The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters

Abstract

This paper commences a series of investigations into the stellar populations of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evolution histories of present-day elliptical galaxies. The primary galaxy sample analyzed is that of González (1993, G93), which consists of 39 ellipticals drawn primarily from the local field and nearby groups, plus the bulge of Messier 31. Single-stellar-population (SSP) equivalent ages, metallicities, and abundance ratios are derived from Hβ, Mg b, and Fe line strengths using an extension of the Worthey (1994) models that incorporates non-solar line-strength "response functions" by Tripicco & Bell (1995). These functions account for changes in the Lick/IDS indices caused by non-solar abundance ratios, allowing us to correct the Worthey (1994) models for the enhancements of Mg and other α-like elements relative to the Fe-peak elements.SSP-equivalent ages of the G93 ellipticals are found to vary widely, 1.5 ∼ < t ∼ < 18 Gyr, while metallicities [Z/H] and enhancement ratios, [E/Fe] are strongly peaked around [Z/H] = +0.26 and [E/Fe] = +0.20 (in an aperture of radius r e /8). The enhancement ratios [E/Fe] are milder than previous estimates, owing to the application of non-solar abundance corrections to both Mg b and Fe for the first time. While [E/Fe] is usually > 0, it is not the "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken Fe but also to strengthen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al. 1998), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the "E" group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age decreasing by 25%, metallicity decreasing by [Z/H] = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius r e /2 for nearly all systems.Our ages have an overall zeropoint uncertainty of at least ∼ 25% due to uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] = 0 on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hβ by blue stragglers or hot horizontal branch stars, or by fill-in of Hβ by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.

The Massive Survey. Iv. The X-Ray Halos of the Most Massive Early-Type Galaxies in the Nearby Universe

“…The other possible second parameter is metallicity. More massive early-type galaxies are also more metal-rich (e.g., Trager et al 2000;McDermid et al 2014;Greene et al 2015), which naturally leads to more mass loss per unit mass. Thus, one possible explanation for the mismatch between the expected luminosity due to mass loss and that observed simply arises due to our presumed model of mass loss.…”

Peculiarities and populations in elliptical galaxies

“…The differential colour indices are sampled in a cylindrical volume of the galaxy under study limited by its de Vaucouleurs radius r e , while the Lick indices are sampled through a central small aperture, expressed in arcsec, and not the same in the various data sources used in our compilation: for one of these only, the Lick indices are corrected to an r e /8 aperture (Trager et al 2000a). The indices are used concurrently in the derivation of the 3 parameters defining the intruding SSP, leading to possible uncertainties.…”

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

“…In fact, one of the most important results has been the finding that massive early-type galaxies (ETGs) show an overabundance of [Mg/Fe] (e.g., Peletier 1989;Worthey, Faber & González 1992). Such departure from the solar abundance pattern is generally attributed to short formation time-scales (<1 Gyr) for the bulk of their stellar populations (see the reviews of e.g., Trager et al 1998Trager et al , 2000Renzini 2006;Conroy 2013, and references therein). The Magnesium, an α-element, is ejected into the ISM by type-II supernovae resulting from massive stars on time-scales of ∼10 Myr, whereas most of the iron is released on time-scales of ∼1 Gyr, when SNIa explosions of intermediate-mass stars in binary systems occur.…”

“…The responses obtained for the Lick indices to variations of the individual element abundances, i.e. the so called response functions, were used to correct the model predictions based on the empirical stellar libraries (e.g., Vazdekis et al 1997;Tantalo, Chiosi & Bressan 1998;Trager et al 2000;Tantalo & Chiosi 2004;Thomas et al 2003;Schiavon 2007;Chung et al 2013). These models allowed us to achieve a better understanding of what these line-strengths are made of, as none of these rather strong atomic and molecular lines are contributed to by a single element.…”