FIRST-AUTHOR PAPERS
Yates R. M., Hendriks D., Vijayan A. P., Izzard R. G., Thomas P. A., Das P.
We present detailed implementations of (a) binary stellar evolution (using binary_c) and (b) dust production and destruction into the cosmological semi-analytic galaxy evolution simulation, L-Galaxies. This new version of L-Galaxies is compared to a version assuming only single stars and to global and spatially-resolved observational data across a range of redshifts (𝑧). We find that binaries have a negligible impact on the stellar masses, gas masses, and star formation rates of galaxies if the total mass ejected by massive stars is unchanged. This is because massive stars determine the strength of supernova (SN) feedback, which in turn regulates galaxy growth. Binary effects, such as common envelope ejection and novae, affect carbon and nitrogen enrichment in galaxies, however heavier alpha elements are more affected by the choice of SN and wind yields. Unlike many other simulations, the new L-Galaxies reproduces observed dust-to-metal (DTM) and dust-to-gas (DTG) ratios at 𝑧 ∼ 0 − 4.
This is mainly due to shorter dust accretion timescales in dust-rich environments. However, dust masses are under-predicted at 𝑧 ≳ 4, highlighting the need for enhanced dust production at early times in simulations, possibly accompanied by increased star formation. On sub-galactic scales, there is very good agreement between L-Galaxies and observed dust and metal radial profiles at 𝑧 = 0. A drop in DTM ratio is also found in diffuse, low-metallicity regions, contradicting the assumption of a universal value. We hope that this work serves as a useful template for binary stellar evolution implementations in other cosmological simulations in future.
Yates R. M., Peroux, C., Nelson D.
We contrast the latest observations of the cosmic metal density in neutral gas (𝜌_met,neu) with three cosmological galaxy evolution simulations: L-Galaxies 2020, TNG100, and EAGLE. We find that the fraction of total metals that are in neutral gas is < 40 per cent at 3 < 𝑧 < 5 in these simulations, whereas observations of damped Lyman-𝛼 (DLA) systems suggest >85 per cent. In all three simulations, hot, low-density gas is also a major contributor to the cosmic metal budget, even at high redshift. By considering the evolution in cosmic SFR density (𝜌_sfr), neutral gas density (𝜌_HI), and mean gas-phase metallicity ([<M/H>]_neu), we determine two possible ways in which the absolute 𝜌_met,neu observed in DLAs at high redshift can be matched by simulations: (a) the 𝜌_sfr at 𝑧~3 is greater than inferred from current far ultra violet
observations, or (b) current high-redshift DLA metallicity samples have a higher mean host mass than the overall galaxy population. If the first is correct, TNG100 would match the ensemble data best, however there would be an outstanding tension between the currently observed 𝜌_sfr and 𝜌_met,neu. If the second is correct, L-Galaxies 2020 would match the ensemble data best, but would require an increase in neutral gas mass inside subhaloes above 𝑧 > 2.5. If neither is correct, EAGLE would match the ensemble data best, although at the expense of over-estimating [<M/H>]_neu. Modulo details related to numerical resolution and HI mass modelling in simulations, these incompatibilities highlight current tensions between key observed cosmic properties at high redshift.
Yates R. M., Henriques B. M. B., Fu J., Kauffmann G., Thomas P. A., Guo Q., White S. D. M., Schady, P.
We present a modified version of the L-GALAXIES 2020 semi-analytic model of galaxy evolution, which includes significantly increased direct metal enrichment of the circumgalactic medium (CGM) by supernovae (SNe). These more metal-rich outflows do not require increased mass-loading factors, in contrast to some other galaxy evolution models. This modified L-GALAXIES 2020 model is able to simultaneously reproduce the gas-phase metallicity (Zg) and stellar metallicity (Z*) radial profiles observed in nearby disc galaxies by MaNGA and MUSE, as well as the observed mass - metallicity relations for gas and stars at z = 0 and their evolution back to z ∼ 2 - 3. A direct CGM enrichment fraction of ∼90 per cent for SNe-II is preferred. We find that massive disc galaxies have slightly flatter Zg profiles than their lower-mass counterparts in L-GALAXIES 2020, due to more efficient enrichment of their outskirts via inside-out growth and metal-rich accretion. Such a weak, positive correlation between stellar mass and Zg profile slope is also seen in our MaNGA-DR15 sample of 571 star-forming disc galaxies, although below log(M∗/M⊙) ∼ 10.0 this observational result is strongly
dependent on the metallicity diagnostic and morphological selection chosen. In addition, a lowered maximum SN-II progenitor mass of 25 M⊙, reflecting recent theoretical and observational estimates, can also provide a good match to observed Zg and Z* profiles at z = 0 in L-GALAXIES 2020. However, this model version fails to reproduce an evolution in Zg at fixed mass over cosmic time, or the magnesium abundances observed in the intracluster medium (ICM).
