Metallicity gradients in disks

Abstract

Aims. We examine radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient slope and reconcile the differences existing between extant models and observations within the canonical "inside-out" disk growth paradigm. Methods. A suite of 25 cosmological disks is used to examine the evolution of metallicity gradients; this consists of 19 galaxies selected from the RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code ramses, including eight drawn from the "field" and six from "loose group" environments. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code gasoline. Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. Results. We first show that generically flatter gradients are observed at redshift zero when comparing older stars with those forming today, consistent with expectations of kinematically hot simulations, but counter to that observed in the Milky Way. The vertical abundance gradients at ∼1−3 disk scalelengths are comparable to those observed in the thick disk of the Milky Way, but significantly shallower than those seen in the thin disk. Most importantly, we find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. Conclusions. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical "inside-out" growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z ∼ 1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required.

The evolution of the oxygen abundance radial gradient in the Milky Way Galaxy disk

“…From a MWG-like model, we obtained the evolution of the radial gradient of Oxygen abundances, finding a gradient of ∼ −0.2 dex kpc −1 for z = 2 which flattens with time until reaching the present time value ∼ −0.05 − 0.06 dex kpc −1 . This behavior was in agreement with the cosmological simulations from Pilkington et al (2012) and also with the Yuan et al (2011) value found at z ∼ 1.5. However, it is necessary to take into account that this gradient was calculated with the O abundances of all radial regions † Recent works conclude that there are at least two stellar generations in these old objects.…”

The CALIFA survey across the Hubble sequence

“…This assumption is a natural outcome of the mass, momentum, and energy conservation laws imposed in the simulations of disks in a cosmological context (Brook et al 2011(Brook et al , 2012Few et al 2012;Pilkington et al 2012a,b;Gibson et al 2013). Pilkington et al (2012a) examined a set of 25 simulations, from several groups, using different codes and initial conditions (Stinson et al 2010;Rahimi et al 2011;Kobayashi & Nakasato 2011;Few et al 2012) to predict the present-day metallicity gradient in MW-like galaxies and its evolution. Although the evolution of the simulated metallicity gradients depends strongly on the choice of the subgrid physics employed, most of the simulated galaxies tend to a similar present-day gradient of ∼−0.05 dex kpc −1 , in agreement with the Chiappini et al (2001) and Mollá & Díaz (2005) models for normal galaxies as the MW.…”

“…Recently, a new set of cosmological hydrodynamic simulations have started to predict how the spatially resolved information of the properties of stellar populations in galaxies can constrain the complex interplay between gas infall, outflows, stellar migration, radial gas flows, and star formation efficiency, in driving the inside-out growth of galactic disks (Brook et al 2012;Gibson et al 2013;Few et al 2012;Pilkington et al 2012a;Minchev et al 2014). Radiative cooling, star formation, feedback from supernovae, and chemical enrichment are also included in simulations to predict radial metallicity gradients as a function of merging history.…”

Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115−2124 (the Bird)

“…However, a similarly lower [Fe/H] could also be expected if the Head was a remote part of an progenitor disk involving the Body -stellar metallicity gradient determinations (e.g. Pilkington et al 2012;Sánchez-Blázquez et al 2014) are quite noisy, however, and moreover, in that case the Tail would be expected to have a similar lower value, which it does not have.…”