On the stellar content of the starburst galaxy IC10
N. Sanna, G. Bono, P. B. Stetson, A. Pietrinferni, M. Monelli, S. Cassisi, R. Buonanno, E. Sabbi, F. Caputo, M. Castellani, C. E. Corsi, S. Degl'Innocenti, I. Drozdovsky, I. Ferraro, G. Iannicola, M. Nonino, P. G. Prada Moroni, L. Pulone, M. Romaniello, A. R. Walker
aa r X i v : . [ a s t r o - ph . S R ] M a y Draft version 2018 October 31
Preprint typeset using L A TEX style emulateapj v. 6/22/04
ON THE STELLAR CONTENT OF THE STARBURST GALAXY IC10 N. Sanna , G. Bono , P. B. Stetson , A. Pietrinferni , M. Monelli , S. Cassisi , R. Buonanno , E. Sabbi F.Caputo , M. Castellani , C. E. Corsi , S. Degl’Innocenti I. Drozdovsky , I. Ferraro , G. Iannicola , M.Nonino , P. G. Prada Moroni , L. Pulone , M. Romaniello , and A. R. Walker (Dated: drafted 2018 October 31 / Received / Accepted) Draft version 2018 October 31
ABSTRACTWe investigate the stellar content of the starburst dwarf galaxy IC10 using accurate and deep opticaldata collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2on board the Hubble Space Telescope. The comparison between theory and observations indicates aclear change in age distribution when moving from the center toward the external regions. Moreover,empirical calibrators and evolutionary predictions suggest the presence of a spread in heavy elementabundance of the order of one-half dex. The comparison between old and intermediate-age core He-burning models with a well defined overdensity in the color-magnitude diagram indicates the presenceof both intermediate-age, red clump stars and of old, red horizontal branch stars.
Subject headings: galaxies: individual (IC10) — galaxies: stellar content — Local Group — stars:evolution INTRODUCTION
Dwarf galaxies are ubiquitous stellar systems outnum-bering giant systems in the Local Group (LG, Mateo1998), in the Local Volume ( d ≤
10 Mpc, Vaduvescu &McCall 2008), and in the nearby Universe (Popesso etal. 2006; Milne et al. 2007). Recent evidence indicatesthat the Local Group includes at least 62 dwarfs andamong them 26 ±
5% are dwarf irregulars (dIrrs, Grebel2003; McConnachie et al. 2008). However, we still lackfirm criteria discriminating between dIrrs and Blue Com-pact Galaxies (BCDs). According to Thuan (1985) andto van den Bergh (2000) the BCDs are dIrrs that are ex-periencing a significant burst of star formation. On theother hand, Richer & McCall (1995) found that the metalabundance of BCDs is more similar to dwarf spheroidalsthan to dIrrs, and Papaderos et al. (1996) pointed outthe lack of an evolutionary link among BCDs, dIrrs anddwarf ellipticals (dEs). This key issue is far from beingsettled, and indeed in a recent detailed photometric andspectroscopic investigation Vaduvescu & McCall (2008)suggested that BCDs, dIrrs and dEs define the same fun-damental plane.Dwarf irregulars also play a key role in constraining theimpact that structural parameters and intrinsic proper-ties have on the evolution (initial mass function, star for-mation history) of complex systems (Massey et al. 2007). Based on observations collected with the ACS and the WFPC2on board of the HST. Univ. Roma ToV, via della Ricerca Scientifica 1, 00133 Rome,Italy; [email protected] INAF–OAR, via Frascati 33, Monte Porzio Catone, Rome,Italy DAO–HIA, NRC, 5071 West Saanich Road, Victoria, BC V9E2E7, Canada INAF–OACTe, via M. Maggini, 64100 Teramo, Italy IAC, Calle Via Lactea, E38200 La Laguna, Tenerife, Spain STScI, 3700 San Martin Drive, Baltimore, MD, 21218, USA Univ. Pisa, Largo B. Pontecorvo 2, 56127 Pisa, Italy INFN, Sez. Pisa, via E. Fermi 2, 56127 Pisa, Italy INAF–OAT, via G.B. Tiepolo 11, 40131 Trieste, Italy ESO, Karl-Schwarzschild-Str. 2, 85748 Garching beiMunchen, Germany CTIAO–NOAO, Casilla 603, La Serena, Chile
Moreover, they are fundamental laboratories for investi-gating the evolution of massive stars in systems that areundergoing significant bursts of star formation (Crowther2007).Among the nearby dIrrs IC10 is an interesting sys-tem, since it is one of the most massive (log
