Stellar Mass-metallicity Relation Research Articles

PGC 38025: A Star-forming Lenticular Galaxy with an Off-nuclear Star-forming Core

Abstract Lenticular galaxies (S0s) were considered mainly as passive evolved spirals due to environmental effects for a long time; however, most S0s in the field cannot fit into this common scenario. In this work, we study one special case, Sloan Digital Sky Survey J120237.07+642235.3 (PGC 38025), a star-forming field S0 galaxy with an off-nuclear blue core. We present optical integral field spectroscopic observation with the 3.5 meter telescope at Calar Alto Observatory, and high-resolution millimeter observation with the NOrthern Extended Millimeter Array. We estimated the star formation rate (SFR = 0.446 M ⊙ yr−1) and gaseous metallicity (12 + log(O/H) = 8.42) for PGC 38025, which follows the star formation main sequence and stellar mass–metallicity relation. We found that the ionized gas and cold molecular gas in PGC 38025 show the same spatial distribution and kinematics, while rotating misaligned with stellar component. The off-nuclear blue core is locating at the same redshift as PGC 38025 and its optical spectrum suggest it is H ii region. We suggest that the star formation in PGC 38025 is triggered by a gas-rich minor merger, and the off-nuclear blue core might be a local star formation happened during the accretion/merger process.

The weak imprint of environment on the stellar populations of galaxies

ABSTRACT We investigate the environmental dependence of the stellar populations of galaxies in Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Echoing earlier works, we find that satellites are both more metal-rich (<0.1 dex) and older (<2 Gyr) than centrals of the same stellar mass. However, after separating star-forming, green valley, and passive galaxies, we find that the true environmental dependence of both stellar metallicity (<0.03 dex) and age (<0.5 Gyr) is in fact much weaker. We show that the strong environmental effects found when galaxies are not differentiated result from a combination of selection effects brought about by the environmental dependence of the quenched fraction of galaxies, and thus we strongly advocate for the separation of star-forming, green valley, and passive galaxies when the environmental dependence of galaxy properties are investigated. We also study further environmental trends separately for both central and satellite galaxies. We find that star-forming galaxies show no environmental effects, neither for centrals nor for satellites. In contrast, the stellar metallicities of passive and green valley satellites increase weakly (<0.05 and <0.08 dex, respectively) with increasing halo mass, increasing local overdensity and decreasing projected distance from their central; this effect is interpreted in terms of moderate environmental starvation (‘strangulation’) contributing to the quenching of satellite galaxies. Finally, we find a unique feature in the stellar mass–stellar metallicity relation for passive centrals, where galaxies in more massive haloes have larger stellar mass (∼0.1 dex) at constant stellar metallicity; this effect is interpreted in terms of dry merging of passive central galaxies and/or progenitor bias.

The Mean Absorption-line Spectra of a Selection of Luminous z ∼ 6 Lyman Break Galaxies

Abstract We examine the absorption-line spectra of a sample of 31 luminous ( ) Lyman break galaxies at redshift z ≃ 6 using data taken with the FOCAS and OSIRIS spectrographs on the Subaru and GTC telescopes. For two of these sources we present longer exposure data taken at higher spectral resolutions from ESO’s X-shooter spectrograph. Using these data, we demonstrate the practicality of stacking our lower-resolution data to measure the depth of various interstellar and stellar absorption lines to probe the covering fraction of low-ionization gas and the gas phase and stellar metallicities near the end of the era of cosmic reionization. From maximum absorption-line depths of Si ii λ1260 and C ii λ1334, we infer a mean covering fraction of ≥0.85 ± 0.16 for our sample. This is larger than that determined using similar methods for lower-luminosity galaxies at slightly lower redshifts, suggesting that the most luminous galaxies appear to have a lower escape fraction than fainter galaxies, and therefore may not play a prominent role in concluding reionization. Using various interstellar absorption lines we deduce gas-phase metallicities close to solar, indicative of substantial early enrichment. Using selected stellar absorption lines, we model our spectra with a range of metallicities using techniques successfully employed at lower redshift and deduce a stellar metallicity of solar, consistent with the stellar mass—stellar metallicity relation recently found at z ∼ 3–5. We discuss the implications of these metallicity estimates for the typical ages of our luminous galaxies and conclude our results imply initial star formation at redshifts z ∼ 10, consistent with independent analyses of earlier objects.

