2002),andalsoremovedaninstrumen-
rected for gain variations along the spectral axis (using the tal fingerprint (which had a spectral signature and ap-
response curves provided by the GCAL flat images), cor- peared in the form of vertical stripes across the images),
rectedforgainvariationsalongtheFOVandforillumination using the Principal Component Analysis (PCA) Tomogra-
patterns of the instrument (with response maps obtained phy technique (Steineret al. 2009). Finally, in order to im-
with the twilight flat) and wavelength calibrated. The av- prove the spatial resolution of the observation, we applied
erage spectrum obtained with the sky FOV was subtracted a Richardson-Lucy deconvolution (Richardson 1972; Lucy
fromsciencedata.Itisimportanttomention,however,that, 1974) to all the images of thedata cube.
beinglocatedat1arcminfromthenucleusofNGC1313,the The Richardson-Lucy deconvolution requires an esti-
sky FOV could be contaminated by the emission from the mateoftheoriginalpoint-spreadfunction(PSF)ofthedata
galaxy’sdisc.However,acarefulinspection oftheskyspec- cubeatacertainwavelength.AsexplainedinSection3,this
trum revealed no emission lines other than the sky lines. data cube has a point-like source, east from the nucleus,
In addition, we performed a cross-correlation between the which is a supergiant/hypergiant star candidate. The spec-
sky spectrum and an average spectrum of the science data trumofthissourceshowsastrongHαemission line,with a
and found no indication of a significant contamination of broad component. An image of the blue wing of this broad
theskyspectrumbytheemissionfromthegalaxy’sdisc.Fi- component(afterthesubtractionofanimageoftheadjacent
nally, the telluric absorptions were removed, the data were stellar continuum) provided a reliable estimate of the PSF
flux calibrated (with also a correction for the atmospheric at6568˚A,whichcanbewelldescribedbyaGaussianwitha
extinction) and the data cubes were constructed, with spa- fullwidthathalf-maximum(FWHM)of∼0.63arcsec.This
tial pixels (spaxels) of 0.05 arcsec. value is consistent with the estimate obtained from the ac-
We treated the reduced data cubes with the proce- quisition images and from a comparison between theimage
dure described in Menezes, Steiner & Ricci (2014) and of the data cube, collapsed along the spectral axis, with a
Menezes et al.(2015).First,thedifferentialatmosphericre- convolvedV bandHST imageofthisgalaxy,obtainedwith
fraction effect was removed from the data cubes. Then, theAdvancedCameraforSurveys(ACS).Itisimportantto
a median of the data cubes was calculated. After that, emphasize that the wavelength of this reference PSF does
we removed high spatial-frequency noise from the images not take intoaccount any redshift correction.
(cid:13)c 2017RAS,MNRAS000,1–??
4 Menezes & Steiner
Figure 3. Integrated flux maps of the main emission features in the data cube of NGC 1313, after the starlight subtraction. The
isocontours are shown in green. The four circular regions from which spectra were extracted are superposed to the Hα image at the
bottom.Allthefluxvaluesarein10−15 ergcm−2 s−1.
In order to apply the Richardson-Lucy deconvolution, deconvolveddata cubeand aconvolved V bandHST/ACS
it is also necessary to determinetheway theFWHMof the image of this galaxy.
PSFvarieswiththewavelength.Anequationdescribingthe
variationofthePSFwiththewavelengthwasobtainedfrom
thedatacubeofthestandardstarLTT1020,whichwasused
Fig.2showstheimageofthetreateddatacubeofNGC
inthedatareductiontoperform thefluxcalibration. Using
1313,collapsedalongthespectralaxis,anditsaveragespec-
the image of the blue wing of the broad Hα component,
trum.Twobrightareascanbeseenintheimage,thebright-
we verified that the FWHM of the PSF of the treated data
cube was ∼0.52 arcsec, which is, again, consistent with an estonecoincidingwiththestellarnucleusofthegalaxy.The
average spectrum reveals prominent narrow emission lines,
estimate obtained from a comparison betwen the collapsed
with noapparent broad component.
(cid:13)c 2017RAS,MNRAS000,1–??
The emitting areas in the nucleus of NGC 1313 5
3 DATA ANALYSIS AND RESULTS region 4 is due to the presence of a massive emission-line
star(likealuminousbluevariable -LBV,aB[e] supergiant
Since this paper is focused on the emission-line spectrum oraBhypergiant)inthisarea(seethediscussioninSection
of the data cube of NGC 1313, the first step of our analy- 4).
sis consisted of the starlight subtraction of the data cube.
