The Star-Forming Histories of the Nucleus, Bulge, and Inner Disk of NGC 5102: Clues to the Evolution of a Nearby Lenticular Galaxy
aa r X i v : . [ a s t r o - ph . GA ] D ec The Star-Forming Histories of the Nucleus, Bulge, and Inner Diskof NGC 5102: Clues to the Evolution of a Nearby LenticularGalaxy.
T. J. Davidge
Dominion Astrophysical Observatory,National Research Council of Canada, 5071 West Saanich Road,Victoria, BC Canada V9E [email protected]
ABSTRACT
Long slit spectra recorded with GMOS on Gemini South are used to exam-ine the star-forming history of the lenticular galaxy NGC 5102. Structural andsupplemental photometric information are obtained from archival Spitzer [3.6]images. Absorption features at blue and visible wavelengths are traced out alongthe minor axis to galactoentric radii ∼
60 arcsec ( ∼ . ∼ +0 . − . Gyr, and the integrated light is dominated by starsthat formed over a time period of only a few hundred Myr. For comparison, theluminosity-weighted ages of the bulge and disk are ∼ +0 . − . Gyr and 10 +2 − Gyr,respectively. The g ′ − [3 .
6] colors of the nucleus and bulge are consistent withthe spectroscopically-based ages. In contrast to the nucleus, models that assume Based on observations obtained at the Gemini Observatory, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (United States), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia), Minist´erio da Ciˆencia, Tecnologiae Inova¸c˜ao (Brazil) and Ministerio de Ciencia, Tecnolog´ıa e Innovaci´on Productiva (Argentina). This research used the facilities of the Canadian Astronomy Data Centre operated by the NationalResearch Council of Canada with the support of the Canadian Space Agency. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the JetPropulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics andSpace Administration.
Subject headings: galaxies:evolution – galaxies:elliptical and lenticular, cD –galaxies: individual (NGC 5102)
1. INTRODUCTION
NGC 5102 is a lenticular galaxy that is part of the Cen A group (Karachentsev et al.2002). As one of the nearest lenticular galaxies, it is an important laboratory for investigatingthe origins of this galaxy type in group environments. As is typical of lenticular galaxies,there is only modest star-forming activity in the disk (Davidge 2010; Karachentsev et al.2002), and the present-day star formation rate (SFR) is ∼ .
02 M ⊙ year − (Davidge 2008).The current star-forming activity is largely confined to the south east quadrant of the galaxy,where 5 of the 7 known HII regions are located (McMillan et al. 1994).With a total stellar mass of ∼ × M ⊙ (Davidge 2008), the SFR averaged over aHubble time is ∼ . ⊙ year − , and so it is not surprising that that there are signs that thedisk of NGC 5102 experienced past levels of star-forming activity at rates that far surpassthose seen today. Davidge (2008) investigated the properties of AGB stars in NGC 5102and concluded that ∼
20% of the disk mass formed within the past Gyr, which is twice thepace that would be expected if the galaxy had experienced a constant SFR throughout itsentire lifetime. The number density of C stars provides additional evidence of an elevatedSFR during intermediate epochs (Davidge 2010). The decline in the disk SFR during recent 3 –epochs has been charted by Beaulieu et al. (2010), who investigated the star-forming history(SFH) of the inner disk of NGC 5102 (their F3). They find that the mean SFR between40 and 120 Myr in the past was only one third that 120 - 200 Myr in the past, while theSFR during the past 40 Myr has been negligible. NGC 5102 has a high HI gas mass whencompared with other lenticular galaxies (van Woerden et al. 1993), and it is possible thatthis reservoir of cool gas may eventually re-kindle global star-forming activity.The stellar content of the nucleus of NGC 5102 also indicates that this was an area ofhigh levels of star-forming activity during intermediate epochs. The nucleus has a blue colorat visible wavelengths (Pritchet 1979), and Bica (1988) finds that the central SFR peaked0.5 Gyr in the past based on the integrated visible/red spectrum. Deharveng et al. (1997)conclude that large scale star-forming activity near the center of NGC 5102 commenced 0.5- 0.8 Gyr in the past, while Miner et al. (2011) find a luminosity-weighted age ∼ . . µ m are of interest for isophotal studies as they samplethe red stars that trace stellar mass.Details of the observations and the processing of the data are provided in Section 2.The isophotal properties of NGC 5102 are examined in Section 3, and the results are usedto define angular intervals from which representative spectra of the nucleus, bulge, and diskcan be extracted. An initial assessment of stellar content is made from Lick indices andbroad-band colors in Section 4. Different SFHs are explored in Section 5, where the spectraare compared with models on a pixel-by-pixel (PBP) basis. The paper closes in Section 6with a discussion and summary of the results. 5 –
2. OBSERVATIONS & INITIAL PROCESSING
The data were recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gem-ini South during the night of March 18, 2008 as part of program GS-2008A-Q-72 (PI:Davidge). The sky was clear and the image quality entry in the headers indicates that85%ile seeing prevailed, which notionally corresponds to ∼ . × . µ msquare pixel sampled 0.073 arcsec on a side. Additional details concerning the design andperformance of GMOS have been given by (Crampton et al. 2000).Spectra were recorded through a 1 arcsec wide slit that was positioned along the minoraxis of NGC 5102. The light was dispersed with the B1200 grating ( λ blaze = 4630˚ A ), andthe expected resolution of this grating + slit combination is 1872. The grating was rotatedto deliver a central wavelength of 4750˚A, and a g ′ filter was used to suppress signal fromhigher spectral orders. Twelve 960 sec exposures were recorded, yielding a total observingtime of 3.2 hours. The detector was binned 2 ×
3. ISOPHOTAL MEASUREMENTS
The isophotal properties of a galaxy are one part of its fossil record, and these havebeen investigated in NGC 5102 at visible wavelengths by Sohn, Chun, & Byun (1992). Thatstudy used information gleaned from a deep photographic IIa-O image to characterize thebulge and disk components. The bulge was found to follow an r / light profile and tocontribute the same amount of light as the disk at a major axis distance of ∼
60 arcsec.The isophotal properties at visible wavelengths can be skewed by even modest numbersof young stars, as the brightest members of a young population can dominate the light eventhough they may contribute modestly to the total stellar mass (e.g. Serra & Trager 2007).Line emission may also complicate efforts to trace the distribution of stellar mass, as it isexpected to be concentrated around ionizing sources. Given that the central regions of NGC5102 harbor a substantial population of moderately young stars and there is a complex webof line emission (McMillan et al. 1994), then it is of interest to conduct an isophotal analysisof NGC 5102 in the infrared, where the light better traces total stellar mass. For the currentstudy the isophotal properties of NGC 5102 are examined using archival SPITZER [3.6]data. A Post-Basic Calibrated Dataset (PBCD) mosaic that was constructed from [3.6] im-ages that were recorded for program 80072 (PI: Tully) was downloaded from the SpitzerIPAC archive . Isophote properties were measured with the program ellipse (Jedrzejewski1987), as implemented in the IRAF STSDAS package. The mean surface brightness, theellipticity, and the coefficient of the fourth order cosine term in the Fourier expansion ofthe isophotes – B4 – were measured. The latter distinguishes between ‘boxy’ and ‘disky’ http://irsa.ipac.caltech.edu/Missions/spitzer.html ellipse . The x-axis has been scaled so that an r / law producesa linear trend in the light profile.The bulge is characterised by an r / relation and dominates the minor axis lightprofile between 2 and 30 arcsec. The dotted line in Figure 1 is an r / relation that was fitto the bulge light profile – this relation has an effective surface brightness of 22 . ± . and an effective scale length along the minor axis of 25 . ± .