High Molecular Gas Masses in Absorption-selected Galaxies at z ≈ 2

Abstract We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3−2) or (4−3) emission from the fields of 12 high-metallicity ([M/H] ≥ −0.72 dex) damped Lyα absorbers (DLAs) at z ≈ 1.7–2.6. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, . The impact parameters of the CO emitters to the QSO sight line lie in the range b ≈ 5.6–100 kpc, with the three new CO detections having . The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H] ≳ −0.3 dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass–metallicity relation and a high molecular gas-to-star mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[ii] 158 μm detection rate in high-z, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-z DLAs. The H i-absorption criterion identifying DLAs selects the entire high-z galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.

Evolution of dwarf galaxy observable parameters

ABSTRACT We present a semi-analytic model of isolated dwarf galaxy evolution and use it to study the build-up of observed correlations between dwarf galaxy properties. We analyse the evolution using models with averaged and individual halo mass assembly histories in order to determine the importance of stochasticity on the present-day properties of dwarf galaxies. The model has a few free parameters, but when these are calibrated using the halo mass–stellar mass and stellar mass–metallicity relations, the results agree with other observed dwarf galaxy properties remarkably well. Redshift evolution shows that even isolated galaxies change significantly over the Hubble time and that ‘fossil dwarf galaxies’ with properties equivalent to those of high-redshift analogues should be extremely rare, or non-existent, in the local Universe. A break in most galaxy property correlations develops over time, at a stellar mass $M_* \simeq 10^7 \, {\rm M_{\odot }}$. It is caused predominantly by the ionizing background radiation and can therefore in principle be used to constrain the properties of reionization.

The Relationship between Galaxy ISM and Circumgalactic Gas Metallicities

Abstract We present interstellar matter (ISM) and circumgalactic medium (CGM) metallicities for 25 absorption systems associated with isolated star-forming galaxies ( ) with 9.4 ≤ log(M */M ⊙) ≤ 10.9 and with absorption detected within (200 kpc). Galaxy ISM metallicities were measured using Hα/[N ii] emission lines from Keck/ESI spectra. CGM single-phase low-ionization metallicities were modeled using Markov Chain Monte Carlo and Cloudy analysis of absorption from HST/COS and Keck/HIRES or VLT/UVES quasar spectra. We find that the star-forming galaxy ISM metallicities follow the observed stellar mass–metallicity relation (1σ scatter 0.19 dex). CGM metallicity shows no dependence with stellar mass and exhibits a scatter of ∼2 dex. All CGM metallicities are lower than the galaxy ISM metallicities and are offset by log(dZ) = −1.17 ± 0.11. There is no obvious metallicity gradient as a function of impact parameter or virial radius (<2.3σ significance). There is no relationship between the relative CGM-galaxy metallicity and azimuthal angle. We find the mean metallicity differences along the major and minor axes are −1.13 ± 0.18 and −1.23 ± 0.11, respectively. Regardless of whether we examine our sample by low/high inclination or low/high impact parameter, or low/high N(H i), we do not find any significant relationship with relative CGM-galaxy metallicity and azimuthal angle. We find that 10/15 low column density systems (logN(H i) < 17.2) reside along the galaxy major axis while high column density systems (logN(H i) ≥ 17.2) reside along the minor axis. This suggests N(H i) could be a useful indicator of accretion/outflows. We conclude that CGM is not well mixed, given the range of galaxy-CGM metallicities, and that metallicity at low redshift might not be a good tracer of CGM processes. On the other hand, we should replace integrated line-of-sight, single-phase metallicities with multiphase, cloud–cloud metallicities, which could be more indicative of the physical processes within the CGM.

Evolution of the Stellar Mass–Metallicity Relation. II. Constraints on Galactic Outflows from the Mg Abundances of Quiescent Galaxies

Abstract We present the stellar mass–[Fe/H] and mass–[Mg/H] relation of quiescent galaxies in two galaxy clusters at z ∼ 0.39 and z ∼ 0.54. We derive the age, [Fe/H], and [Mg/Fe] for each individual galaxy using a full-spectrum fitting technique. By comparing with the relations for z ∼ 0 Sloan Digital Sky Survey galaxies, we confirm our previous finding that the mass–[Fe/H] relation evolves with redshift. The mass–[Fe/H] relation at higher redshift has lower normalization and possibly steeper slope. However, based on our sample, the mass–[Mg/H] relation does not evolve over the observed redshift range. We use a simple analytic chemical evolution model to constrain the average outflow that these galaxies experience over their lifetime, via the calculation of mass-loading factor. We find that the average mass-loading factor η is a power-law function of galaxy stellar mass, . The measured mass-loading factors are consistent with the results of other observational methods for outflow measurements and with the predictions where outflow is caused by star formation feedback in turbulent disks.