Wecorrectedthespectraofregions1,2,3and4forthe
This procedure was applied using synthetic stellar spectra
provided by the Starlight software (Cid Fernandes et al. interstellarextincton(intheobservedgalaxy),usingtheAV
values provided by the spectral synthesis of these spectra
2005), which performs a spectral synthesis using template
and also the reddening law of Cardelli et al. (1989). Af-
stellarspectrafromapre-establishedbase.Forthiswork,we
ter this correction, we verified that the Balmer decrements
choseabaseofstellarpopulationspectrabasedonMedium-
(Hα/Hβ) of the extracted spectra were all compatible, at
resolution Isaac Newton Telescope Library of Empirical
1σ level, with 2.86, which is the expected value for Case B
Spectra (MILES; S´anchez-Bla´zquez et al. 2006). This base recombination, considering an electron density of 102 cm−3
has a spectral resolution of FWHM = 2.3˚A, which is close and a temperature of 104 K (Osterbrock & Ferland 2006).
to our spectral resolution (FWHM = 1.8˚A). The spectra of
We calculated the luminosities of the blue and red bumps
the data cube were corrected for the Galactic extinction,
forthespectrumofregion1(assumingadistanceof4.5Mpc
using AV = 0.299 mag (NASA Extragalactic Database - forthegalaxy)andobtainedL =(7.7±0.4)×1036
blue bump
NED)andthereddeninglawofCardelli, Clayton & Mathis ergs−1andL =(5.17±0.26)×1036 ergs−1.Wealso
red bump
(1989),andalsoshiftedtotherestframe,usingz=0.001568
calculated the emission-line ratios [N II] λ6584/Hα, ([S II]
(NED),before thespectral synthesiswas applied.
λ6716+λ6731)/Hαand[OIII]λ5007/Hβ forthespectraof
Bysubtractingthesyntheticstellarspectraprovidedby regions1,2and3.Table1showstheemission-lineratios,to-
the spectral synthesis from the observed ones, we obtained getherwiththeluminositiesoftheHαlineandtheprojected
a data cube with only emission lines. In this residual data distances between each region and the stellar nucleus.
cube,besidesthenarrowemissionlinesvisibleintheaverage We constructed the diagnostic diagrams [O III]
spectrumoftheoriginaldatacube(Fig.2),wealsodetected λ5007/Hβ × [N II] λ6584/Hα and [O III] λ5007/Hβ × ([S
two broad features: a “blue bump” representing a blend of II] λ6716+λ6731)/Hα and included the points represent-
emission lines around 4650˚A (including lines such as N III ing regions 1, 2 and 3. In these same diagrams, we also in-
λλλ4628,4634,4640, C III λ4650, C IV λ4658 and He II cluded the points corresponding to the objects analysed by
4686)anda“redbump”representingtheCIVλλ5801,5812 Ho, Filippenko& Sargent (1997). The results are shown in
lines. Fig. 3 shows integrated flux maps of the blue and Fig. 6. It is easy to see that the analysed areas are H II
red bumps and of the [O III] λ5007 and Hα emission lines, regions, with a decreasing ionization degree from region 1
with isocontours. The blue and red bumpsare emitted in a to 3. The values of the emission-line ratio [S II] λ6716/[S
compactarea,whosepositioniscompatiblewiththeposition II] λ6731, along the entire FOV, were consistent with 1.44,
of thebrightest region in theHα and [O III]λ5007 images. whichisthelow-densitylimitofthisratio,assumingatem-
TheHαemissioncomesmainlyfromfourareas,thebrightest peratureof104 K.Thisindicatesthattheelectrondensities
of them located west and at a projected distance of 0.36± along theFOV must satisfy ne <∼10 cm−3.
0.06 arcsec (which corresponds to 7.9±1.3 pc) from the The analysis of thespectrum of region 4 requires more
stellarnucleus.Thespatialmorphologiesofthe[OIII]λ5007 caution.Inordertodisentanglethebroadandnarrowcom-
and Hα emission lines are relatively similar to each other. ponents of the Hα line in this area, we fitted the [N II]
The main difference between the images of these lines is λλ6548,6583andHαlineswithasetofthreenarrowGaus-
an isolated emitting region, located east and at a projected sians,withthesamewidthandradialvelocity,andonebroad
distanceof1.24±0.06arcsec(26.9±1.3pc)fromthestellar Gaussian.Weassumedthattheintensityofthe[NII]λ6548
nucleus, which is clearly visible in Hα but does not appear lineisequalto0.328oftheintensityofthe[NII]λ6583line
in [O III] λ5007. (Osterbrock & Ferland 2006). The[N II]and Hα lines were
We extracted the spectra from four circular regions of well reproduced by the fit, shown in Fig. 7. The FWHM
the data cube of NGC 1313, before the starlight subtrac- of the broad and narrow Gaussians in the fit, corrected for
tion, centred on each one of the Hα emitting areas. The the instrumental resolution, are FWHMbroad = 540 ±30
circular regions have a radius of 0.4 arcsec and are illus- km s−1 and FWHMnarrow = 120±6 km s−1, respectively.