M/M ⊙ =8.49,Vaduvescu et al. 2007), and the comparison between the H α and the B -band luminosity indicates that it is expe-riencing a starburst phase (Hunter et al. 1993). More-over, it has been suggested that IC10 harbors a largenumber of young Wolf-Rayet stars (Massey & Holmes2002) and intermediate-age carbon stars (Demers et al.2004). However, we still lack detailed knowledge ofthe stellar content of IC10. In particular, Vacca et al.(2007), using deep optical and near-infrared data, pro-posed that the isochrone fit to IC10—at fixed distanceand metal content—would require different reddeningvalues for Main Sequence (MS) and Red Giant Branch(RGB) stars.In a previous investigation (Sanna et al. 2008) weprovided a new estimate of the distance modulus( µ =24.60 ± .
15 mag) based on a new calibration ofthe tip of the RGB, and of the reddening (E( F W - F W )=1.16 ± .
06 mag) based on empirical calibra-tors. Here we address the galaxy’s stellar content. RESULTS AND DISCUSSION
The photometric catalog we adopt is based on archivaloptical images from the Advanced Camera for Surveys(ACS) and the Wide Field Planetary Camera 2 (WFPC2;see top panel of Fig. 1 and Sanna et al. 2008). Thefinal catalog includes ∼ ,
000 stars with at least onemeasurement in each of two different bands. The ACSdata in the F W and F W bands were placed onthe VEGAMAG system following Sirianni et al. (2005).The F W -band images collected with the ACS weretransformed into the F W -band using local standards,and the same approach was adopted to transform the F W and the F W images collected with WFPC2into the corresponding ACS bands. On average the star-to-star precision of the above transformations is better Sanna et al.than 0.02 mag (Sanna et al. 2009, in preparation). Thefinal catalog was split into two different regions: region C) covers the galaxy center and includes both ACS andWFPC2 data, while region E) lies at a radial distancegreater than two arcminutes and only includes ACS data(see the blue and red polygons in the top panel of Fig. 1and Sanna et al. 2008).Spectroscopic estimates of IC10’s metallicity, basedon HII regions, indicate a metal content ([ F e/H ] ∼− . ± .
14, Lee et al. 2003) similar to the Small Magel-lanic Cloud (SMC, [
F e/H ] ≈ − .
7, Zaritsky et al. 1994also based on HII regions or [
F e/H ] ∼ − . ± . α -enhanced (old ages) isochrones from theBaSTI database plus a few young isochrones specif-ically computed for this project. In particular, weadopted isochrones based on evolutionary tracks ac-counting for mass-loss ( η =0.4), neglecting both convec-tive overshooting during the core H-burning phases andatomic diffusion (Pietrinferni et al. 2004, 2006). We haveassumed a true distance modulus of µ =24.60 and a red-dening E( F W - F W )=1.16 mag. Data plotted inthe bottom left panel of Fig. 1 show that young scaled-solar isochrones (red lines) at fixed metal and heliumcontent ([ M/H ]=–0.66, Y =0.251) and ages ranging from6 Myr ( M ( T urn − Of f [ T O ]) /M ⊙ ∼ M ( T O ) /M ⊙ ∼ α -enhanced isochrone (t ∼
14 Gyr). Notethat the global metallicity of this isochrone is [
M/H ]=–0.66 with [
F e/H ]=–1.01 and [ α/F e ] ∼ σ F W = σ F W ≤ separation ( sep ≥
4) and sharp-ness ( | sha |≤ separation index quantifies the de-gree of crowding (Stetson et al. 2003). The current valuecorresponds to stars that have required a correction ofless than a few percent for light contributed by knownneighbours. The sharpness index quantifies the similar-ity between the shape of the measured objects and of thePoint-Spread-Function (PSF). It is the quadrature differ-ence between the one-sigma-half-width of the measuredobject and the one-sigma-half-width of the core of thePSF (Stetson & Harris 1988). The bottom right panelof Fig. 1 shows that the CMD of the external regions isdeeper, since these regions are less affected by crowding.The comparison with the central regions indicates a sig-nificant change in age distribution, and indeed, massiveMS stars almost disappear when moving toward the ex-ternal regions. The MS stars are well fit by isochroneswith ages ranging from 12 Myr ( M ( T O ) /M ⊙ ∼ M ( T O ) /M ⊙ ∼ ≈ thelast half Gyr. Fig. 1 also indicates that IC10’s stel-lar populations show a spread in metal content. Thewidth in color of RGB stars is well fit by isochroneswith a single age (14 Gyr) and metal contents rang-ing from [ M/H ] ∼ − .