EDGE: the mass–metallicity relation as a critical test of galaxy formation physics

ABSTRACT We introduce the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low-mass halo ($M_{\rm halo}=10^{9}{\, \rm M}_\odot$), simulated to redshift z = 0 at a mass and spatial resolution of $\sim 20{\, \rm M}_\odot$ and ∼3 pc. We consider different star formation prescriptions, supernova feedback strengths, and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm (∼104 K) even before cosmic reionization. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, V-band magnitudes, and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass–metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass–metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.

Recalibration of [O ii]λ3727 as a Star Formation Rate Estimator for Active and Inactive Galaxies

We investigate the use of the [O ii] λ3727 emission line as a star formation rate (SFR) estimator using Sloan Digital Sky Spectra for nearly 100,000 star-forming galaxies and 5500 galaxies with narrow-line active galactic nuclei. Consistent with previous work, we find that the [O ii]/Hα ratio in star-forming galaxies depends strongly on gas-phase metallicity. Using metallicities derived from the [N ii] λ6584/[O ii] λ3727 method, we refine a metallicity-dependent SFR estimator based on [O ii] that is calibrated within a scatter of 0.056 dex against the more commonly used SFR indicator based on Hα emission. The scatter increases to only 0.12 dex if the metallicity is estimated using the stellar mass–metallicity relation. With the aim of extending the [O ii]-based SFR estimator to active galaxies, we calculate radiation pressure-dominated photoionization models to constrain the amount of [O ii] emission arising from the narrow-line region. We use the sample of active galaxies to demonstrate that the SFRs derived from [O ii], after accounting for nonstellar contamination, are consistent with independent SFR diagnostics estimated from the stellar continuum of the host galaxies.

Stellar Metallicities and Elemental Abundance Ratios of z ∼ 1.4 Massive Quiescent Galaxies*

Abstract The chemical composition of galaxies has been measured out to z ∼ 4. However, nearly all studies beyond z ∼ 0.7 are based on strong-line emission from H ii regions within star-forming galaxies. Measuring the chemical composition of distant quiescent galaxies is extremely challenging, as the required stellar absorption features are faint and shifted to near-infrared wavelengths. Here, we present ultradeep rest-frame optical spectra of five massive quiescent galaxies at z ∼ 1.4, all of which show numerous stellar absorption lines. We derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies; the remaining two galaxies have too young luminosity-weighted ages to yield robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears positively correlated with stellar mass, while [Fe/H] is approximately constant with mass. These results may imply that the stellar mass–metallicity relation was already in place at z ∼ 1.4. While the [Mg/Fe]−mass relation at z ∼ 1.4 is consistent with the z < 0.7 relation, [Fe/H] at z ∼ 1.4 is ∼0.2 dex lower than at z < 0.7. With a [Mg/Fe] of the most massive galaxy may be more α-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z = 2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low-mass, less α-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star formation timescales of 0.2–1.0 Gyr.

The Origin of the Stellar Mass–Stellar Metallicity Relation in the Milky Way Satellites and Beyond

Abstract Observations and semianalytical galaxy formation and evolution models (SAMs) have suggested the existence of a stellar mass–stellar metallicity relation (MZR), which is shown to be universal for different types of galaxies over a large range of stellar masses (M * ∼ 103–1011 ) and dark matter halo masses (M halo ∼ 109–1015 h −1 ). In this work, we construct a chemical evolution model to investigate the origin of the MZR, including both the effects of gas inflows and outflows in galaxies. We solve the MZR from the chemical evolution model by assuming that the cold gas mass (M cold) and the stellar feedback efficiency (β) follow some power-law scaling relationships with M * during the growth of a galaxy, i.e., and . We use the SAM to obtain these power-law scaling relations, which appear to be roughly universal over a large range of stellar masses for both satellites and central galaxies within a large range of halo masses. The range of MZRs produced by our models is in a narrow space, which provides support to the universality of the MZRs. The formation of the MZR is a result caused jointly by the cold gas fraction decreasing with increasing M * and by the stellar feedback efficiency decreasing with increasing M * in the galaxy growth, and the exponent in the MZR is around −α βs or 1 − α gs. The MZR represents an “average” evolutionary track for the stellar metallicity of a galaxy. The comparison of our model with some previous models for the origin of MZRs is also discussed.

