The Mass-Metallicity Gradient Relation of Early-Type Galaxies

Abstract

We present a newly observed relation between galaxy mass and radial metallicity gradients of earlytype galaxies. Our sample of 51 early-type galaxies encompasses a comprehensive mass range from dwarf to brightest cluster galaxies. The metallicity gradients are measured out to one effective radius by comparing nearly all of the Lick absorption-line indices to recent models of single stellar populations. The relation shows very different behavior at low and high masses, with a sharp transition being seen at a mass of ∼ 3.5 × 10 10 M ⊙ (velocity dispersion of ∼ 140 km s −1 , M B ∼ −19). Low-mass galaxies form a tight relation with mass, such that metallicity gradients become shallower with decreasing mass and positive at the very low-mass end. Above the mass transition point several massive galaxies have steeper gradients, but a clear downturn is visible marked by a broad scatter. The results are interpreted in comparison with competing model predictions. We find that an early star-forming collapse could have acted as the main mechanism for the formation of low-mass galaxies, with star formation efficiency increasing with galactic mass. The high-mass downturn could be a consequence of merging and the observed larger scatter a natural result of different merger properties. These results suggest that galaxies above the mass threshold of ∼ 3.5 × 10 10 M ⊙ might have formed initially by mergers of gas-rich disk galaxies and then subsequently evolved via dry merger events. The varying efficiency of the dissipative merger-induced starburst and feedback processes have shaped the radial metallicity gradients in these high-mass systems.

MASSIV: Mass Assembly Survey with SINFONI in VVDS

“…Looking further into its integrated spectrum (global and nuclear, see Fig. 3), we noticed that (i) the emission lines are broad, which is a possible sign of nuclear activity; (ii) the two nitrogen lines are clearly visible, as is the sulfur doublet and the [O i]6300 line, which together are characteristics of LINER galaxies, often associated to AGNs (Heckman 1980) -or violent episodes of star-formation in highmetallicity galaxies (Terlevich & Melnick 1985).…”

“…They appear to correlate with macroscopic properties such as stellar mass (e.g. Spolaor et al 2009). It has been demonstrated that a metallicity gradient in the stellar populations could survive a major merger event, although it will be weakened.…”

The accretion histories of brightest cluster galaxies from their stellar population gradients

“…In the canonical scenario, a galaxy's initial metallicity gradient is set by an initial starburst at z 3 and the metallicity decreases in the outskirts, as metallicity follows the changes in the gravitational potential (Scott et al 2009;McDermid et al 2012). This gradient can be disrupted by violent merging events (major mergers), or reinforced by minor mergers (Kobayashi 2004;Spolaor et al 2009;Cooper et al 2010;Pipino et al 2010). Hirschmann et al (2014) analysed the stellar populations of 10 massive halos (10 12 < M halo < 10 13 M ) from the high-resolution cosmological simulation of Hirschmann et al (2013).…”

Distances and stellar population properties for 12 elliptical galaxies

“…These SBF gradients represent a powerful and independent tool to understand how stellar populations have evolved within galaxies. As discusses by Cantiello et al (2005), the radial change of SBF magnitudes, or SBF colours, in a galaxy can be a valuable tool to constrain formation mechanism theories: steep gradientscorrelated with the galactic mass -support dissipative collapse scenarios (Carlberg 1984;Davies et al 1993;Spolaor et al 2009), while shallow gradients are expected in the hierarchical merging picture (Kauffmann et al 1993;Bekki & Shioya 2001;La Barbera et al 2004). Therefore, the radial behaviour of stellar population properties contains a footprint of the galaxy star-formation history but, while modern versions of the monolithic or hierarchical formation scenarios have shown that they are able to reproduce in part the observed behaviour of earlytype galaxies, none of the proposed scenarios has proved to be decisively successful over the other (Kampakoglou et al 2008;Foster et al 2009).…”