Yates R. M., Schady P., Chen T.-W., Schweyer T., Wiseman P.
Aims: We investigate electron temperature (Te) and gas-phase oxygen abundance (ZTe) measurements for galaxies in the local Universe (z < 0.25). Our sample comprises spectra from a total of 264 emission-line systems, ranging from individual HII regions to whole galaxies, including 23 composite HII regions from star-forming main sequence galaxies in the MaNGA survey.
Methods: We utilise 130 of these systems with directly measurable Te(OII) to calibrate a new metallicity-dependent Te(OIII)-Te(OII) relation that provides a better representation of our varied dataset than existing relations from the literature. We also provide an alternative Te(OIII)-Te(NII) calibration. This new Te method is then used to obtain accurate ZTe estimates and form the mass - metallicity relation (MZR) for a sample of 118 local galaxies.
Results: We find that all the Te(OIII)-Te(OII) relations considered here systematically under-estimate ZTe for low-ionisation systems by up to 0.6 dex. We determine that this is due to such systems having an intrinsically higher O+ abundance than O++ abundance, rendering
ZTe estimates based only on [OIII] lines inaccurate. We therefore provide an empirical correction based on strong emission lines to account for this bias when using our new Te(OIII)-Te(OIII) and Te(OIII)-Te(NII) relations. This allows for accurate metallicities (1σ = 0.08 dex) to be derived for any low-redshift system with an [OIII]λ4363 detection, regardless of its physical size or ionisation state. The MZR formed from our dataset is in very good agreement with those formed from direct measurements of metal recombination lines and blue supergiant absorption lines, in contrast to most other Te-based and strong-line-based MZRs. Our new Te method therefore provides an accurate and precise way of obtaining ZTe for a large and diverse range of star-forming systems in the local Universe.
set to groups and clusters in the L-GALAXIES galaxy evolution model. Our homogenized data set reveals a tight T-ZFe relation for clusters, with a scatter in ZFe of only 0.10 dex and a slight negative gradient. After examining potential measurement biases, we conclude that some of this negative gradient has a physical origin. Our model suggests greater accretion of hydrogen in the hottest systems, via stripping from infalling satellites, as a cause. In groups, L-GALAXIES over-estimates ZFe, indicating that metal-rich gas removal (via e.g. AGN feedback) is required. L-GALAXIES is consistent with the observed ZFe in the intracluster medium (ICM) of the hottest clusters at z = 0, and shows a similar rate of ICM enrichment as that observed from at least z ∼ 1.3 to the present day. This is achieved without needing to modify any of the galactic chemical evolution (GCE) model parameters. However, the ZFe in intermediate-T clusters could be under-estimated in our model. We caution that modifications to the GCE modelling to correct this disrupt the agreement with observations of galaxies' stellar components.
Yates R. M., Thomas P. A., Henriques B. M. B.
We present an analysis of the iron abundance in the hot gas surrounding galaxy groups and clusters. To do this, we first compile and homogenize a large data set of 79 low-redshift (tilde{z} = 0.03) systems (159 individual measurements) from the literature. Our analysis accounts for differences in aperture size, solar abundance, and cosmology, and scales all measurements using customized radial profiles for the temperature (T), gas density (ρgas), and iron abundance (ZFe). We then compare this data
Yates R. M., Kauffmann G.