96 ( Y =0.248, green line) to[ M/H ] ∼ − .
35 ( Y =0.256, blue line). The above com- parison between theory and observations has to be con-sidered as a preliminary guideline. These estimates ofage and metal content are affected by empirical (distancemodulus, reddening, photometric zero-points) and theo-retical (mixing length, color-temperature relations) un-certainties. Firm constraints on these parameters requiredeep and accurate photometry down to the TO of the oldpopulation. Note that the possible presence of differen-tial reddening amounting to ±
10% would not account forthe observed spread in color (see the arrows in the rightpanel of Fig. 1 and Sanna et al. 2008). However, IC10 hasan extended HI envelope, a large number of HII regions(Hidalgo-Gamez 2005) and molecular clouds (Leroy et al.2006). This means that internal spatial variations of thereddening are quite probable. To partially overcome thisproblem, we adopted as representative of the IC10 stellarcontent the stars located in a small external region (E,see the red polygon in the top panel of Fig. 1).To further characterize the stellar content of IC10 wealso adopted empirical tracers. Fig. 2 shows the compar-ison between field E) of IC10 and the ACS photometry ofan SMC field provided by Sabbi et al. (2007; red dots).Note that in this figure we plotted a number of IC10stars, randomly selected, similar to the number of starspresent in the SMC field. The SMC sample was plot-ted by assuming for the SMC a true distance modulus of µ = 18 . E ( B − V ) = 0 .
08 mag. Theages of the main stellar components in this SMC fieldrange from a few tens of Myr (bright MS) to a few Gyr(red clump, RC). To investigate the possible presence ofan old stellar population we also compared IC10 to theglobular cluster NGC 6362. The
V, I -band photometry(Stetson 2000) for this cluster was transformed into theACS photometric system using the transformations bySirianni et al. (2005). NGC 6362 is an almost metal-rich cluster ([
F e/H ] = − . ± .