trated in an Hα image in Fig. 3. Figs. 4 and 5 show the The width of the [S II] λλ6716,6731 emission lines is com-
extracted spectra, together with the fits provided by the patible with the width of the narrow Gaussians used to fit
spectral synthesis and the fit residuals. The blue and red the[N II] and Hα lines. The luminosity of the broad Gaus-
bumps discussed above can be easily seen in the spectrum sian in the fit is L(broad Hα−1stfit)=(5.4±0.7)×1036
of region 1. These broad features are typically seen in the erg s−1, while its equivalent width (calculated using the
spectra of WRs, which indicates thepresence of these mas- continuum of the spectrum before the starlight subtrac-
sive evolved stars in region 1. The values of the FWHM tion) is EW(broad Hα − 1stfit) = −16 ± 4˚A. In addi-
of the blue and red bumps (corrected for the spectral res- tion, the luminosity of the narrow component of Hα is
olution) are FWHM = 4900 ± 150 km s−1 and L(narrow Hα−1stfit)=(7.4±0.4)×1036 erg s−1.
blue bump
FWHM = 4200 ± 200 km s−1, respectively. The ThebroadcomponentofHαisoriginatedbytheproba-
red bump
spectra of regions 2 and 3 are similar to each other and blemassiveemission-linestarinthisarea.Thenarrowcom-
reveal only narrow emission lines. On the other hand, the ponent of Hα may be nebular; however, we cannot exclude
spectrum of region 4 is considerably different from the oth- the possibility that part of this narrow component is also
ers,asitshowsanapparentbroadcomponentoftheHαline. emitted by the emission-line star. In order to evaluate the
Basedonitscharacteristics,webelievethatthespectrumof contributionofapossiblebackgroundemissiontothenarrow
(cid:13)c 2017RAS,MNRAS000,1–??
6 Menezes & Steiner
Figure 4.Differentmagnificationsandwavelengthrangesofthespectraextractedfromregions1and2ofthedatacubeofNGC1313.
The circular regions from which the spectra were extracted have a radius of 0.4 arcsec. The synthetic stellar spectra provided by the
spectral synthesis areshowninredand theemission-linespectraobtained bysubtracting thesynthetic spectrafromthe observed ones
areshowningreen.
components of the emission lines in region 4, we extracted [NII]λλ6548,6583andHαlineswithasumofthreenarrow
thespectrumfromanannularregion,locatedaroundregion Gaussians and one broad Gaussian, considering the same
4, of the data cube of NGC 1313, after the starlight sub- assumptions mentioned before. The fit is shown in Fig. 7.
traction. This “background” was then subtracted from the Thewidthsofthelinesandtheintegratedfluxofthebroad
spectrum ofregion 4, which resulted in aresidual spectrum component of Hα did not change significantly in this sec-
that still shows the [O III] λ5007, [N II] λ6584 and [S II] ond fit and the new luminosity of the narrow component
λλ6716,6731 emission lines. After that, we fitted again the of Hα (L(narrow Hα−2ndfit) = (4.2±0.4)×1036 erg
(cid:13)c 2017RAS,MNRAS000,1–??
The emitting areas in the nucleus of NGC 1313 7
Figure 5. The same as Fig. 4 but for the spectra extracted from regions 3 and 4. Two additional magnifications of the emission-line
spectrumofregion4areshownatthebottom.
(cid:13)c 2017RAS,MNRAS000,1–??
8 Menezes & Steiner
Figure6.Diagnosticdiagramswiththepointsrepresentingregions1,2and3ofthedatacubeofNGC1313.Theotherpointscorrespond
to objects analysed by Ho et al. (1997). The open circles in red, green, blue and magenta represent H II regions, transition objects,
lowionizationnuclearemission-lineregions(LINERs)andSeyfertgalaxies,respectively.Thebluecurverepresentsanempiricaldivision
between H II regions and AGNs (Kauffmannetal. 2003), the black curve corresponds to the maximum limit for the ionization by a
starburst(Kewleyetal.2001)andtheredcurverepresentsadivisionbetween LINERsandSeyfertgalaxies(Kewleyetal.2006).