16 arcsec. Theellipticity changes with radius in the bulge, with the isophotes becoming progressively flatteras radius increases. While B4 has both disky (B4 >
0) and boxy (B4 <
0) values in thebulge, disk-shaped isophotes predominate.The outer boundary of the nucleus and the inner boundary of the disk are identified asthe points where the light profile departs from the r / relation. The nucleus departs fromthe light profile of the surroundings at radii < is 0.84 arcsec, then significant blurringof the structural properties by the instrument PSF is expected within the nucleus. Keepingthis caveat in mind, the ellipticity jumps by ∼ .
05 (i.e. the isophotes become flatter) nearthe nucleus/bulge boundary, while the B4 measurements are indicative of boxy isophotes inthe nucleus, as opposed to the disky isophotes that prevail in the bulge.The disk dominates the infrared light at radii >
30 arcsec, and the change in the lightprofile from an r / relation is accompanied by changes in other structural characteristicsnear the bulge/disk boundary. The radial trend in ellipticity that prevails throughout thebulge changes near the bulge/disk boundary, in the sense that the tendency for the ellipticity http://http://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/iracinstrumenthandbook/5/ r >
40 arcsec.
4. LINE STRENGTHS & GRADIENTS4.1. Spectral Extraction and Index Measurement
Spectra that sample different radii were extracted from the final two-dimensional spec-trum to examine radial trends in line strengths, and gauge spectroscopic uniformity withradius. The S/N ratio of the two-dimensional spectrum decreases with increasing distancefrom the galaxy center and – when necessary – the extracted spectra were binned to obtain aS/N ratio ≥
15. While little or no binning was required to meet this criterion in the nucleusand throughout much of the bulge, the extracted spectra in the disk include data that spansseveral arcsec on the sky.The slit passes through an object located 42 arcsec south of the center of NGC 5102along the minor axis. This object has a stellar morphology and a velocity that is comparableto NGC 5102, suggesting that it may be a compact star cluster. The Lick indices measuredfor this object differ from those in the adjacent disk, and the spatial interval containing thisobject was excluded when extracting spectra.An initial reconnaisance of stellar content is conducted by examining Lick indices(Worthey et al. 1994). These are well-calibrated indices that multiplex information overmoderately broad wavelength intervals, facilitating the study of absorption features in spec-tra that may have low S/N ratios. While the indices are centered on prominent absorptionfeatures that originate from one element or molecule, in reality they are contaminated bylines from other species (e.g. Worthey et al. 1994), and this complicates their use as probes ofstellar content. The indices were measured using the bandpasses defined on Guy Worthey’s 9 –website .The indices measured in NGC 5102 probe age (H β ) and chemical composition (Mg ,Mgb, Fe5270, Fe5335). These indices sample some of the strongest absorption features inthe visible part of the spectra of composite stellar systems, and have comparatively robusttransformation properties (Puzia et al. 2013). The Fe5270 and Fe5335 indices are averagedtogether for this study to boost the S/N ratio of the Fe features, and the mean index isreferred to as < F e > . The [MgFe]’ index defined by Thomas, Maraston, & Bender (2003),which combines the Mgb, Fe5270, and Fe5335 indices to track total metallicity in a mannerthat is not sensitive to variations in [ α /Fe], is also calculated.Figure 2 of McMillan et al. (1994) indicates that line emission spanning arcmin angularscales – which corresponds to kpc spatial scales at the distance of NGC 5102 – is presentnear the center of NGC 5102. It is thus not surprising that emission features are seen insome of the extracted spectra. An index to gauge the strength of the [OIII]5007 line, whichis a prominent emission feature in many of the extracted spectra, was defined. The strengthof the [OIII]5007 line is measured in the interval 4995 – 5020 ˚A, with continua measuredin the intervals 4973 – 4995 ˚A and 5020 – 5047 ˚A. The [OIII] index is measured in ˚A, andbecomes more negative as the emission line increases in strength.The extracted spectra were subject to additional processing prior to measuring indices.First, an empirical correction for the wavelength response of the instrument and optics wasconstructed from the galaxy spectra and applied to the data. Indices that cover broadwavelength intervals, such as Mg , can be affected by such response functions if the spectrumis dominated by stars with cool temperatures. In an effort to minimize the impact of thecontinuum correction on molecular features, the continuum was measured from the nuclearspectrum, which has weak metallic features and is dominated by deep Balmer absorption http://astro.wsu.edu/worthey/html/index.table.html
10 –lines that can be filtered out when obtaining a response function.Second, each extracted spectrum was corrected for the bulk radial velocity of the galaxyand for differences in velocity due to rotation internal to NGC 5102. The results were thensmoothed with a Gaussian to match the spectral resolution of the Lick system ( ∼ A ). Thewidth of the Gaussian varied with location in the galaxy to correct for gradients in velocitydispersion.Examples of extracted spectra in the 4500 - 5400˚A wavelength interval are comparedin Figure 2. Spectroscopic features sampled by the indices that are examined in this studyare indicated. Emission from [OIII]5007 and H β is seen in the +13 arcsec spectrum, andso the Lick H β index measured from this spectrum will likely underestimate the depth ofH β absorption. While the Lick indices that sample metallic features have passbands thatavoid prominent emission features like [OIII]5007, they are still subject to contaminationfrom continuum emission, which veils absorption features and causes their strengths to beunderestimated.Puzia et al. (2013) investigate the transformation of GMOS data into the Lick system.They consider two instrumental configurations that involve a common grating (B600) butthat have different resolutions due to pixel binning and slit width. The offsets