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.
ABSTRACT
We contrast the latest observations of the cosmic metal density in neutral gas ($\rho _{ {met,neu}}$) 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 ($\rho _{ {\rm {\small {sfr}}}$), neutral gas density ($\rho _{ {HI}}$), and mean gas-phase metallicity ($\langle {} {M/H}\rangle _{ {neu}}$), we determine two possible ways in which the absolute $\rho _{ {met,neu}}$ observed in DLAs at high redshift can be matched by simulations: (i) the $\rho _{ {\rm {\small {sfr}}}$ at $z$ ≳ 3 is greater than inferred from current FUV observations, or (ii) 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 $\rho _{ {\rm {\small {sfr}}}$ and $\rho _{ {met,neu}}$. 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 overestimating $\langle {} {M/H}\rangle _{ {neu}}$. Modulo details related to numerical resolution and H i mass modelling in simulations, these incompatibilities highlight current tensions between key observed cosmic properties at high redshift.
It has been estimated that the world's governments spend approximately half a trillion US dollars each year on subsidizing fossil fuels. This subsidy has recently been described as a reckless use of ...public funds. Although many governments would like to reduce fuel subsidies, such subsidies are so popular with the general public, that their reduction can cause civil unrest -- as already seen in Egypt, Indonesia and Nigeria. Given the large market failures in the provision of health services, there is a strong economic rationale for governments to subsidize health care. The subsidy of health services therefore appears to be a sensible policy. Some developing countries are already promoting the benefits of free health care, especially free medicines, to justify reductions in fuel subsidies.
ABSTRACT
We present a variation of the recently updated Munich semi-analytical galaxy formation model, L-Galaxies, with a new gas stripping method. Extending earlier work, we directly measure the ...local environmental properties of galaxies to formulate a more accurate treatment of ram-pressure stripping for all galaxies. We fully recalibrate the modified L-Galaxies model using a Markov Chain Monte Carlo (MCMC) method with the stellar mass function and quenched fraction of galaxies as a function of stellar mass at 0 ≤ z ≤ 2 as constraints. Due to this recalibration, global galaxy population relations, including the stellar mass function, quenched fractions versus galaxy mass, and H i mass function are all largely unchanged and remain consistent with observations. By comparing to data on galaxy properties in different environments from the SDSS and HSC surveys, we demonstrate that our modified model improves the agreement with the quenched fractions and star formation rates of galaxies as a function of environment, stellar mass, and redshift. Overall, in the vicinity of haloes with total mass 1012 to $10^{15}\, \rm M_{\odot }$ at z = 0, our new model produces higher quenched fractions and stronger environmental dependencies, better recovering observed trends with halocentric distance up to several virial radii. By analysing the actual amount of gas stripped from galaxies in our model, we show that those in the vicinity of massive haloes lose a large fraction of their hot halo gas before they become satellites. We demonstrate that this affects galaxy quenching both within and beyond the halo boundary. This is likely to influence the correlations between galaxies up to tens of megaparsecs.
ABSTRACT
The relatively red wavelength range (4800–9300 Å) of the VLT Multi Unit Spectroscopic Explorer (MUSE) limits which metallicity diagnostics can be used; in particular excluding those ...requiring the ${\rm O\, {\small II}}$λλ3726,29 doublet. We assess various strong line diagnostics by comparing to sulphur $T_{\rm {\it e}}$-based metallicity measurements for a sample of 671 H ii regions from 36 nearby galaxies from the MUSE Atlas of Disks (MAD) survey. We find that the O3N2 and N2 diagnostics return a narrower range of metallicities that lie up to ∼0.3 dex below $T_{\rm {\it e}}$-based measurements, with a clear dependence on both metallicity and ionization parameter. The N2S2H α diagnostic shows a near-linear relation with the $T_{\rm {\it e}}$-based metallicities, although with a systematic downward offset of ∼0.2 dex, but no clear dependence on ionization parameter. These results imply that the N2S2H α diagnostic produces the most reliable results when studying the distribution of metals within galaxies with MUSE. On sub-H ii region scales, the O3N2 and N2 diagnostics measure metallicity decreasing towards the centres of H ii regions, contrary to expectations. The S-calibration and N2S2H α diagnostics show no evidence of this, and show a positive relationship between ionization parameter and metallicity at $12 + \log (\rm {O/H})$ > 8.4, implying the relationship between ionization parameter and metallicity differs on local and global scales. We also present hiidentify, a python tool developed to identify H ii regions within galaxies from H α emission maps. All segmentation maps and measured emission line strengths for the 4408 H ii regions identified within the MAD sample are available to download.
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We perform a comparison, object by object and statistically, between the Munich semi-analytical model, L-GALAXIES, and the IllustrisTNG hydrodynamical simulations. By running L-GALAXIES on ...the IllustrisTNG dark matter-only merger trees, we identify the same galaxies in the two models. This allows us to compare the stellar mass, star formation rate, and gas content of galaxies, as well as the baryonic content of subhaloes and haloes in the two models. We find that both the stellar mass functions and the stellar masses of individual galaxies agree to better than ${\sim} 0.2\,$dex. On the other hand, specific star formation rates and gas contents can differ more substantially. At z = 0, the transition between low-mass star-forming galaxies and high-mass quenched galaxies occurs at a stellar mass scale ${\sim} 0.5\,$dex lower in IllustrisTNG than that in L-GALAXIES. IllustrisTNG also produces substantially more quenched galaxies at higher redshifts. Both models predict a halo baryon fraction close to the cosmic value for clusters, but IllustrisTNG predicts lower baryon fractions in group environments. These differences are primarily due to differences in modelling feedback from stars and supermassive black holes. The gas content and star formation rates of galaxies in and around clusters and groups differ substantially, with IllustrisTNG satellites less star forming and less gas rich. We show that environmental processes such as ram-pressure stripping are stronger and operate to larger distances and for a broader host mass range in IllustrisTNG. We suggest that the treatment of galaxy evolution in the semi-analytic model needs to be improved by prescriptions that capture local environmental effects more accurately.