06, [
M/H ] ∼ -0.75, seeTable 1) and its Horizontal Branch (HB) morphologyis characterized by both red and blue stars (Brocato etal. 1999). Data plotted in this figure, in particular inthe helium burning region (i.e., RC and red HB stars,25.8 . F W . . F W − F W . M/H ] ∼ –0.47, 47 Tuc; –0.85, NGC 2808; –0.93,NGC 1851, see references listed in Table 1) and HB mor-phologies (only red HB stars, 47 Tuc; red, blue HB andRR Lyrae stars, NGC 1851; red, blue HB and extremeHB stars, NGC 2808). Note that in this figure we plot-ted a number of IC10 stars, randomly selected, similar tothe number of stars present in the GC NGC 2808. Datain Fig. 3 further support the evidence (see Fig. 1) thatRGs in IC10 cover at least one-half dex in metal content(–0.4 . [ M/H ] . –1). Moreover, a fraction of the stars lo-cated near F W ≈
26 and 1.5 . F W − F W . F W − F W ≈ Fig. 1.—
Top – The coverage of the HST data sets collected with the ACS and with the WFPC2 (black lines). The blue and red polygons,superimposed to IC10, mark fields C) and E), respectively. The background is a MegaCam@CFHT image of 6 × F W , F W - F W CMD of IC10 central regions. Red lines display scaled-solar isochrones (BaSTI)at fixed chemical composition and different ages (see labeled values). The circle ( M ( T O ) /M ⊙ =30.5), the diamond ( M ( T O ) /M ⊙ =7.7) andthe triangle ( M ( T O ) /M ⊙ =4.3) mark the Turn-Off (TO) of three young isochrones. Bottom right – Same as the left, but for the IC10external regions. The cross ( M ( T O ) /M ⊙ =14.2), the asterisk ( M ( T O ) /M ⊙ =5.0) and the square ( M ( T O ) /M ⊙ =2.5) mark the TO of threeyoung-intermediate age isochrones. The green and the blue lines show two old ( t =14 Gyr) isochrones at different metal contents. Thearrows in the top right corner show the shift in color and in magnitude caused by the possible occurrence of a differential reddening of ± adopted different sets of evolutionary models constructed assuming both old and intermediate-age progenitors. Sanna et al. Fig. 2.— F W , F W - F W CMD of IC10 external regions(black dots) compared with an SMC field (red dots) and with theGC NGC 6362 (green dots). The number of IC10 stars plottedin this figure is similar to the number of stars in the SMC field.See text and Table 1 for more details concerning the true distancemoduli and the reddenings adopted for these systems.
The top panel of Fig. 4 shows α -enhanced Zero-Age-Horizontal-Branch (ZAHB, solid line) models togetherwith the He-exhaustion locus (dashed, 10% of centralHe still available) at fixed metal and helium content([ M/H ]=–0.66, Y =0.251) for an old ( sim
14 Gyr) pro-genitor ( M pr =0.80 M ⊙ ). To cover the age range ofIC10 stars we also adopted core He-burning (solid) andHe-exhaustion (dashed, 10% of He left) models for aset of intermediate-age (160 Myr ≤ t ≤ ≤ M pr ≤ M ⊙ ). The evolutionary proper-ties of He-burning, low-to-intermediate mass stars havebeen thoroughly investigated in the literature (Sweigart,Greggio & Renzini 1990; Castellani et al. 2000; Pietrin-ferni et al. 2004,2006; Bertelli et al. 2008). Here wenote that the ratio between He- and H-burning life-times is quite constant ( t He /t H =0.006) when movingfrom M ( HB )=0.60 to 0.80 M ⊙ (old progenitor). On theother hand, the same ratio changes from t He /t H =0.11( M ( RC )=1.83 M ⊙ ) to 0.39 ( M ( RC )=2.18 M ⊙ ) and to0.34 ( M ( RC )=2.78 M ⊙ ) for scaled-solar, intermediate- The use of He lifetimes at He-exhaustion, i.e., no He left inthe core, changes the quoted ratios by a few thousandths and a fewhundredths for old and intermediate-age progenitors, respectively.