between theinstrumental and standard values for the majority of indices examined by Puzia et al. (2013)differ significantly between the two set-ups. There are exceptions, and Puzia et al. (2013)identify H β , Mgb, Mg , Fe 5270, and Fe 5335 as having ‘small and well-defined correctionterms’. The stable nature of the transformations for these indices is probably due to therelative strengths of the targeted absorption features, which makes them less susceptible toblending from weak contaminating features that fall within the index passbands.The linear δ offsets listed in Table 5 of Puzia et al. (2013) apply to the B600 gratingwith a 0.5 arcsec wide slit and 1 × × The H β , Mg , Mgb, < F e > , and [MgFe]’ indices obtained from the extracted spectraare shown in Figure 3. The comparatively sparse angular coverage at large radii reflects thebinning applied to meet the minimum S/N criterion described in the previous section. Theerror bars show the random uncertainties in the indices computed from the signal in thepassbands – these are smallest near the galaxy center, where the signal is highest. The H β index peaks in the central few arcsec of NGC 5102, indicating that the nucleushas a younger luminosity-weighted age than the surroundings. The region with deep H β absorption and the area associated with the nucleus in the SPITZER [3.6] image have similarangular extents, suggesting that the prominence of the nucleus in the Spitzer data is related– at least in part – to stellar population factors, and is not driven solely by stellar massconcentration. The entries in the bottom panel of Figure 3 suggest that [OIII] emission near 12 –the nucleus is modest when compared with what is seen at some points outside of the nucleus.That the H β indices in the nucleus are not skewed greatly by H β emission is demonstratedin Section 5, where model spectra are compared directly with the observations.Weak metallic absorption features are detected in the nucleus (e.g. Figure 2). Thestrengths of the H β and metallic indices are anti-correlated near the nucleus, in the sensethat the metallic indices strengthen as H β weakens with increasing radius. This behaviour isdue to the veiling of metallic absorption features in the spectra of cool stars by the continuumfrom hot main sequence stars. The H β index is systematically higher in the interval –5 to –20 arcsec than in thecorresponding range of radii on the other side of the nucleus. This may suggest that thereare younger stars immediately to the south of the nucleus than there are to the north ofthe nucleus. However, the dip in H β in the interval 10 – 15 arcsec is also an area of [OIII]emission, raising the possibility that line emission has filled the H β absorption feature inthis angular interval. Thus, the radial age properties of the bulge may be more uniform thanindicated in Figure 2.H β and the majority of metallic indices are stable within their uncertainties in thebulge when | r | >
20 arcsec. While the Mg index is asymmetric about the nucleus, in thesense that between –20 and –40 arcsec it is a few hundredths of a magnitude larger thanbetween 20 and 40 arcsec, this may be due to low-level scattered light (see below). In fact,the symmetric nature of the [MgFe]’ index throughout the bulge suggests that there is nota radial metallicity gradient – in terms of overall metallicity the bulge of NGC 5102 appearsto be well-mixed. 13 – The Mgb and < F e > indices have different radial behaviours in the disk. While theMgb index stays constant throughout the bulge and disk, the < F e > index drops at thebulge/disk interface. A calculation of mean indices confirms that the < F e > distributionsin the disk and bulge differ, while Mgb stays constant. The mean < F e > is 0 . ± . A inthe disk and 1 . ± . A in the bulge, where the quoted uncertainties are the formal errorsin the means. The < F e > point at the southernmost end of the disk, which is by far thehighest < F e > measurement in the NGC 5102 dataset, was not included when calculatingthe mean < F e > for the disk. This point is the outlier in the upper right hand corner of the[MgFe]’ vs < F e > diagram in Figure 5 (discussed below). For comparison, the means of theMgb indices in the disk and bulge are 2 . ± . A and 2 . ± . A , respectively. Like Mgb,the averages of the [MgFe]’ indices in the bulge and disk do not show statistically significantdifferences, suggesting that the overall metallicities of the bulge and disk are similar.The Mg indices in the southern disk tend to be larger than those on the other sideof the galaxy, continuing the trend in the bulge noted in the previous section. The Mgbindices do not show a north/south asymmetry, and we suspect that the high Mg indicesin the south may be an artifact of uncertainties related to continuum shape. The spectrawere recorded during moderately bright moon conditions, and the behaviour of the Mg indices may reflect subtle differences in scattered light across the GMOS detector that areonly evident in indices that span 100 + ˚A. 14 – Indices were measured from model spectra of simple stellar populations (SSPs) thatwere downloaded from the Bag of Stellar Tricks and Isochrones (BaSTI) data base (Manzatoet al. 2008; Percival et al. 2009). The models include evolution on the AGB. SSPs are thebasic building blocks for models of more complicated SFHs and yield luminosity-weightedeffective ages when used on their own.The choice of metallicities for the models was based on the colors of resolved RGBstars in the disk of NGC 5102. Davidge (2008) found that RGB stars in NGC 5102 have[M/H] between –0.9 and –0.1, and that the metallicity distribution function peaks near –0.6.Based on these results, models with two metallicities were considered: solar and Z=0.004(i.e. [M/H] ∼ − . < F e > indices mightbe expected.The model spectra were processed to duplicate the steps applied to the extractedGMOS spectra. In particular, the models were re-sampled to match the dispersion of theGMOS data, and then continuum-corrected by applying a fit that was obtained from ayoung population using the same fitting parameters (i.e. the same function type, order,and rejection limits) that were applied to the GMOS data. Finally, the model spectra weresmoothed to match the spectral resolution of the Lick system.The observed and model indices are compared in Figure 4. It is evident that (1) thecentral regions of the nucleus have a luminosity-weighted age at visible wavelengths of a fewhundred Myr, and (2) the disk and bulge having substantially older ages than the nucleus. http://albione.oa-teramo.inaf.it/