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We implement a detailed dust model into the L-Galaxies semi-analytical model which includes: injection of dust by type II and type Ia supernovae (SNe) and AGB stars; grain growth in ...molecular clouds; and destruction due to supernova-induced shocks, star formation, and reheating. Our grain growth model follows the dust content in molecular clouds and the inter-cloud medium separately, and allows growth only on pre-existing dust grains. At early times, this can make a significant difference to the dust growth rate. Above z ∼ 8, type II SNe are the primary source of dust, whereas below z ∼ 8, grain growth in molecular clouds dominates, with the total dust content being dominated by the latter below z ∼ 6. However, the detailed history of galaxy formation is important for determining the dust content of any individual galaxy. We introduce a fit to the dust-to-metal (DTM) ratio as a function of metallicity and age, which can be used to deduce the DTM ratio of galaxies at any redshift. At z ≲ 3, we find a fairly flat mean relation between metallicity and the DTM, and a positive correlation between metallicity and the dust-to-gas (DTG) ratio, in good agreement with the shape and normalization of the observed relations. We also match the normalization of the observed stellar mass–dust mass relation over the redshift range of 0–4, and to the dust mass function at z = 0. Our results are important in interpreting observations on the dust content of galaxies across cosmic time, particularly so at high redshift.
ABSTRACT
Hot ionized gas is important in the baryon cycle of galaxies and contributes the majority of their ‘missing baryons’. Until now, most semi-analytic models of galaxy formation have paid ...little attention to hot gaseous haloes and their X-ray emission. In this paper, we adopt the one-dimensional model from Sharma et al. instead of the isothermal sphere to describe the radial distribution of hot gas in the L-Galaxies semi-analytic model. The hot gas halo can be divided into two parts according to the ratio of the local thermal instability time-scale and the free-fall time-scale: a cool core with tTI/tff = 10 and a stable outer halo with tTI/tff > 10. We update the prescriptions of cooling, feedback, and stripping based on the new hot gas profiles, and then reproduce several X-ray observational results like the radial profiles of hot gas density, and the scaling relations of X-ray luminosity and temperature. We find: (1) Consistent with observations, flatter density profiles in halo centers produce lower X-ray emission than an isothermal sphere; (2) Cool core regions prone to precipitation have higher gas temperature than the virial temperature, and a larger TX/T200 ratio in smaller haloes leads to a steeper slope in the LX–TX relation; (3) The ionized gas in the unbounded reservoir and low-temperature intergalactic gas in low-mass haloes could be the main components of the halo ‘missing baryons’. Our model outputs can predict the observations of hot gas in the nearby universe and produce mock surveys of baryons probed by future X-ray telescopes.
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We have updated the Munich galaxy formation model, L-galaxies, to follow the radial distributions of stars and atomic and molecular gas in galaxy discs. We include an H2-based star-formation ...law, as well as a detailed chemical-enrichment model with explicit mass-dependent delay times for SN-II, SN-Ia, and AGB stars. Information about the star formation, feedback, and chemical-enrichment histories of discs is stored in 12 concentric rings. The new model retains the success of its predecessor in reproducing the observed evolution of the galaxy population, in particular, stellar mass functions and passive fractions over the redshift range 0 ≤ z ≤ 3 and mass range $8\le \log (M_*/\hbox{$\rm \, M_{\odot }$})\le 12$, the black hole-bulge mass relation at z = 0, galaxy morphology as a function of stellar mass and the mass–metallicity relations of both stellar and gas components. In addition, its detailed modelling of the radial structure of discs allows qualitatively new comparisons with observation, most notably with the relative sizes and masses of the stellar, atomic, and molecular components in discs. Good agreement is found with recent data. Comparison of results obtained for simulations differing in mass resolution by more than two orders of magnitude shows that all important distributions are numerically well converged even for this more detailed model. An examination of metallicity and surface-density gradients in the stars and gas indicates that our new model, with star formation, chemical enrichment, and feedback calculated self-consistently on local disc scales, reproduces some but not all of the trends seen in recent many-galaxy IFU surveys.
ABSTRACT
Radial metallicity trends provide a key indicator of physical processes such as star formation and radial gas migration within a galaxy. Large integral field unit surveys allow for detailed ...studies of these radial variations, with recent observations detecting central dips in the metallicity, which may trace the impact of various evolutionary processes. However, the origin of these dips has not been conclusively determined, with suggestions that they may be diagnostic dependent. In this paper, we use the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey to investigate whether the observed dips represent genuine decreases in the central metallicity, or whether they could be an artefact of the diagnostic used. Using a sub-sample of 758 local star-forming galaxies at low inclinations, we investigate in detail the impact of using different strong line diagnostics on the shapes of the returned profiles, and the prevalence of dips. We find no clear evidence of the dips being caused by changing values of the ionization parameter within galaxies. To investigate physical causes, we explore both global and spatially resolved parameters, finding that galaxies exhibiting central dips in the O3N2 metallicity profile have on average lower H α equivalent width values out to $R/R_{\rm {e}} \sim 1.5$, and higher values of DN(4000) in the central regions. We additionally find a higher prevalence of dips in galaxies with high stellar mass, and lower values of global specific star formation rate, suggesting a possible link to central quenching. Nevertheless, these results are dependent on the diagnostic used, suggesting caution should be taken when interpreting observed features in galaxy metallicity gradients.
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