[Paper Review] Metallicity Gradients in Disks: Do Galaxies Form Inside-Out?

K. Pilkington, C. G. Few+14 more|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Jan 30, 2012

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy68 references118 citations

TL;DR

This study investigates metallicity gradients in disk galaxies using 25 cosmological hydrodynamical simulations and two chemical evolution models, revealing that star formation efficiency profiles drive differences in gradient evolution despite all models adhering to the 'inside-out' formation paradigm. It finds that SPH-based simulations produce steeper high-redshift gradients consistent with recent z ~ 1.5 observations, suggesting a need to revise classical chemical evolution models to better match observed gradient evolution.

ABSTRACT

We examine radial and vertical metallicity gradients using a suite of disk galaxy simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient and reconcile differences between extant models and observations within the `inside-out' disk growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+) and Kobayashi & Nakasato (GRAPE-SPH). 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. 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. 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.

Motivation & Objective

• To resolve discrepancies between observed metallicity gradients in disk galaxies and predictions from classical inside-out formation models.
• To investigate how sub-grid physics in hydrodynamical simulations influences the evolution of radial and vertical metallicity gradients.
• To compare the time evolution of metallicity gradients in cosmological simulations with those from analytical chemical evolution models.
• To assess whether observed gradients at high redshift (z ~ 1.5) are consistent with simulated gradient evolution patterns.
• To determine the role of star formation efficiency profiles in shaping the observed diversity of gradient evolution across different simulation codes.

Proposed method

• Utilized 25 cosmological disk galaxy simulations: 19 from the RaDES sample (RAMSES-AMR code) and 6 from the MUGS sample (GASPER-SPH code), plus two additional SPH-based simulations (gcd+ and grape-SPH).
• Compared simulated radial and vertical metallicity gradients with those from two classical chemical evolution models (Chiappini et al. 2001; Mollá & Díaz 2005).
• Tracked the evolution of metallicity gradients over time, focusing on differences between young and old stellar populations and across redshifts.
• Analyzed vertical abundance gradients at 1–3 disk scalelengths to compare with Milky Way thick disk observations.
• Quantified the rate of change of gradient slope and assessed the impact of sub-grid physics on gradient evolution.
• Used observational constraints from high-redshift galaxies (e.g., Yuan et al. 2011) to test model consistency with z ~ 1.5 data.

Experimental results

Research questions

• RQ1 How do radial metallicity gradients evolve in cosmological hydrodynamical simulations compared to classical chemical evolution models?
• RQ2 Why do simulations with different sub-grid physics produce divergent gradient evolution patterns despite all following the inside-out paradigm?
• RQ3 Are the steeper metallicity gradients observed in high-redshift (z ~ 1.5) disk galaxies consistent with predictions from current simulation codes?
• RQ4 To what extent do the simulated gradients of old versus young stars reflect the observed flattening of gradients in the Milky Way?
• RQ5 What role does the radial dependence of star formation efficiency play in determining the shape and evolution of metallicity gradients?

Key findings

• SPH-based simulations (MUGS, gcd+, grape-SPH) produce systematically steeper radial metallicity gradients at high redshift (z ~ 1.5) than AMR-based simulations (RaDES), consistent with observed gradients in high-z Grand Design spirals.
• The MUGS simulations exhibit much more rapid evolution in their metallicity gradients compared to RaDES and chemical evolution models, driven by differences in sub-grid physics and star formation efficiency profiles.
• Despite divergent evolutionary paths, all models converge to similar present-day radial gradients (~ -0.05 dex/kpc), indicating self-similarity in late-type disks.
• Vertical abundance gradients in simulations are comparable to those observed in the Milky Way’s thick disk at 1–3 disk scalelengths, though resolution limits thin/thick disk discrimination.
• The Chiappini et al. (2001) chemical evolution model uniquely starts with a positive gradient that inverts to negative over time, unlike other models.
• The diversity in gradient evolution across simulations highlights that sub-grid physics—particularly star formation and feedback—determines the magnitude and direction of gradient evolution, not just the inside-out paradigm alone.