15 –The luminosity-weighted age of the nucleus increases with radius within the nucleus, andthis may be due in part to seeing. In addition, that the innermost few parsecs of NGC 5102may harbor a population with an even younger luminosity-weighted age than is found herecan not be ruled out given the limited angular resolution of these data. The effective ageof the nucleus deduced from the indices is not greatly different from that found by Mineret al. (2011), who sampled wavelengths where the light originates from a different mix ofstellar types than at visible wavelengths. That the nucleus contains a concentration of starswith ages in excess of a few hundred Myr provides a natural explanation for the prominentnature of the nucleus in the [3.6] SPITZER images, as a large population of luminous AGBstars are expected in such a system (Section 4.4).With the exception of the measurements on the H β vs Mg plane, the observed in-dices throughout the nucleus and the inner regions of the bulge are consistent with a sub-solar metallicity, with the nucleus and inner bulge indices intersecting or falling close to theZ=0.004 sequences. In the previous section it was demonstrated that the Mgb and < F e > indices in the bulge and disk have different distributions. This is also evident in Figure 4,where the bulge and disk points on the H β vs Mgb diagram overlap on the Mgb axis, whilethere is a clear tendency for the disk points in the H β vs < F e > diagram to fall to the leftof the bulge measurements.The H β indices throughout much of the disk are smaller than predicted by the 10Gyr models. The fitting of model spectra to the observations in Section 5 reveals that H β emission may partially fill in H β absorption, causing points in the various panels of Figure4 to fall below where they would if there was no emission. Given that H β can be affectedby line emission, it is then of interest to make comparisons with model sequences using onlymetallic indices.Comparisons between the observations and models that involve only metallic featuresare made in Figure 5. The overall trends defined by the Z=0.004 and solar models are not 16 –markedly different in this figure. The modest offsets between the two model sequences inthe various panels are due in part to differences in the temperature of the main sequenceand red giant branch, and the resulting effect on the depth of metallic features.The NGC 5102 Mg indices are clearly offset from the model indices on the [MgFe]’ vs Mg plane, although the trends defined by the observations and models parallel each other.The application of an offset of a few hundredths of a magnitude to Mg will force agreementbetween the models and observations in the left hand panel of Figure 5. This same correctionwould also produce agreement between the observations and Z=0.004 models in the upperleft hand panel of Figure 4.The solar metallicity models extend to much higher index values than the Z=0.004models in Figure 5, and the full extents of the solar metallicity sequences are not shown inthis figure. It is thus significant that the range of indices in NGC 5102 matches that predictedby the Z=0.004 models. Moreover, the NGC 5102 data fall within a few tenths of an ˚A ofthe Z=0.004 models on the H β vs Mgb and H β vs < F e > planes. Whereas the majority ofpoints fall to the left of the Z=0.004 sequence on the [MgFe]’ vs Mgb diagram, the majorityof points on the [MgFe]’ vs < F e > diagrams fall to the left of the model sequence. Thisagain signals that the chemical mixture in NGC 5102 likely differs from solar.
Broad-band photometry provides a check on the stellar content information that isobtained from spectra. In general, leverage for probing stellar content increase when thewavelength range is broadened, and so photometry that covers a wide range of wavelengthsis prefered. For the current study, colors are measured from visible and mid-infrared images.Photometric measurements at visible wavelengths were obtained from the 120 sec g ′ acquisition exposure that was recorded immediately before the GMOS spectra. As per 17 –Gemini acquisition procedures, only the central GMOS CCD was read out. The data wereflat-fielded using a sky flat that was constructed from other GMOS g ′ acquisition imagesthat were recorded during March 2008. Photometry that sampled longer wavelengths wasobtained from the [3.6] SPITZER image discussed in Section 3, and the [4.5] SPITZER imageobtained from data recorded for the same program. The [3.6] and [4.5] observations have anangular resolution that is not greatly different from that of the GMOS data, simplifying thecomparison between colors and line indices.The SPITZER images were transformed to match the pixel scale and orientation of the g ′ image. The g ′ and [3.6] images were then smoothed to match the angular resolution of the[4.5] data, which has the poorest angular resolution of the image trio. All three images weresky-subtracted using measurements along the minor axis made near the edges of the fields.The g ′ − [3 .
6] and [3.6]–[4.5] colors are shown in Figure 6. The radial coverage isrestricted to ±
25 arcsec about the nucleus as the S/N of the SPITZER data plunges atlarger radii. The SPITZER observations were calibrated using zeropoints from Reach et al.(2005), while the g ′ measurements were calibrated using archived photometric zeropoints.Jorgensen (2009) investigates the time variation of GMOS zeropoints, and these variationssuggest that the use of archived zeropoints introduces an uncertainty of ± . g ′ − [3 .
6] calibration.The H β measurements from Figure 3 are re-plotted in the top panel of Figure 6 to assistin the interpretation of the results. The mix of stellar types that contributes to the integratedlight changes with wavelength, and so the amplitude of any color variations depends on thewavelength range that is examined. In the absence of emission and absorption by dust atvisible wavelengths the nucleus of NGC 5102 should have a bluer color than the bulge, andthis is observed to be the case (Pritchet 1979). However, the nucleus in g ′ − [3 .
