AGB stars as tracers of metallicity and mean age across M 33

Context. Wide-field JHKs near-infrared observations covering an area of 1.8 ◦ × 1.8 ◦ centred on M 33 were obtained using WFCAM at UKIRT. These data show a large population of intermediate-age asymptotic giant branch (AGB) stars. Aims. We have used both C-type and M-type AGB stars to determine spatial variations in metallicity and mean age across the galaxy. Methods. We distinguished between C-type and M-type AGB stars from their location in the colour-magnitude diagram (J − Ks, Ks). The distribution of these stars is supported by a cross-identification between our sample and a catalogue of optically confirmed, long-period variable stars, as well as with the list of sources detected by Spitzer in the mid-infrared. We calculated the C/M ratio and the orientation of the galaxy in the sky, and compared the Ks magnitude distribution with theoretical distributions spanning a range of metallicities and star formation rates (SFRs). Results. The C/M ratio surface map confirms a metallicity gradient in the galaxy corresponding to a spread in [Fe/H] = 0.6 dex with substructures in the inner and outer galaxy. Magnitude and colour variations suggest orientation and extinction effects on the galaxy disc. Maps showing the distribution of mean age and metallicity obtained from the Ks method suggest that: the outer galaxy disc/halo is metal poorer than the nuclear region and metal-rich clumps in the inner galaxy change location with time. The average outer ring and nuclear stellar population is ∼6 Gyr old while central regions are a few Gyr younger.

[1]  B. Salasnich,et al.  Theoretical isochrones in several photometric systems , 2003 .

[2]  M.-R.L. CioniH.J. Habing,et al.  AGB stars in the Magellanic Clouds I: The C/M ratio , 2003, astro-ph/0302051.

[3]  David W. Hogg.  Confusion errors in astrometry and counterpart association , 2001 .

[4]  Martin Groenewegen.  Long Period Variables in the Magellanic Clouds: OGLE +2 MASS + DENIS , , 2004 .

[5]  P. Harding,et al.  The Stellar Populations in the Outer Regions of M33. III. Star Formation History , 2006, astro-ph/0611892.

[6]  Andrew W. Stephens,et al.  The Structure and Stellar Content of the Central Region of M33 , 2002, astro-ph/0206344.

[7]  T. Davidge.  The Outer Regions of the Nearby Sc Galaxies NGC 2403 and M33: Evidence for an Intermediate-Age Population at Large Radii , 2003, astro-ph/0304194.

[8]  K. Dawson,et al.  Determination of the Cosmic Distance Scale from Sunyaev-Zel’dovich Effect and Chandra X-Ray Measurements of High-Redshift Galaxy Clusters , 2005, astro-ph/0512349.

[9]  N. Tanvir,et al.  Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II , 2004 .

[10]  Belgium,et al.  Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP- , 2007, 0711.4922.

[11]  Near-IR observations of NGC 6822: AGB stars, distance, metallicity and structure , 2004, astro-ph/0409294.

[12]  L. Girardi,et al.  Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets , 2002, astro-ph/0205080.

[13]  Ata Sarajedini,et al.  The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function , 2004, astro-ph/0403525.

[14]  E. Hatziminaoglou,et al.  Star counts in the Galaxy - Simulating from very deep to very shallow photometric surveys with the TRILEGAL code , 2005, astro-ph/0504047.

[15]  D. Bersier,et al.  The First DIRECT Distance Determination to a Detached Eclipsing Binary in M33 , 2006 .

[16]  R. Humphreys,et al.  The M33 Variable Star Population Revealed by Spitzer , 2007, 0704.3026.

[17]  Carbon Stars and Other Luminous Stellar Populations in M33 , 2004, astro-ph/0411095.

[18]  A. T. Tokunaga,et al.  The Mauna Kea observatories near-infrared filter set. II. Specifications for a new JHKL ' M ' filter set for infrared astronomy , 2001 .

[19]  J. Frogel,et al.  Carbon Stars in the Large Magellanic Cloud: Luminosities, Colors, and Implications for the History of Star Formation , 1996 .

[20]  M. Schultheis,et al.  Period–magnitude relations for M giants in Baade's Window NGC 6522 , 2003 .

[21]  P. Harding,et al.  The Stellar Populations in the Outer Regions of M33. II. Deep ACS Imaging , 2007 .