We investigate whether dilution in some elliptical galaxies is the cause of a positive correlation between specific star formation rate (sSFR) and gas-phase metallicity (Zg) at high stellar mass in the local Universe. In the Munich semi-analytic model of galaxy formation, L-GALAXIES, massive, low-sSFR, elliptical galaxies are seen to undergo a gradual dilution of their interstellar medium, via accretion of metal-poor gas in cold-gas clumps and low-mass satellites. This occurs after a merger-induced starburst and the associated supernova feedback have quenched most of the original gas reservoir. Signatures of this evolution are present in these model galaxies at z = 0, including low gas fractions, large central black holes, old ages, and importantly, low (Zg-Z*). Remarkably, all of these properties are also found in massive, low-sSFR, elliptical galaxies in the sloan digital sky survey data release 7 (SDSS-DR7). This provides strong, indirect evidence that gradual dilution is also occurring in nearby ellipticals in the real Universe. This scenario provides an explanation for the positive correlation between SFR and Zg measured in high-M* galaxies, and therefore has consequences for the local fundamental metallicity relation, which assumes a weak anticorrelation above ∼10^10.5 M☉.
Yates R. M., Henriques B. M. B., Thomas P. A., Kauffmann G., Johansson J., White S. D. M.
We update the treatment of chemical evolution in the Munich semi-analytic model, L-GALAXIES. Our new implementation includes delayed enrichment from stellar winds, Type II supernovae (SNe-II) and Type Ia supernovae (SNe-Ia), as well as metallicity-dependent yields and a reformulation of the associated supernova feedback. Two different sets of SN-II yields and three different SN-Ia delay-time distributions (DTDs) are considered, and 11 heavy elements (including O, Mg and Fe) are self-consistently tracked. We compare the results of this new implementation with data on (a) local, star-forming galaxies, (b) Milky Way disc G dwarfs and (c) local, elliptical galaxies. We find that the z = 0 gas-phase mass-metallicity relation is very well reproduced for all forms of DTD considered, as is the [Fe/H] distribution in the Milky Way disc. The [O/Fe] distribution in the Milky Way disc is best reproduced when using a DTD with ≤50 per cent of SNe-Ia exploding within ∼400 Myr. Positive slopes in the mass-[α/Fe] relations of local ellipticals are also obtained when using a DTD with such a minor `prompt' component. Alternatively, metal-rich winds that drive light α elements directly out into the circumgalactic medium also produce positive slopes for all forms of DTD and SN-II
yields considered. Overall, we find that the best model for matching the wide range of observational data considered here should include a power-law SN-Ia DTD, SN-II yields that take account of prior mass-loss through stellar winds and some direct ejection of light α elements out of galaxies.
Rob Yates, Guinevere Kauffmann & Qi Guo
We study relations between stellar mass, star formation and gas-phase metallicity in a sample of 177 071 unique emission line galaxies from the Sloan Digital Sky Survey Data Release 7, as well as in a sample of 43 767 star-forming galaxies at z= 0 from the cosmological semi-analytic model L-GALAXIES. We demonstrate that metallicity is dependent on star formation rate at fixed mass, but that the trend is opposite for low and for high stellar mass galaxies. Low-mass galaxies that are actively forming stars are more metal poor than quiescent low-mass galaxies. High-mass galaxies, on the other hand, have lower gas-phase metallicities if their star formation rates are small. Remarkably, the same trends are found for our sample of model galaxies. By examining the evolution of the stellar component, gas and metals as a function of time in these galaxies, we gain some insight into the physical processes that may be responsible for these trends. We find that massive galaxies with low gas-phase metallicities have undergone a gas-rich merger in the past, inducing a starburst which exhausted their
cold gas reservoirs and shutdown star formation. Thereafter, these galaxies were able to accrete metal-poor gas, but this gas remained at too low a density to form stars efficiently. This led to a gradual dilution in the gas-phase metallicities of these systems over time. These model galaxies are predicted to have lower-than-average gas-to-stellar mass ratios and higher-than-average central black hole masses. We use our observational sample to confirm that real massive galaxies with low gas-phase metallicities also have very massive black holes. We propose that accretion may therefore play a significant role in regulating the gas-phase metallicities of present-day massive galaxies.