6] is redderthan the surroundings. This is due to luminous AGB stars that dominate the infrared light inthe nucleus. Evidence for a significant AGB component is seen in the near-infrared spectrum 18 –presented by Miner et al. (2011).Additional evidence for a nuclear concentration of massive AGB stars is found at longerwavelengths. Images from the Wide-field Infrared Survey Explorer (WISE: Wright et al.2010), downloaded from the NASA/IPAC Infrared Science Archive , reveal pronouncedemission from the central regions of NGC 5102 at 12 µ m and 22 µ m. The emission at 22 µ mis of particular interest as it has a compact morphology. The WISE images thus reveal thatthe nucleus of NGC 5102 harbors significant quantities of warm dust. However, the bluecolor of the nucleus at visible wavelengths suggests that large quantities of dust likely do notpervade the interstellar medium throughout the nucleus. Rather, the warm dust detectedin the WISE images is probably concentrated in circumstellar envelopes that gird individualAGB stars.Models of the integrated photometric properties of SSPs from Marigo et al. (2008)were downloaded from the Padova observatory website . The models assume Z = 0.004,a Chabrier (2001) mass function, and an 85% AMC +15% silicate model for circumstellardust (Groenewegen 2006). This particular circumstellar composition was adopted becauseof the spectroscopic evidence for bright C stars found by Miner et al. (2011). However, thedust composition is expected to be a major factor only at longer wavelengths than thoseconsidered here (e.g. Davidge 2014).The comparisons in Figure 4 indicate that the nucleus has a luminosity-weighted ageof a few hundred Myr, while the bulge has an effective age of a few Gyr. The models predictthat a population with an age > g ′ − [3 . ∼ . − .
5, and this is consistentwith the g ′ − [3 .
6] color of the bulge in Figure 6. The models also predict that a populationwith an age 0.5 – 1.0 Gyr, like that expected in the nucleus of NGC 5102, will have g ′ − [3 . http://irsa.ipac.caltech.edu/Missions/wise.html http://stev.oapd.inaf.it/cgi-bin/cmd
19 –colors that are a few tenths of a magnitude redder than the bulge, and this is consistent withthe relative g ′ − [3 .
6] colors of the bulge and nucleus. Models that are generated without anAGB component do not have a red nucleus.While less sensitive to age than g ′ − [3 . ± .
01 magnitude for ages >
5. PIXEL-BY-PIXEL COMPARISONS WITH MODEL SPECTRA
The Lick indices combine signal over many ˚A, and the resulting decrease in spectralresolution can introduce ambiguities in age and metallicity measurements. In contrast, directPBP comparisons with model spectra provide a means of probing stellar content withoutcompromising spectral resolution. In this section PBP comparisons are made with modelsthat are based on two SFH families that might be expected in NGC 5102.
One family of models assumes SSPs. These models might be expected to representthe visible spectrum if ∼
10% or more of the stellar mass formed within the past few Gyr(Serra & Trager 2007). We note that the mean star burst amplitude in the sample of dwarfgalaxies studied by McQuinn et al. (2010) is < b > ∼
6, and so large bursts that can beapproximated as SSPs might be expected. The SSP models assume a Chabrier (2001) massfunction.The SFHs compiled by Weisz et al. (2011) indicate that if there have been multiple star- 20 –forming bursts then the SFHs of gas-rich dwarf galaxies can be approximated by a constantstar formation rate (cSFR) when averaged over suitably long timespans. The stellar massof NGC 5102 is comparable to that of the LMC (Davidge 2010), and galaxies of this sizemay experience a higher number of star bursts than more massive systems (e.g. Huang etal. 2013). Therefore, a second family of models that assume a cSFR was also explored. Starformation was assumed to start 12 Gyr ago, and the models were constructed by combiningSSP models. A Chabrier (2001) mass function was adopted. Given the evidence that star-forming activity in NGC 5102 plummeted during recent or intermediate epochs, then thecSFR models also assume that star formation was shut off in the past and was not re-started.With the metallicity, SFR, and the IMF set then the sole free parameter of these ‘truncated’cSFR models is the termination time of star-forming activity.The models were constructed from the same family of BaSTI spectra that were usedin Section 4. In light of the comparisons with the Lick indices, only models with Z=0.004are considered for the PBP comparisons. The use of solar metallicity models would resultin ages that are ∼ . indices discussedin Section 4, coupled with the suspected influence of scattered light on the spectra extractedfor the southern disk, a high order continuum function was fit to each spectrum individually,rather than basing the continuum on only one spectrum that has weak metallic features.While the removal of a high-order continuum function alters the depths of broad molecularabsorption bands, the age estimates should not be biased if the continua in the model andobserved spectra are measured and removed in a consistent manner.Spectra of the nucleus, bulge, and disk of NGC 5102 were constructed by co-adding 21 –all extracted spectra in these regions. While sacrificing angular information, the combinedspectra have high S/N ratios. Given the modest angular extent of the nucleus and theblurring introduced by seeing then the characteristic ages found for the nucleus are expectedto be upper limits. The co-added spectra were re-sampled to match the 1 ˚A spacing of theBaSTI models, and the results are shown in Figure 7.SSP and cSFR models that minimized the residuals in the wavelength interval 4750- 5400 ˚A were identified. This wavelength interval was selected as it has the highest S/Nratio. Still, the results do not change significantly if the comparisons are made in the 4100- 5400˚A interval – the key criterion is the inclusion of a Balmer line (H β in this case) toprovide leverage for age estimates.Line and continuum emission can skew age estimates, and so residuals were measuredafter supressing points that exceeded the initial dispersion by more than the 2 . σ level. Theapplication of this sigma-clipping criterion did not affect the age estimates of the nucleus anddisk. However, there are prominent emission lines in the bulge spectrum (see below), andsigma-clipping resulted in a slightly lower age estimate than when no clipping was applied.The uncertainties in the residuals were estimated by adding noise to the best-fittingmodel spectrum using the IRAF mknoise task to simulate the S/N ratio of the spectrumbeing modelled. The dispersion in the age estimate could then be found after running anumber of such realizations and comparing the results. The uncertainties found in this waytake into account random noise, but do not include systematic errors introduced by meanmetallicity or chemical composition. Thus, the uncertainties are lower limits. The effective ages of the SSP models that best match the observations are listed inTable 1. The nucleus is significantly younger than either