The VANDELS survey: the stellar metallicities of star-forming galaxies at $\mathbf {2.5\,\, \lt\,\, z\,\, \lt\,\, 5.0}$

We present the results of a study utilising ultra-deep, rest-frame UV, spectroscopy to quantify the relationship between stellar mass and stellar metallicity for 681 star-forming galaxies at $2.5<z<5.0$ ($\langle z \rangle = 3.5 \pm 0.6$) drawn from the VANDELS survey. Via a comparison with high-resolution stellar population models, we determine stellar metallicities for a set of composite spectra formed from subsamples selected by mass and redshift. Across the stellar mass range $8.5 < \mathrm{log}(\langle M_{\ast} \rangle/\rm{M}_{\odot}) < 10.2$ we find a strong correlation between stellar metallicity and stellar mass, with stellar metallicity monotonically increasing from $Z_{\ast}/\mathrm{Z}_{\odot} < 0.09$ at $\langle M_{\ast} \rangle = 3.2 \times 10^{8} \rm{M}_{\odot}$ to $Z_{\ast}/Z_{\odot} = 0.27$ at $\langle M_{\ast} \rangle = 1.7 \times 10^{10} \rm{M}_{\odot}$. In contrast, at a given stellar mass, we find no evidence for significant metallicity evolution across the redshift range of our sample. However, comparing our results to the $z=0$ stellar mass-metallicity relation, we find that the $\langle z \rangle = 3.5$ relation is consistent with being shifted to lower metallicities by $\simeq 0.6$ dex. Contrasting our derived stellar metallicities with estimates of gas-phase metallicities at similar redshifts, we find evidence for enhanced $\rm{O}/\rm{Fe}$ ratios of the order (O/Fe) $\gtrsim 1.8$ $\times$ (O/Fe)$_{\odot}$. Finally, by comparing our results to simulation predictions, we find that the $\langle z \rangle = 3.5$ stellar mass-metallicity relation is consistent with current predictions for how outflow strength scales with galaxy mass. This conclusion is supported by an analysis of analytic models, and suggests that the mass loading parameter ($\eta=\dot{M}_{\mathrm{outflow}}/M_{\ast}$) scales as $\eta \propto M_{\ast}^{\beta}$ with $\beta \simeq -0.4$.

Is the Stellar Mass–Stellar Metallicity Relation Universal in the Milky Way Satellites and Beyond?

Abstract Observations reveal a universal stellar mass–stellar metallicity relation (MZR) existing in Local Group dwarfs of different types, Z * ∝ M * α with α = 0.30 ± 0.02. In this work, we investigate the “universality” of the MZRs for both satellites and central galaxies in a large number of different host dark matter halos covering a large mass range of 109–1015 h −1 M ⊙ , by using a semianalytical galaxy formation and evolution model. We obtain the following results: (1) The exponents (α) for the MZRs of the satellites in halos with the same mass as the Milky Way halo, but different individual assembly histories, are mostly ∼0.2–0.4, i.e., having a scatter of ∼0.2; and the scatter of α increases with decreasing halo masses. (2) The MZR relations are changed little by the variation of halo masses and the classification between central galaxies and satellites, if many halos with the same mass are stacked together. (3) A double power law exists in the MZR relations for both central galaxies and stacked satellites, with α ∼ 0.2–0.4 at 103 M ⊙ ≲ M * ≲ 108 M ⊙ and a relatively higher α ∼ 0.5 at 108 M ⊙ ≲ M * ≲ 1011 M ⊙ . (4) The high-mass satellites (M * ≳ 108 M ⊙ ) existing mostly in high-mass halos can lead to an apparent increase of α (from ∼0.2 to ∼0.4) with increasing host halo masses shown in the single-power-law fitting results of stacked satellites. The universality of the MZR suggests the common physical processes in stellar formation and chemical evolution of galaxies can be unified over a large range of galaxy masses and halo masses.

