Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators

We have obtained blue integrated spectra of 175 nearby early-type galaxies, covering a wide range in galaxy velocity dispersion and emphasizing those with σ < 100 km s-1. Galaxies have been observed both in the Virgo Cluster and in lower density environments. The main goals are the evaluation of higher order Balmer lines as age indicators and differences in stellar populations as a function of mass, environment, and morphology. In this first paper, our emphasis is on presenting the methods used to characterize the behavior of the Balmer lines through evolutionary population synthesis models. Lower σ galaxies exhibit a substantially greater intrinsic scatter, in a variety of line-strength indicators, than do higher σ galaxies, with the large intrinsic scatter setting in below a σ of 100 km s-1. Moreover, a greater contrast in scatter is present in the Balmer lines than in the lines of metal features. Evolutionary synthesis modeling of the observed spectral indexes indicates that the strong Balmer lines found primarily among the low-σ galaxies are caused by young age, rather than by low metallicity. Thus we find a trend between the population age and the central velocity dispersion, such that low-σ galaxies have younger luminosity-weighted mean ages. We have repeated this analysis using several different Balmer lines and find consistent results from one spectral indicator to another.

[1]  F. Grundahl,et al.  The ages of the globular clusters M$\,$71 and 47 Tuc from Strömgren $uvby$ photometry - Evidence for high ages , 2002 .

[2]  H. Kuntschner,et al.  Early-type galaxies in low-density environments , 2002, astro-ph/0207504.

[3]  M. Geha,et al.  Internal Dynamics, Structure, and Formation of Dwarf Elliptical Galaxies. II. Rotating versus Nonrotating Dwarfs , 2002, astro-ph/0304537.

[4]  S. Pedraz,et al.  Evidence of fast rotation in dwarf elliptical galaxies , 2002, astro-ph/0204173.

[5]  A. Sansom,et al.  A comparison of stellar populations in galaxy spheroids across a wide range of Hubble types , 2002, astro-ph/0202390.

[6]  D. Forbes,et al.  A catalogue and analysis of local galaxy ages and metallicities , 2001, astro-ph/0110413.

[7]  R. Schiavon,et al.  Population Synthesis in the Blue. II. The Spectroscopic Age of 47 Tucanae , 2001, astro-ph/0109365.

[8]  R. Schiavon,et al.  Population Synthesis in the Blue. I. Synthesis of the Integrated Spectrum of 47 Tucanae from Its Color-Magnitude Diagram , 2001, astro-ph/0109364.

[9]  R. Mignani,et al.  Blue Stragglers, Young White Dwarfs, and UV-Excess Stars in the Core of 47 Tucanae , 2001, astro-ph/0107056.

[10]  O. Nakamura,et al.  On the Origin of the Color-Magnitude Relation in the Virgo Cluster , 2001, astro-ph/0104084.

[11]  N. Arimoto,et al.  47 Tucanae: The Spectroscopic versus Color-Magnitude Diagram Age Discrepancy , 2001 .

[12]  R. Rich,et al.  The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age , 2001, astro-ph/0101485.

[13]  C. Baugh,et al.  A comparison of semi-analytic and smoothed particle hydrodynamics galaxy formation , 1999, astro-ph/9912220.

[14]  J. Lucey,et al.  On the dependence of spectroscopic indices of early-type galaxies on age, metallicity and velocity dispersion , 2000, astro-ph/0011234.

[15]  B. Chaboyer,et al.  The Relative Age of the Thin and Thick Galactic Disks , 2000, astro-ph/0007193.

[16]  R. Propris.  How galaxies disguise their ages , 2000, astro-ph/0005562.

[17]  R. Gilliland,et al.  Resolving the Controversy over the Core Radius of 47 Tucanae (NGC 104) , 2000, astro-ph/0005192.

[18]  N. Caldwell,et al.  Spectral Variations in Early-Type Galaxies as a Function of Mass , 2000, The Astrophysical journal.

[19]  D. Schlegel,et al.  Streaming motions of galaxy clusters within 12 000 km s−1 — I. New spectroscopic data , 2000 .

[20]  C. Maraston,et al.  Strong Balmer Lines in Old Stellar Populations: No Need for Young Ages in Ellipticals? , 2000, astro-ph/0004145.

[21]  UCOLick,et al.  The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations , 2000, astro-ph/0004095.

[22]  D. Alexander,et al.  Models for Old, Metal-poor Stars with Enhanced α-Element Abundances. I. Evolutionary Tracks and ZAHB Loci; Observational Constraints , 2000 .

[23]  G. Worthey,et al.  Evidence for a Young Stellar Population in NGC 5018 , 2000, astro-ph/0002321.

[24]  H. Kuntschner.  The Stellar Populations of Early-Type Galaxies in the Fornax Cluster , 2000, astro-ph/0001210.

[25]  S. Faber,et al.  The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters , 2000, astro-ph/0001072.

[26]  N. Arimoto,et al.  A Robust Age Indicator for Old Stellar Populations , 1999 .

[27]  B. Gibson,et al.  The Spectroscopic Age of 47 Tucanae , 1999, astro-ph/9906095.

