Abstract
We describe the use of our chemical evolution model to reproduce the abundance patterns observed in a catalog of elliptical galaxies from the SDSS DR4. The model assumes ellipticals form by fast gas accretion, and suffer a strong burst of star formation followed by a galactic wind which quenches star formation. Models with fixed IMF failed in simultaneously reproducing the observed trends with the galactic mass. So, we tested a varying IMF; contrary to the diffused claim that the IMF should become bottom heavier in more massive galaxies, we find a better agreement with data by assuming an inverse trend, where the IMF goes from being bottom heavy in less massive galaxies to top heavy in more massive ones. This naturally produces a downsizing in star formation, favoring massive stars in largest galaxies. Finally, we tested the use of the Integrated Galactic IMF, obtained by averaging the canonical IMF over the mass distribution function of the clusters where star formation is assumed to take place. We combined two prescriptions, valid for different SFR regimes, to obtain the IGIMF values along the whole evolution of the galaxies in our models. Predicted abundance trends reproduce the observed slopes, but they have an offset relative to the data. We conclude that bottom-heavier IMFs do not reproduce the properties of the most massive ellipticals, at variance with previous suggestions. On the other hand, an IMF varying with galactic mass from bottom-heavier to top-heavier should be preferred
Highlights
There are models based on the assumption that elliptical galaxies form following the fast, monolithic collapse of a gas cloud; the increased density resulting from such a collapse leads to a period of intense star formation, until the onset of a galactic wind, powered by the thermal energy injected into the interstellar medium (ISM) by SNe and stellar winds, drives the remaining gas away, quenching star formation
In a first series of tests, we compared the observed data to different Models, with the following assumptions: (i) initial infall masses in the 5 × 109–1012 M range; (ii) effective radius increasing with the galactic mass; (iii) according to the inverse wind model prescription, increasing star formation efficiency ν and decreasing infall time-scale τ for more massive galaxies
The results described in the previous section show how the attempts of reproducing the data trends by modifying the star formation efficiency and the infall time-scale in the models proved to be unsuccessful; for this reason, we decided to test the effect of varying the initial mass function (IMF) used in the models
Summary
The origin and evolution of elliptical galaxies has been long debated, in an attempt to explain the existence of a certain number of common properties.To name a few, we remember the so-called Fundamental Plane (FP), i.e. the tight correlation existing between the central velocity dispersion, the surface brightness, and the effective radius of ellipticals (Dressler et al 1987; Djorgovski & Davis 1987; Bender, Burstein & Faber 1992; Jorgensen, Franx & Kjaergaard 1996; Burstein et al 1997); the mass–metallicity relation (MZR), i.e. the trend of increasing absorption metal lines strength with galactic velocity dispersion, usually interpreted as an increase of the metal content in more massive galaxies (Lequeux et al 1979; Garnett & Shields 1987; Zaritsky, Kennicutt & Huchra 1994; Garnett 2002; Pilyugin, Vılchez & Contini 2004; Tremonti et al 2004; Kewley & Ellison 2008; Mannucci et al 2010); and the colour–magnitude relation (CMR), i.e. the observed reddening of higher mass galaxies (Bower, Lucey & Ellis 1992).Two main scenarios have been proposed to model the formation of elliptical galaxies.On one hand, models based on the hierarchical clustering of dark matter (DM) haloes picture ellipticals as the result of several merging events of spiral galaxies (Kauffmann & White 1993; Kauffmann & Charlot 1998), with the consequence that more massive ellipticals should be formed at a lower redshift.On the other hand, there are models based on the assumption that elliptical galaxies form following the fast, monolithic collapse of a gas cloud; the increased density resulting from such a collapse leads to a period of intense star formation, until the onset of a galactic wind, powered by the thermal energy injected into the interstellar medium (ISM) by SNe and stellar winds, drives the remaining gas away, quenching star formation.C 2017 The Author(s). Two main scenarios have been proposed to model the formation of elliptical galaxies. Models based on the hierarchical clustering of dark matter (DM) haloes picture ellipticals as the result of several merging events of spiral galaxies (Kauffmann & White 1993; Kauffmann & Charlot 1998), with the consequence that more massive ellipticals should be formed at a lower redshift. There are models based on the assumption that elliptical galaxies form following the fast, monolithic collapse of a gas cloud; the increased density resulting from such a collapse leads to a period of intense star formation, until the onset of a galactic wind, powered by the thermal energy injected into the interstellar medium (ISM) by SNe and stellar winds, drives the remaining gas away, quenching star formation
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