the bulge or disk, while the bulge 22 –is in turn significantly younger than the disk. There is thus a radial gradient in the effectiveages obtained from the SSP models, in the sense of progressively younger ages towardssmaller radii.The residuals that remain after subtracting the best-matching SSP models from thenucleus and bulge spectra are shown in Figure 8. [OIII] 4959 and [OIII] 5007 lines areseen in the bulge and nucleus residuals, and H β emission is also seen in the bulge residuals.The relative strengths of the [OIII] lines provide a check on the agreement between theobservations and models, as the [OIII] 4959 line should have a strength that is ∼ . × thatof [OIII] 5007 (Osterbrock 1989). The line strengths in the lower panel of Figure 8 areconsistent with the predicted ratio. Finally, while the model fits were restricted to the 4750 -5400˚A interval, the residuals near H γ (not shown) are similar to those near H β , even thoughline emission is expected to be less of an issue for H γ . For the present work, cSFR models in which star formation starts 12 Gyr ago andterminates at 0.2, 0.4, 0.7, 0.9, 1.3, 1.8, 2.5, 4, 6, 8, and 9.5 Gyr in the past are considered.While there is evidence for low-level star-forming activity at the present epoch in NGC5102 (e.g. Section 1), the integrated spectra indicate that this activity does not contributesignificantly to the visible light. Therefore, cSFR models in which star formation terminateswithin the past 0.2 Gyr are not considered.The truncated cSFR model that provides the best match to the nucleus spectrum yieldsresiduals that are larger than those of the best-matching SSP model at the many tens of σ level. The vastly superior agreement with an SSP model suggests that the visible light fromthe nucleus is dominated by stars that formed during a large-scale star-forming event thatoccured over a time interval of no more than a few hundred Myr. However, the situation is 23 –very different in the bulge. The cSFR model in which star formation is truncated 0.7 Gyrin the past produces a match to the bulge spectra that is 5 σ better than the best-matchingSSP model. The bulge of NGC 5102 thus contains a mix of stars that span a range of ages,with no single age group dominating. Given that star formation in the bulge proceeded upto ∼ . ∼ .
6. DISCUSSION & SUMMARY
Deep long-slit spectra that cover the blue/visible wavelength region are used to probethe SFHs of the nucleus, bulge, and inner disk of the lenticular galaxy NGC 5102. Thewavelength interval that is sampled contains a number of prominent atomic and moleculartransitions that are traditional probes of stellar content. Absorption features are tracedout to ∼ ∼ . The Lick indices indicate that the visible light from the nucleus is dominated by starsthat have roughly the same metallicity as the stars that dominate the visible light in thebulge and disk. The indices are consistent with Z ∼ . ≤ ∼ V − [3 .
6] color of the nucleus, which is attributed here to a largepopulation of luminous AGB stars. A large AGB component can also explain the emission 25 –detected by WISE at 12 µ and 22 µ m. Direct evidence of a large AGB component is seen inthe near-infrared absorption spectrum (Miner et al. 2011). The age and metallicity of thenucleus measured from the GMOS spectrum are such that a large C star population mightbe expected (Maraston 2005), and Miner et al. (2011) find spectroscopic evidence of such acomponent.While low levels of nuclear star formation may have continued to very recent epochs,the SFR during recent times has been comparatively low. Indeed, the ultraviolet SED of thecenter of NGC 5102 is consistent with only a modest contribution from main sequence starsyounger than 0.3 Gyr (Kraft et al. 2005). Still, there are hints that star-forming activitymight be experiencing a revival near the center of NGC 5102. DeHarveng et al. (1997)identify bright stars near the nucleus that may be as young as 15 Myr, although they notethat these could also be older post asymptotic giant branch objects.Additional evidence of recent central star-forming activity comes from Beaulieu et al.(2010), who investigate the SFHs of three fields along the major axis of NGC 5102. Onlythe brightest stars in each field are resolved due to crowding, and this limits investigations ofSFHs to the past few hundred Myr. Field F1 studied by Beaulieu et al. (2010) samples thecentral regions of the galaxy, and the SFH constructed from their data shows a more-or-lessflat SFR 40 – 200 Myr in the past, but a five-fold increase in the SFR during the past 20Myr. The level of star-forming activity during the past ∼
10 Myr could be estimated fromthe strengths of emission lines. However, the GMOS spectra are not well-suited to this task.The GMOS slit samples only a tiny part of the galaxy, whereas the line emission in thecentral regions of NGC 5102 has a complex spatial distribution (McMillan et al. 1994) – thismakes it difficult to estimate a total SFR from only a single slit pointing. Some of the lineemission may also not be related to recent star formation. Finally, the GMOS spectra donot sample H α , which is the primary emission line SFR diagnostic at visible wavelengths. 26 –SFRs estimated from H β are subject to significant uncertainties due to extinction, whilethe strengths of the [OIII] lines depend on metallicity and the ionization parameter, withthe result that they do not correlate as well with SFR as other indicators (Moustakas et al.2006).What could cause a recent uptick in central star-forming activity? The frequency ofblue nuclei among nearby galaxies suggests that star formation near the centers of thesesystems has a duty cycle of ≤ ⊙ ) = 0.2 (i.e. Z ∼ . ⊙ )= 1.5. The GMOS spectra do not show clear evidence of an older, metal-rich component.The bulge dominates the light along the minor axis between 3 and 35 arcsec in the[3.6] images. The luminosity-weighted age of the bulge at visible wavelengths obtained fromthe GMOS spectrum is ∼ ∼ . >
35 arcsec, and has aluminosity-weighted age ∼
10 Gyr if the metallicity is Z=0.004. Comparisons with cSFRmodels in which star formation proceeded for only 2 – 3 Gyr yield slightly better agreementwith the disk spectrum than SSP models. The duration of star-forming activity notwith-standing, large-scale star formation in the disk of NGC 5102 evidently shut down many Gyrin the past, well before the drop in SFR in the nucleus and bulge. The disk is not completelydevoid of recent star-forming activity, as there is a diffuse population of bright blue stars(Davidge 2010).An old luminosity-weighted age for the disk of NGC 5102 is at odds with the conclusionreached by Davidge (2008; 2010) that the large number of AGB stars found in the disk belongto a dominant population with an age of a few Gyr. However, as demonstrated by Davidge(2014), photometric variabity among AGB stars should be considered when estimating ages,and failure to do so will result in age estimates that are biased to significantly younger values.Davidge (2008) did not account for photometric variability, and so likely underestimated the 28 –ages of AGB stars.