[28]  J. Rose,et al.  The Integrated Spectra of M32 and of 47 Tucanae: A Comparative Study in the Mid-Ultraviolet With IUE , 1999 .

[29]  I. Jørgensen.  E and S0 galaxies in the central part of the Coma cluster: ages, metal abundances and dark matter , 1999, astro-ph/9902250.

[30]  A. Vazdekis,et al.  Evolutionary Stellar Population Synthesis at 2 Å Spectral Resolution , 1999, astro-ph/9901181.

[31]  U. California,et al.  Semi-analytic modelling of galaxy formation: The local Universe , 1998, astro-ph/9802268.

[32]  G. Kauffmann,et al.  Chemical enrichment and the origin of the colour-magnitude relation of elliptical galaxies in a hierarchical merger model , 1997, astro-ph/9704148.

[33]  D. Fabricant,et al.  The FAST Spectrograph for the Tillinghast Telescope , 1998 .

[34]  P. Prugniel,et al.  Kinematical data on early-type galaxies. VI , 1997 .

[35]  D. Ottaviani,et al.  Hγ and Hδ Absorption Features in Stars and Stellar Populations , 1997 .

[36]  R. Peletier,et al.  A New Chemo-evolutionary Population Synthesis Model for Early-type Galaxies. II. Observations and Results , 1997, astro-ph/9701036.

[37]  R. G. Gratton,et al.  Abundances for globular cluster giants: I. homogeneous metallicities for 24 clusters , 1996, astro-ph/9607078.

[38]  M. Dopita,et al.  A DATABASE FOR GALAXY EVOLUTION MODELING , 1996 .

[39]  Matthew A. Bershady,et al.  Linear Regression for Astronomical Data with Measurement Errors and Intrinsic Scatter , 1996, astro-ph/9605002.

[40]  J. Rose,et al.  The Ages of Starbursts in Post-Starburst Galaxies , 1995, astro-ph/9510033.

[41]  G. Kauffmann.  The age of elliptical galaxies and bulges in a merger model , 1995, astro-ph/9502096.

[42]  R. Bell,et al.  Modeling the LICK/IDS Spectral Feature Indices Using Synthetic Spectra , 1995 .

[43]  D. Mcelroy.  A Catalog of Stellar Velocity Dispersions. II. 1994 Update , 1995 .

[44]  G. Worthey,et al.  New Age Indicators for Old Stellar Populations , 1995 .

[45]  C. Tout,et al.  Approximate input physics for stellar modelling , 1995, astro-ph/9504025.

[46]  G. Kauffmann.  The Ages of elliptical galaxies in a merger model , 1995 .

[47]  G. Worthey.  Comprehensive stellar population models and the disentanglement of age and metallicity effects , 1994 .

[48]  David Burstein,et al.  Old stellar populations. 5: Absorption feature indices for the complete LICK/IDS sample of stars , 1994 .

[49]  J. Rose.  The integrated spectra of M32 and of 47 Tuc: A comparative study at high spectral resolution , 1994 .

[50]  Robert L. Kurucz,et al.  SYNTHE Spectrum Synthesis Programs and Line Data. , 1993 .

[51]  G. Wallerstein,et al.  High-resolution CCD spectra of stars in globular clusters. VII : Abundances of 16 elements in 47 Tuc, M4, and M22 , 1992 .

[52]  Sandra M. Faber,et al.  MG and Fe absorption features in elliptical galaxies , 1992 .

[53]  D. Osterbrock,et al.  Book-Review - Astrophysics of Gaseous Nebulae and Active Galactic Nuclei , 1989 .

[54]  D. Osterbrock,et al.  Astrophysics of Gaseous Nebulae and Active Galactic Nuclei , 1989 .

[55]  M. Tripicco.  Spectroscopy of metal-rich M31 globular clusters , 1986 .

[56]  Elaine M. Sadler,et al.  Ionized gas in elliptical and S0 galaxies. I - A survey for H-alpha and forbidden N II emission , 1986 .

[57]  J. Rose,et al.  Constraints on stellar populations in elliptical galaxies. , 1985 .

[58]  A. Sandage,et al.  Studies of the Virgo Cluster. II - A catalog of 2096 galaxies in the Virgo Cluster area. , 1985 .

[59]  G. Efstathiou,et al.  The evolution of large-scale structure in a universe dominated by cold dark matter , 1985 .

[60]  David Burstein,et al.  Old stellar populations. II. an analysis of K-giant spectra. , 1985 .

[61]  David Burstein,et al.  Old stellar populations. I. A spectroscopic comparison of galactic globular clusters, M 31 globular clusters, and elliptical galaxies. , 1984 .

[62]  N. Caldwell.  A SURVEY OF 0II EMISSION IN ELLIPTICAL GALAXIES. , 1984 .

[63]  John L. Tonry,et al.  X-Ray Spectra of Active Galactic Nuclei. , 1983 .

[64]  J. E. Gunn,et al.  Stellar spectrophotometric atlas, wavelengths from 3130 to 10800 A , 1983 .

[65]  John L. Tonry,et al.  A survey of galaxy redshifts: 4. The data. , 1983 .

[66]  R. O’Connell,et al.  Galaxy spectral synthesis. I. Stellar populations in the nuclei of giant ellipticals , 1976 .