The SAMI galaxy survey: exploring the gas-phase mass–metallicity relation

We present a detailed exploration of the stellar mass vs. gas-phase metallicity relation (MZR) using integral field spectroscopy data obtained from ~1000 galaxies observed by the SAMI Galaxy survey. These spatially resolved spectroscopic data allow us to determine the metallicity within the same physical scale (Reff) for different calibrators. The shape of the MZ relations is very similar between the different calibrators, while there are large offsets in the absolute values of the abundances. We confirm our previous results derived using the spatially resolved data provided by the CALIFA and MaNGA surveys: (1) we do not find any significant secondary relation of the MZR with either the star formation rate (SFR) nor the specific SFR (SFR/Mass) for any of the calibrators used in this study, based on the analysis of the {individual} residuals, (2) if there is a dependence with the SFR, it is weaker than the reported one ($r_c\sim -$0.3), it is confined to the low mass regime (M*<10$^9$Msun) or high SFR regimes, and it does not produce any significant improvement in the {description of the average population of galaxies. The aparent disagreement with published results based on single fiber spectroscopic data could be due to (i) the interpretation of the secondary relation itself, (ii) the lower number of objects sampled at the low mass regime by the current study, or (iii) the presence of extreme star-forming galaxies that drive the secondary relation in previous results

Mass–Metallicity Relation and Fundamental Metallicity Relation of Metal-poor Star-forming Galaxies at 0.6 &lt; Z &lt; 0.9 from the eBOSS Survey

Abstract The stellar mass–metallicity relation (M ⋆–Z; MZR) indicates that the metallicities of galaxies increase with increasing stellar masses. The fundamental metallicity relation (FMR) suggests that galaxies with higher star formation rates (SFRs) tend to have lower metallicities for a given stellar mass. To examine whether the MZR and FMR still hold at poorer metallicities and higher redshifts, we compile a sample of 35 star-forming galaxies (SFGs) at 0.6 &lt; z &lt; 0.9 using the public spectral database (v5_10_0) of emission-line galaxies from the extended Baryon Oscillation Spectroscopic Survey. These galaxies are identified for their significant auroral [O iii]λ4363 emission line (S/N ≥ 3). With the electronic temperature metallicity calibration, we find nine SFGs that are extremely metal-poor galaxies with 12 + log(O/H) ≤ 7.69 (1/10 Z ⊙). The metallicity of the most metal-deficient galaxy is 7.35 ± 0.09 (about 1/20 Z ⊙). Compared to the SFGs with normal metallicities in the local and high-redshift universe, our metal-poor SFGs have more than 10 times higher SFRs at a fixed stellar mass. We create a new mass–SFR relation for these metal-poor galaxies at 0.6 &lt; z &lt; 0.9. Due to the higher SFRs and younger stellar ages, our metal-poor SFGs deviate from the MZR and FMR in the local universe toward lower metallicities, confirming the existence of FMR, as well as the cosmic evolution of MZR and FMR with redshift.

NIHAO XV: the environmental impact of the host galaxy on galactic satellite and field dwarf galaxies

We study the impact of the host on dwarf galaxy properties using four new Milky Way-like, ultra high-resolution simulations, ($N_{\rm part} >10^7$) from the NIHAO project. We split our sample in satellite ($R<R_{200}$), nearby ($1<R/R_{200}<2.5$), and field ($R>2.5 R_{\rm 200}$) galaxies. Simulated galaxies from all three groups are in excellent agreement with Local Group dwarf galaxies in terms of: stellar mass-velocity dispersion, stellar mass-metallicity relation, star formation histories, and stellar mass functions. Satellites and nearby galaxies show lower velocity dispersions and gas fractions compared to field galaxies. While field galaxies follow global abundance matching relations, satellites and nearby galaxies deviate from them, showing lower dark matter masses for given stellar mass. The reason for this deficit in dark matter mass is substantial mass loss experienced by satellites and $\sim80$\% of the nearby galaxies, while orbiting inside $R_{200}$ at earlier times. However, both satellites and nearby objects fall back onto the relation for field galaxies if we use the maximum of their virial mass instead of the present-day value. This allows us to provide estimates for the peak masses of observed Local Group galaxies. Finally, using radial velocities, distances, and the velocity dispersion-stellar mass relation from our simulations, we derive a metric to distinguish between galaxies harassed by the central object and unaffected ones. Applying this metric to observed objects we find that even far away dwarf galaxies like Eri II ($D\approx$ 370 kpc) have a strong probability ($\approx 83$\%) of having been affected by the Milky Way in the past. This naturally explains the lack of gas and recent star formation seen in Eri II.