Given that a significant fraction of the bulge population in NGC 5102 does not havea primordial origin, but formed a few Gyr in the past, then the bulge-to-disk ratio of NGC5102 at the present day almost certainly differs from what prevailed 5 – 10 Gyr in thepast. Thus, the morphology of NGC 5102 has likely changed during the past few Gyr.The past morphology of NGC 5102 is of interest as it provides clues into (1) the types ofgalaxies that might eventually evolve into lenticular systems in low density environmentslike the Centaurus Group, and (2) the mechanisms that cause such a transformation. Twopossible early morphologies for NGC 5102 are considered here. The first is a late type spiralgalaxy. Such a system might transform into a lenticular galaxy if it experienced a merger,as suggested by Davidge (2008; 2010) and Beaulieu et al. (2010) for NGC 5102.Interactions and mergers have the potential to trigger galaxy-wide star-forming activityand alter morphology, and so play a key role in galaxy evolution. Tidal interactions duringsuch an event can form a bar that channels gas into the central regions of the primary galaxy(e.g. Hopkins et al. 2009). The area of active star formation shrinks in size as gas is depletedand/or is removed from the outer regions of a disk (e.g. Soto & Martin 2010), and the innerregions of the galaxy are expected to contain the last remnants of elevated levels of starformation. A pseudo-bulge might form when the bar buckles, and this structure will containa mix of virialized stars from the progenitor and stars that formed after the merger. If thegas channeled into the bar originated in the disk of the progenitor then it would have adisk-like chemical mixture and structure. The isophotal analysis in Section 3 indicates thatthe bulge of NGC 5102 has a disky shape.The difference between the effective ages of the bulge and nucleus of NGC 5102 is 29 –comparable to the damping time for starbursts. Indeed, the timescales of bursts in nearbydwarf galaxies range from 0.45 to 1.3 Gyr (McQuinn et al. 2010). However, a merger fails toexplain the old age of the NGC 5102 disk. Indeed, if the dominant burst of star formation inthe nucleus was triggered by a merger, then the disk might be expected to show evidence oflarge scale star formation ∼ − ≤ . +0 . − . Bulge 2 . +0 . − . Disk 10 +2 − Table 1: SSP AGES FROM PIXEL FITTING 33 –
REFERENCES
Beaulieu, S. F., Freeman, K. C., Hidalgo, S. L., Norman, C. A., & Quinn, P. J. 2010, AJ,139, 984Bica, E. 1988, A&A, 195, 76Bouchard, A., Progniel, P., Koleva, M., & Sharina, M. 2010, A&A, 513, 54Bresolin, F. 2013, ApJ, 772, L23Carter, D. 1978, MNRAS, 182, 797Chabrier, G. 2001, ApJ, 554, 1274Crampton, D., et al. 2000, SPIE, 4008, 114Davidge, T. J., & Courteau, S. 2002, AJ, 123, 1438Davidge, T. J. 2008, AJ, 135, 1636Davidge, T. J. 2010, AJ, 139, 680Davidge, T. J. 2014, ApJ, 791, 66Deharveng, J.-M., Jedrzejewski, R., Crane, P., Disney, M. J., & Rocca-Volmerange, B. 1997,A&A, 326, 528Groenewegen, M. A. T. 2006, A&A, 448, 181Harris, J., & Zaritsky, D. 2009, AJ, 138, 1243Hopkins, P. F., Cox, T. J., Younger, J. D., & Hernquist, L. 2009, ApJ, 691, 1168Jedrzejewski, R. I. 1987, MNRAS, 226, 747Jorgensen, I. 2009, PASA, 26, 17Karachentsev, I. D., et al. 2002, A&A, 385, 21Kim, J. H., Peirani, S., Kim, S., Ann, H. B., An, S.-H., & Yoon, S.-J. 2014, ApJ, 789, A90 34 –Kormendy, J., & Kennicutt, R. C. 2004, ARA&A, 42, 603Kraft, R. P., Nolan, L. A., Ponman, T. J., Jones, C., & Raychaudhury, S. 2005, ApJ, 625,785Manzato, P., Pietrinferni, A., Gasparo, F., Taffoni, G., & Cordier, D. 2008, PASP, 120, 922Maraston, C. 2005, MNRAS, 362, 799Marigo, P., Girardi, L., Bressan, A., et al. 2008, A&A, 482, 883McMillan, R., Ciardullo, R., & Jacoby, J. H. 1994, AJ, 108, 1610McQuinn, K. B. W., Skillman, E. D., Cannon, J. M., et al. 2010, ApJ, 724, 49Meschin, I., Gallart, C., Aparicio, A., Hidalgo, S. L., Monelli, M., Stetson, P. B., & Carrera,R. 2014, MNRAS, 438, 1067Miner, J., Rose, J. A., Cecil, G. 2011, ApJ, 727, L15Moustakas, J., Kennicutt, R. C. Jr., & Tremonti, C. A. 2006, ApJ, 642, 775Osterbrock, D. E. 1989, Astrophysics of Gaseous Nebulae (San Francisco: W. H. Freeman&Company)Percival, S. M., Salaris, M., Cassisi, S., & Pietrinferni, A. 2009, ApJ, 690, 427Piatti, A. E., & Geisler, D. 2013, AJ, 145, 17Pritchet, C. 1979, ApJ, 231, 354Puzia, T. H., Miller, B. W., Trancho, G., Basarab, B., Mirocha, J. T., & Butler, K. 2013,AJ, 145, 164Reach, W. T., Megeath, S. T., Cohen, M., et al. 2005, PASP, 117, 978Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525Serra, P., & Trager, S. C. 2007, MNRAS, 374, 769 35 –Sohn, Y.-J., Chun, M.-S., & Byun, Y.-I. 1992, in Relationships between active galacticnuclei and starburst galaxies, ed. A. V. Filippenko, ASP Conference Series (ASP:San Francisco), Vol 31, pp. 105.Soto, K. T., & Martin, C. L. 2010, ApJ, 716, 332Thomas, D., Maraston, C., & Bender, R. 2003, MNRAS, 339, 897Weisz, D. R., Dalcanton, J. J., Williams, B. F., et al. 2011, ApJ, 739, 5Williams, B. E., Dalcanton, J. J., Gilbert, K., et al. 2010, ApJ, 716, 71Worthy, G., Faber, S. M., Gonzalez, J. J., & Burstein, D. 1994, ApJS, 94, 687Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868
This preprint was prepared with the AAS L A TEX macros v5.2.
