skip to main content

Title: Spiral arms are metal freeways: azimuthal gas-phase metallicity variations in flocculent discs in the FIRE-2 cosmological zoom-in simulations

We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way-mass disc galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the discs at z ≈ 0 with pixel sizes ranging from 250 to 750 pc, analogous to modern integral field unit galaxy surveys, mapping the gas-phase metallicities in both the cold and dense gas and the ionized gas correlated with H ii regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity of the order of ≲0.1 dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at z ≈ 0: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving radially inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We more » propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards.

« less
; ; ; ; ; ; ; ; ;
Publication Date:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range or eLocation-ID:
p. 3708-3726
Oxford University Press
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT We present an analysis of spatially resolved gas-phase metallicity relations in five dwarf galaxies ($\rm \mathit{M}_{halo} \approx 10^{11}\, {\rm M}_\odot$, $\rm \mathit{M}_\star \approx 10^{8.8}{-}10^{9.6}\, {\rm M}_\odot$) from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, which include an explicit model for sub-grid turbulent mixing of metals in gas, near z ≈ 0, over a period of 1.4 Gyr, and compare our findings with observations. While these dwarf galaxies represent a diverse sample, we find that all simulated galaxies match the observed mass–metallicity (MZR) and mass–metallicity gradient (MZGR) relations. We note that in all five galaxies, the metallicities are effectively identical between phases of the interstellar medium (ISM), with 95 ${{\ \rm per\ cent}}$ of the gas being within ±0.1 dex between the cold and dense gas (T < 500 K and nH > 1 cm−3), ionized gas (near the H αT ≈ 104 K ridge-line), and nebular regions (ionized gas where the 10 Myr-averaged star formation rate is non-zero). We find that most of the scatter in relative metallicity between cold dense gas and ionized gas/nebular regions can be attributed to either local starburst events or metal-poor inflows. We also note the presence of a major merger in one of our galaxies,more »m11e, with a substantial impact on the metallicity distribution in the spatially resolved map, showing two strong metallicity peaks and triggering a starburst in the main galaxy.« less
  2. ABSTRACT The spatial distribution of metals reflects, and can be used to constrain, the processes of chemical enrichment and mixing. Using PHANGS-MUSE optical integral field spectroscopy, we measure the gas-phase oxygen abundances (metallicities) across 7138 H ii regions in a sample of eight nearby disc galaxies. In Paper I, we measure and report linear radial gradients in the metallicities of each galaxy, and qualitatively searched for azimuthal abundance variations. Here, we examine the 2D variation in abundances once the radial gradient is subtracted, Δ(O/H), in order to quantify the homogeneity of the metal distribution and to measure the mixing scale over which H ii region metallicities are correlated. We observe low (0.03–0.05 dex) scatter in Δ(O/H) globally in all galaxies, with significantly lower (0.02–0.03 dex) scatter on small (<600 pc) spatial scales. This is consistent with the measurement uncertainties, and implies the 2D metallicity distribution is highly correlated on scales of ≲600 pc. We compute the two-point correlation function for metals in the disc in order to quantify the scale lengths associated with the observed homogeneity. This mixing scale is observed to correlate better with the local gas velocity dispersion (of both cold and ionized gas) than with the star formation rate. Selecting onlymore »H ii regions with enhanced abundances relative to a linear radial gradient, we do not observe increased homogeneity on small scales. This suggests that the observed homogeneity is driven by the mixing introducing material from large scales rather than by pollution from recent and on-going star formation.« less
  3. Abstract

    The extent to which turbulence mixes gas in the face of recurrent infusions of fresh metals by supernovae (SN) could help provide important constraints on the local star formation conditions. This includes predictions of the metallicity dispersion among metal-poor stars, which suggests that the interstellar medium was not very well mixed at these early times. The purpose of thisLetteris to help isolate, via a series of numerical experiments, some of the key processes that regulate turbulent mixing of SN elements in galactic disks. We study the gas interactions in small simulated patches of a galaxy disk with the goal of resolving the small-scale mixing effects of metals at parsec scales, which enables us to measure the turbulent diffusion coefficient in various galaxy environments. By investigating the statistics of variations ofαelements in these simulations, we are able to derive constraints not only on the allowed range of intrinsic yield variations in SN explosions but also on the star formation history of the Milky Way. We argue that the observed dispersion of [Mg/Fe] in metal-poor halo stars is compatible with the star-forming conditions expected in dwarf satellites or in an early low-star-forming Milky Way progenitor. In particular, metal variations in starsmore »that have not been phase-mixed can be used to infer the star-forming conditions of disrupted dwarf satellites.

    « less
  4. ABSTRACT Understanding the spatial distribution of metals within galaxies allows us to study the processes of chemical enrichment and mixing in the interstellar medium. In this work, we map the 2D distribution of metals using a Gaussian Process Regression (GPR) for 19 star-forming galaxies observed with the Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT–MUSE) as a part of the PHANGS–MUSE survey. We find that 12 of our 19 galaxies show significant 2D metallicity variation. Those without significant variations typically have fewer metallicity measurements, indicating this is due to the dearth of ${\rm H\, {\small II}}$ regions in these galaxies, rather than a lack of higher-order variation. After subtracting a linear radial gradient, we see no enrichment in the spiral arms versus the disc. We measure the 50 per cent correlation scale from the two-point correlation function of these radially subtracted maps, finding it to typically be an order of magnitude smaller than the fitted GPR kernel scale length. We study the dependence of the two-point correlation scale length with a number of global galaxy properties. We find no relationship between the 50 per cent correlation scale and the overall gas turbulence, in tension with existing theoretical models. We also find more actively star-forming galaxies,more »and earlier type galaxies have a larger 50 per cent correlation scale. The size and stellar mass surface density do not appear to correlate with the 50 per cent correlation scale, indicating that perhaps the evolutionary state of the galaxy and its current star formation activity is the strongest indicator of the homogeneity of the metal distribution.« less
  5. ABSTRACT We present the measured gas-phase metal column densities in 155 sub-damped Ly α systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4 ≤ zabs ≤ 4.3. Using all available column densities of the ionic species investigated (mainly C iv, Si ii, Mg ii, Si iv, Al ii, Fe ii, C ii, and O i; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov chain Monte Carlo techniques to explore a large grid of cloudy ionization models. Without accounting for ionization and dust depletion effects, we find that the H i-weighted gas-phase metallicity evolution of subDLAs is consistent with damped Ly α systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal poor than DLAs (between ≈0.5σ and ≈3σ significance) by up to ≈1.0 dex over the redshift range 3 ≤ zabs ≤ 4.3. The correlation of gas phase [Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas phasemore »[C/O] in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments.« less