Table 1. Emission-lineratios, luminosities of the Hαemission lineand projected distances from the stellar nucleus (d) of regions 1, 2
and3ofthedatacubeofNGC1313.
Ratio 1 2 3
[NII]λ6584/Hα 0.121±0.009 0.148±0.010 0.190±0.013
([SII]λ6716+λ6731)/Hα 0.181±0.012 0.267±0.016 0.339±0.021
[OIII]λ5007/Hβ 2.29±0.16 1.44±0.10 1.02±0.07
LHα (1037 ergs−1) 3.05±0.15 1.60±0.08 1.417±0.07
d(arcsec) 0.36±0.06 1.21±0.06 1.10±0.06
Figure 7. Gaussian fit of the [N II] + Hα emission lines of the spectrum of region 4 before (left) and after (right) the background
subtraction. The black points represent the observed values. The narrow Gaussians used in the fit are shown in green and the broad
Gaussianisshowninblue.Thefinalfitisshowninred.
(cid:13)c 2017RAS,MNRAS000,1–??
The emitting areas in the nucleus of NGC 1313 9
Figure 8. Vgas map (top left) and σgas map (top right) obtained with simple Gaussian fits of the Hα emission line in the data cube
of NGC 1313. In both maps, the positions of the four emission-lineregions aremarked with crosses, the position of the stellar nucleus
ismarkedwithasquareandtheVgas centreismarkedwithanellipse.Thehalf-sizesofthecrossesandofthesquareandthesemi-axis
oftheellipseindicatetheuncertainties(1σ)ofthecorrespondingpositions.Thecurvesextracted,alongthelineofnodes,fromtheVgas
mapandfromtheσgas mapareshownatbottom leftandbottom right,respectively.
s−1) represents an upper limit for the narrow Hα emitted uncertainty.Therefore, we decided to not include the point
bythestar(aspartof thisnarrow componentcan still bea representing region 4 in thediagnostic diagrams in Fig. 6.
nebular contribution emitted around the star, not removed
In order to analyse the kinematics of the ionized gas
by the background subtraction). We also applied the back-
in the nuclear region of NGC 1313, we determined the ra-
groundsubtractiontothestellarspectrumofregion4(using
dial velocity and the velocity dispersion of the gas (Vgas
spectrafromthedatacubebeforethestarlightsubtraction).
and σgas, respectively) of each spaxel of the data cube by
Then,usingthecontinuumoftheresultingspectrum,wecal-
fitting a Gaussian function to the Hα emission line of the
culatedtheequivalentwidthofthebroadcomponentofHα
corresponding spectrum. The amplitude/noise (A/N) ratio
providedbythesecondGaussianfitandobtainedavalueof
oftheHαemission lineis∼20inthemost peripheralareas
EW(broadHα−2ndfit)=−20±7˚A.
of the FOV and is larger than 600 near the stellar nucleus.
It is important to mention that the procedure of back- Such large values of the A/N ratio allowed a precise analy-
groundsubtractionusedherehassomeuncertainties.Inpar- sisofthekinematicparameters.However,theanalysisofthe
ticular, the background spectrum may be contaminated by spectraintheareacorrespondingtoregion4ismorecompli-
the emission from stars not related to region 4 (see Fig. 9). cated.Sinceweareperformingthiskinematicanalysisusing
This certainly could affect the values of L(narrow Hα− simple Gaussian fits, we could not reproduce properly the
2ndfit) and EW(broadHα−2ndfit) providedbythefit of profile of the Hα line in the spectra of region 4 (where fits
the background subtracted spectrum. Since we cannot de- involving two Gaussians are required to reproduce the Hα
termine the fractions of the narrow Hα component due to profile,asexplainedbefore).Asaconsequence,thevaluesof
the star and due to the nebular emission, it is not possible σgas obtained for that area were not reliable. On the other
tocalculatetheemission-lineratios[NII]λ6584/Hαand([S hand,weverifiedthatthefitofonlyoneGaussiantotheHα
II] λ6716+λ6731)/Hα for region 4, without a considerable lineinthespectraofregion4provided,atleast,preciseval-
(cid:13)c 2017RAS,MNRAS000,1–??
10 Menezes & Steiner
Figure9.Left:imageofthedatacubeofNGC1313,beforethestarlightsubtraction,collapsedalongthespectralaxis(thesameshown
inFig.2).Middle:HαimageofthedatacubeofNGC1313,afterthestarlightsubtraction(thesameshowninFig.3).Thepositionsof
thefouremittingareasaremarkedwithcrosses.Right:HST imageofNGC1313,intheF336Wfilter(U band),obtainedwithWFC3.