36 –
Fig. 1.— Isophotal properties of NGC 5102 in [3.6]. The radial runs of surface brightness,ellipticity, and B4 – the fourth-order coefficient of the cosine term in the Fourier represen-tation of the isophotes – are shown. The error bars show the uncertainties computed by ellipse . r minor is the radius in arcsec measured along the minor axis. The dotted line in thetop panel is an r / law that was fit to the bulge light profile. The points in the middle andbottom panels indicate that the nucleus/bulge and bulge/disk transitions are accompaniedby changes in isophote shape. The dashed line in the bottom panel marks B4 = 0. Whereasthe isophotes throughout much of the bulge have a ‘disky’ shape, the isophotes in the nucleusand disk tend to be ‘boxy’. 37 – Fig. 2.— Sample extracted spectra. The spectra were produced using the procedure de-scribed in the text. The spectra shown here have been normalized to unity and then shiftedalong the vertical axis to facilitate comparison. Spectral features that are associated withthe indices considered in this study are flagged. Radial offsets along the minor axis fromthe galaxy center are indicated, with positive values to the north of the galaxy center, andnegative values to the south. 38 –
Fig. 3.— Spectral indices. The radial limits of the disk, bulge, and nucleus identified fromthe [3.6] light profile are indicated at the top of the figure. Distances are measured alongthe minor axis from the center of the galaxy – positive values extend to the north, negativevalues to the south. With the exception of Mg , which is in magnitude units, the indices areequivalent widths in ˚A. The error bars show the random uncertainties estimated from thesignal in the various passbands. The median value of each index is indicated by a dottedline. The dip in H β between 10 and 15 arcsec is mirrored by an increase in [OIII] emission,and it is likely that H β absorption at these radii is partially filled by emission. The < F e > and Mgb indices show more-or-less symmetric behaviour throughout the bulge and disk,although the indices at r minor = −
57 arcsec differ markedly from those at other radii. Theasymmetric distribution of the [OIII]5007 index is consistent with the skewed distribution ofline emission mapped by McMillan et al. (1994). The high [OIII]5007 index at r minor > Fig. 4.— Comparisons with model indices. The solid lines show models with Z=0.004,while the dashed lines are models with a solar metallicity. The dotted lines connect pointshaving the same age. A solar chemical mixture is assumed. Indices in the nucleus (red),bulge (green), and disk (black) of NGC 5102 are plotted. These comparisons suggest thata range of luminosity-weighted ages are sampled, from ∼ . β vs Mg diagram, themodels suggest that stars in NGC 5102 have sub-solar metallicities, in agreement with thephotometric properties of resolved RGB stars studied by Davidge (2008). The location ofpoints on the H β vs Mgb and H β vs < F e > planes also suggest slightly different metallicitiesfor the bulge and disk, as expected if the average [Mg/Fe] is non-solar. 40 –
Fig. 5.— Comparisons between models and observations. The solid lines are sequences forZ=0.004, while the dashed lines assume solar metallicity. Indices are shown for the disk(black), bulge (green), and nuclear (red) regions. The NGC 5102 indices fall within therange of values predicted by the Z = 0.004 models. That the Mgb and < F e > points tendto lie on different sides of the Z=0.004 model sequence suggests that [Mg/Fe] in NGC 5102differs from solar. There is a systematic offset of 0.03 - 0.04 magnitudes between the modelsand observations on the [MgFe]’ vs Mg plane. The Mg index uses data over a much widerwavelength interval than the other indices, and this offset may be tied to the relation usedto remove the continuum. 41 – Fig. 6.— Color profiles. The H β indices from Figure 3 are re-plotted in the top panel. Incontrast to the blue color at visible wavelengths, the nucleus has a g ′ − [3 .
6] color that isredder than the surroundings, and the angular extent of the red nucleus matches the region ofdeep H β absorption. The red color of the nucleus is due to bright AGB stars that contributesignificantly to the [3.6] light. While the [3.6]–[4.5] color is only weakly sensitive to stellarcontent, the modest variation in this color between 5 and 15 arcsec from the nucleus suggeststhat the inner bulge has an age that is stable to within ± . Fig. 7.— Co-added spectra. The spectra were produced by summing the extracted spectra inthe nucleus, bulge, and disk regions. The spectra have been re-sampled to 1 ˚A to match themodels, and divided by a high-order continuum function using the same fitting parametersthat were applied to the model spectra. The results have been shifted vertically in this figurefor display purposes. 43 –0.40.60.81 4800 5000 5200 54000.40.60.81