[22]  H. J. Habing,et al.  AGB stars in the Magellanic Clouds II. The rate of star formation across the LMC , 2005, astro-ph/0509881.

[23]  P. Wood,et al.  Evolution of Low- and Intermediate-Mass Stars to the End of the Asymptotic Giant Branch with Mass Loss , 1993 .

[24]  D. Zaritsky,et al.  Kinematics and Composition of H II Regions in Spiral Galaxies. I. M33 , 1989 .

[25]  J. Bergeat,et al.  The effective temperatures of carbon-rich stars , 2001 .

[26]  L. Girardi,et al.  The red tail of carbon stars in the LMC: Models meet 2MASS and DENIS observations , 2003, astro-ph/0302601.

[27]  S. Bergh,et al.  The Galaxies of the Local Group: Dwarf spheroidals in the Andromeda subgroup , 2000 .

[28]  Caltech,et al.  Very luminous carbon stars in the outer disk of the Triangulum spiral galaxy , 2004, astro-ph/0406485.

[29]  J. Beaulieu,et al.  Deep Canada–France–Hawaii Telescope photometric survey of the entire M33 galaxy – I. Catalogue of 36 000 variable point sources★ , 2006 .

[30]  F. Mannucci,et al.  The assessment of the near infrared identification of carbon stars. I. The Local Group galaxies WLM, IC 10 and NGC 6822 , 2007, 0708.1257.

[31]  S. Schneider,et al.  A Warped Disk Model for M33 and the 21 Centimeter Line Width in Spiral Galaxies , 1996, astro-ph/9610087.

[32]  F. Herwig.  Evolution of Asymptotic Giant Branch Stars , 2005 .

[33]  I. McLean,et al.  Very Luminous Giant Stars in the Central Region of M33 , 1996 .

[34]  M. Cioni,et al.  AGB stars in the Magellanic Clouds , 2003 .

[35]  Carbon star populations in systems with different metallicities: statistics in Local Group galaxies , 2002, astro-ph/0209044.

[36]  U. Florida,et al.  Determination of the Distance to M33 Based on the Tip of the Red Giant Branch and the Red Clump , 2001, astro-ph/0110006.

[37]  A. Omont,et al.  Long period variables detected by ISO in the Small Magellanic Cloud , 2003, astro-ph/0304143.

[38]  L. Girardi,et al.  Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 , and from to 0.03 , 1999, astro-ph/9910164.

[39]  Xu Zhou,et al.  SED, age distribution and evolutionary history of M 33 , 2004 .

[40]  -INAF,et al.  Evolution of asymptotic giant branch stars. I. Updated synthetic TP-AGB models and their basic calibration , 2007, astro-ph/0703139.

[41]  The distance of M 33 and the stellar population in its outskirts , 2004, astro-ph/0405465.

[42]  W. Harris,et al.  The Metallicity Distribution in the Outer Halo of M33 , 2004, astro-ph/0406273.

[43]  California,et al.  Keck spectroscopy and Spitzer space telescope analysis of the outer disk of the Triangulum spiral galaxy M 33 , 2007, 0706.0270.

[44]  M. Cioni,et al.  Variability and spectral classification of LMC giants: Results from DENIS and EROS , 2001, astro-ph/0109014.

[45]  The Stellar Halo and Outer Disk of M33 , 2006, astro-ph/0606728.

[46]  P. Battinelli,et al.  The calibration of the metallicity versus C/M relation , 2005 .

[47]  E. Corbelli,et al.  The building up of the disk galaxy M 33 and the evolution of the metallicity gradient , 2007, 0704.3187.

[48]  M. Cioni,et al.  Magellanic Cloud Structure from Near-Infrared Surveys. I. The Viewing Angles of the Large Magellanic Cloud , 2001, astro-ph/0105339.

[49]  M. Bellazzini,et al.  The calibration of the RGB Tip as a Standard Candle Extension to Near Infrared colors and higher metallicity , 2004 .

[50]  H. J. Habing,et al.  AGB stars in the Magellanic Clouds. III. The rate of star formation across the Small Magellanic Cloud , 2006 .

[51]  Doug Geisler,et al.  RR Lyrae Variables in M33. I. Evidence for a Field Halo Population , 2006, astro-ph/0605580.