TheLBV/B[e]supergiant/B hypergiantcandidateismarkedwithagreencircle.
uesforVgas.Fig.8showstheobtainedVgas andσgas maps. 4 DISCUSSION AND COMPARISON WITH
The non-reliable values of σgas in region 4 were masked in PREVIOUS STUDIES
thecorresponding map.
4.1 The WR(s) in region 1
TheVgasmaprevealsapatternconsistentwithrotation, WRs are evolved, central He-burning, massive stars that
although a number of irregularities can be easily seen. The
produce intense stellar winds, which result in considerable
line of nodes has a position angle (PA) of 10◦ ±10◦. We
mass-loss. Such stellar winds affect significantly the sur-
assumed that the centre of the Vgas map (highlighted in rounding medium by releasing mechanical energy and nu-
Fig. 8) is given by thepoint, along theline of nodes, where
clear processed material. The study of WRs is of great im-
the velocity is equal to the average between the maximum
portance, as these objects are believed to be the progen-
and minimum values (Vaverage). The Vgas map shows the itors of type Ib/c supernovae and long gamma-ray bursts
valuesobtainedafterthesubtractionofVaverageor,inother (for more information, see Crowther 2007 and references
words, the values of Vgas relative to the centre of the map. therein).Spectroscopically,WRsarecharacterizedbybroad
The moduli of the radial velocities are lower than 25 km
emission lines of He, N (WNs), C (WCs) or O (WOs). Be-
s−1 along the entire FOV. The systemic velocity of NGC
cause of its easily identifiable properties, WRs have been
1313 was taken as being equal to the sum of Vaverage and detected in the Milky Way (e.g. Crowther et al. 2006;
the velocity corresponding to the redshift (obtained from
Ferna´ndez-Mart´ın et al. 2012; Shara et al. 2012; Smith et
NED)usedtopasstheoriginaldatacubetotherestframe,
al. 2012; Kanarek et al. 2015) and in nearby galaxies (e.g.
which resulted in Vsys =475±6 km s−1. The centre of the Massey&Johnson1998;Castellanos,D´ıaz&Terlevich2002;
Vgas map in Fig.8is notexactly coincidentwith thecentre Massey2003;Hadfield&Crowther2006;Eldridge&Relan˜o
of region 1, although these two positions are compatible at
2011; Neugent & Massey 2011; Bibby & Crowther 2012;
1σ level.Theprojecteddistancebetweenthestellarnucleus
Howarth&Walborn2012;Neugent,Massey&Georgy2012;
and the centre of the Vgas map is 0.31±0.06 arcsec (which Neugent,Massey & Morrell 2012; Kehrig et al. 2013; Shara
corresponds to 6.8±1.3 pc). Regions 2 and 3 are both in
et al. 2013; Gvaramadze et al. 2014a; Massey, Neugent &
blueshift,withsimilarvelocities,whileregion4isinredshift.
Morrell 2015; Sokalet al. 2015).
ThepresenceofWRsclosetothenucleusofNGC1313
Theσgasmapisalsoveryirregular,butwecanseethat wasdetectedbyHadfield & Crowther(2007),inan HIIre-
the dispersion values (corrected for the spectral resolution) gion identified by the authors as 37. We believe that this
appear to be lower at the area where the rotation pattern H II region corresponds to region 1 in our data cube. The
isobserved.Inparticular,thelowest valuesofσgas werede- redbumparound5808˚Ainthespectrumofregion1ischar-
tected very close to the centres of region 1 and of the Vgas acteristic of WCs. A sub-classification of WCs is based on
map, originating what we can call a σgas-drop. The disper- the C IV λ5808/C III λ5696 ratio (Smith,Shara & Moffat
sion values are in the range of 10−43 km s−1 along the 1990).SincetheCIIIλ5696linesarenotvisibleinthespec-
FOV. Fig. 8 also shows curves extracted, along the line of trum of region 1, we believe that a classification of WC4-
nodes, from the Vgas and σgas maps. The uncertainties of 5 is appropriate. Hadfield & Crowther (2007) obtained the
thevalueswereobtained byperformingaMonteCarlosim- sameclassification.TakingLWC4(5808)=3.0×1036 ergs−1
ulation and also taking into account thespectral resolution (Schaerer& Vacca1998) as atypicalluminosity for thered
of the observation. bump(containing theCIV λ5808 lines) of a WC4, wecon-
(cid:13)c 2017RAS,MNRAS000,1–??