Fig. 3.—
Same as Fig. 2, but the comparison is performedwith three GCs: NGC 1851 (green dots), NGC 2808 (red dots)and 47Tuc (blue dots). The number of IC10 stars plotted in thisfigure is similar to the number of stars in NGC 2808. The truedistance moduli and reddenings adopted for these systems are listedin Table 1. From top to bottom the arrows display the position ofthe RGB bumps for NGC 1851 and 47Tuc. mass progenitors (see top panel of Fig. 4). Thisstark difference is caused by the fact that when mov-ing from 0.8 to 2.2
M/M ⊙ the core He-ignition takesplace in structures that are less and less affected byelectron degeneracy. This means that the He coremass at He-ignition, and in turn the luminosity dur-ing core He-burning, decreases from M cHe /M ⊙ =0.485( M pr =0.80 M ⊙ , M F W =–0.35 mag, t H ∼
14 Gyr) to M cHe /M ⊙ =0.467 ( M pr =2.2 M ⊙ , M F W =–0.05 mag, t H =750 Myr). More massive structures are character-ized by a convective core during H-burning phases and afurther increase in stellar mass causes a steady increasein the He core mass and in luminosity ( M pr =2.80 M ⊙ , M cHe =0.370 M ⊙ , M F W =–1.02 mag, t H =280 Myr).The above difference implies that the expected star countratio between MS and He-burning structures increasesby 1–2 orders of magnitude when moving from old tointermediate-mass stars.Therefore, we decided to perform a more detailedcomparison between theory and observations. We se-lected stars in the external regions using severe crite-ria ( σ F W = σ F W ≤ sep ≥ | sha |≤ F W ∼ F W − F W ∼ M/H ]=–0.96, Y =0.248; [ M/H ]=–0.35, Y = 0 . F W ∼
26 and F W − F W ∼ Baade’s red sheet ,i.e., evidence for an old stellar population (Baade 1963)in a starburst galaxy. Current circumstantial evidenceis, indeed, based on intermediate-age (RC) He-burningstars (Aparicio et al. 1997; Schulte-Ladbeck et al. 1998).Moreover, the identification of massive MS stars, old(HB), and intermediate age (RC) helium burning starsindicates that IC10 underwent several star formationepisodes during its life. A CMD a couple of magnitudesdeeper and with a stronger temperature sensitivity couldprovide firm constraints on whether the star formationactivity of this interesting system has been continuous orsporadic.It is a pleasure to thank P. Popesso for several inter-esting discussions concerning dwarf galaxies. We alsothank an anonymous referee for his/her pertinent com-ments and detailed suggestions that helped us to im-prove the content and the readability of the manuscript.This project was partially supported by the grant Montedei Paschi di Siena (P.I.: S. Degl’Innocenti), PRIN-INAF2007 (P.I.: M. Bellazzini), PRIN-MIUR2007 (P.I.:G. Piotto).
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Sanna et al.
Fig. 4.—
Top – Color-Magnitude diagram for predicted He-burning structures at fixed global metallicity ([
M/H ]=–0.66). Thefainter solid and dashed lines show the α -enhanced ZAHB andthe He-exhaustion (10% of He core still available) for an old pro-genitor ( t H =14 Gyr, M pr =0.80 M ⊙ ). The brighter solid anddashed lines show the core He-burning and the He exhaustion forintermediate-age progenitors. The mass of the progenitors, themass at core He-burning and the ratio between He and H lifetimesfor selected structures are labeled and marked with black (old)and red (intermediate-age) symbols, respectively. Bottom – Thered solid polygons display the 35, 60, 80 and 97% isodensity levels.Fainter blue and green lines show the ZAHBs for old, low-massstructures with different chemical compositions. The almost ver-tical green and blue lines display core He-burning structures withthe same compositions, but for intermediate-mass structures. n the stellar content of IC10 7 TABLE 1Intrinsic parameters of the GCs adopted as empirical calibrators. ID µ a E ( B − V ) b [ F e/H ] cspe [ F e/H ] dZW [ F e/H ] dCG [ M/H ] e RGB bump f . ± . g . ± . h − . ± . i − . ± . − . ± . − .
47 13 . ± . . ± . j . ± . j − . ± . k − . ± . − . ± . − .
75 14 . ± . . ± . l . ± . m − . ± . n − . ± . − . ± . − .
85 15 . ± . . ± . o . ± . o − . ± . p − . ± . − . ± . − .
93 15 . ± . a Cluster true distance modulus (mag). b Cluster reddening (mag). c High-resolution spectroscopic iron abundances. d Iron abundances based on Ca triplet measurements provided by Rutledge et al. (1997) in the Zinn & West (1984)and in the Carretta & Gratton (1997) metallicity scale. e Global metallicity based on spectroscopic iron abundances and assuming [ α/F e ] = 0 . f The Johnson-Cousins magnitudes of the RGB bump provided by Di Cecco et al. (2009, in preparation) weretransformed into the ACS F W -band (VEGAMAG) following Sirianni et al. (2005). g Bono et al. (2008). h Salaris et al. (2007). i Koch et al. (2008). j Brocato et al. (1999). k Gratton (1987). l Castellaniet al. (2006). m Bedin et al. (2000). n Carretta (2006). o Saviane et al. (1998). pp