A Study of Two Diffuse Dwarf Galaxies in the Field

Abstract We present optical long-slit spectroscopy and far-ultraviolet to mid-infrared spectral energy distribution fitting of two diffuse dwarf galaxies, LSBG-285 and LSBG-750, which were recently discovered by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We measure redshifts using Hα line emission and find that these galaxies are at comoving distances of ≈25 and ≈41 Mpc, respectively, after correcting for the local velocity field. They have effective radii of r eff = 1.2 and 1.8 kpc and stellar masses of M ⋆ ≈ (2–3) × 107 M ⊙. There are no massive galaxies ( ) within a comoving separation of at least 1.5 Mpc from LSBG-285 and 2 Mpc from LSBG-750. These sources are similar in size and surface brightness to ultradiffuse galaxies, except they are isolated, star-forming objects that were optically selected in an environmentally blind survey. Both galaxies likely have low stellar metallicities [Z ⋆/Z ⊙] &lt; −1.0 and are consistent with the stellar mass–metallicity relation for dwarf galaxies. We set an upper limit on LSBG-750's rotational velocity of ∼50 km s−1, which is comparable to dwarf galaxies of similar stellar mass with estimated halo masses &lt;1011 M ⊙. We find tentative evidence that the gas-phase metallicities in both of these diffuse systems are high for their stellar mass, though a statistically complete, optically selected galaxy sample at very low surface brightness will be necessary to place these results into context with the higher surface brightness galaxy population.

Shark: introducing an open source, free, and flexible semi-analytic model of galaxy formation

We present a new, open source, free semi-analytic model (SAM) of galaxy formation, Shark, designed to be highly flexible and modular, allowing easy exploration of different physical processes and ways of modelling them. We introduce the philosophy behind Shark and provide an overview of the physical processes included in the model. Shark is written in C++11 and has been parallelized with OpenMP. In the released version (v1.1), we implement several different models for gas cooling, active galactic nuclei, stellar and photo-ionisation feedback, and star formation (SF). We demonstrate the basic performance of Shark using the Planck15 cosmology SURFS simulations, by comparing against a large set of observations, including: the stellar mass function (SMF) and stellar-halo mass relation at z=0-4; the cosmic evolution of the star formation rate density (SFRD), stellar mass, atomic and molecular hydrogen; local gas scaling relations; and structural galaxy properties, finding excellent agreement. Significant improvements over previous SAMs are seen in the mass-size relation for disks/bulges, the gas-stellar mass and stellar mass-metallicity relations. To illustrate the power of Shark in exploring the systematic effects of the galaxy formation modelling, we quantify how the scatter of the SF main sequence and the gas scaling relations changes with the adopted SF law, and the effect of the starbursts H$_2$ depletion timescale on the SFRD and $\Omega_{\rm H_2}$. We compare Shark with other SAMs and the hydrodynamical simulation EAGLE, and find that SAMs have a much higher halo baryon fractions due to large amounts of intra-halo gas, which in the case of EAGLE is in the intergalactic medium.

Low Metallicities and Old Ages for Three Ultra-diffuse Galaxies in the Coma Cluster

Abstract A large population of ultra-diffuse galaxies (UDGs) was recently discovered in the Coma cluster. Here we present optical spectra of three such UDGs, DF 7, DF 44, and DF 17, which have central surface brightnesses of μ g ≈ 24.4–25.1 mag arcsec−2. The spectra were acquired as part of an ancillary program within the SDSS-IV MaNGA Survey. We stacked 19 fibers in the central regions from larger integral field units (IFUs) per source. With over 13.5 hr of on-source integration, we achieved a mean signal-to-noise ratio in the optical of 9.5 Å−1, 7.9 Å−1, and 5.0 Å−1, respectively, for DF 7, DF 44, and DF 17. Stellar population models applied to these spectra enable measurements of recession velocities, ages, and metallicities. The recession velocities of DF 7, DF 44, and DF 17 are km s−1, km s−1, and km s−1, spectroscopically confirming that all of them reside in the Coma cluster. The stellar populations of these three galaxies are old and metal-poor, with ages of Gyr, Gyr, and Gyr, and iron abundances of [Fe/H] , , and , respectively. Their stellar masses are (3–6) × 108 M ⊙. The UDGs in our sample are as old or older than galaxies at similar stellar mass or velocity dispersion (only DF 44 has an independently measured dispersion). They all follow the well-established stellar mass–stellar metallicity relation, while DF 44 lies below the velocity dispersion-metallicity relation. These results, combined with the fact that UDGs are unusually large for their stellar masses, suggest that stellar mass plays a more important role in setting stellar population properties for these galaxies than either size or surface brightness.

The Chemical Evolution of Galaxies: The Stellar Mass–Metallicity Relation and Cosmic Downsizing

The Chemical Evolution of Galaxies: The Stellar Mass–Metallicity Relation and Cosmic Downsizing