Stellar Mass-metallicity Relation Research Articles

A Large Sample of Extremely Metal-poor Galaxies at z < 1 Identified from the DESI Early Data

Extremely metal-poor galaxies (XMPGs) at relatively low redshift are excellent laboratories for studying galaxy formation and evolution in the early universe. Much effort has been spent on identifying them from large-scale spectroscopic surveys or spectroscopic follow-up observations. Previous work has identified a few hundred XMPGs. In this work, we obtain a large sample of 223 XMPGs at z < 1 from the early data of the Dark Energy Spectroscopic Instrument (DESI). The oxygen abundance is determined using the direct T e method based on the detection of the [O iii]λ4363 line. The sample includes 95 confirmed XMPGs based on the oxygen abundance uncertainty; the remaining 128 galaxies are regarded as XMPG candidates. These XMPGs are only 0.01% of the total DESI observed galaxies. Their coordinates and other properties are provided in the paper. The most XMPGs have an oxygen abundance of ∼1/34 Z ⊙, a stellar mass of about 1.5 × 107 M ⊙, and a star formation rate of 0.22 M ⊙ yr−1. The two most XMPGs present distinct morphologies suggesting different formation mechanisms. The local environmental investigation shows that XMPGs preferentially reside in relatively low-density regions. Many of them fall below the stellar mass–metallicity relations (MZRs) of normal star-forming galaxies. From a comparison of the MZR with theoretical simulations, it appears that XMPGs are good analogs to high-redshift star-forming galaxies. The nature of these XMPG populations will be further investigated in detail with larger and more complete samples from the ongoing DESI survey.

Chemical evolution of local post-starburst galaxies: implications for the mass–metallicity relation

ABSTRACT We use the stellar fossil record to constrain the stellar metallicity evolution and star-formation histories of the post-starburst (PSB) regions within 45 local PSB galaxies from the MaNGA survey. The direct measurement of the regions’ stellar metallicity evolution is achieved by a new two-step metallicity model that allows for stellar metallicity to change at the peak of the starburst. We also employ a Gaussian process noise model that accounts for correlated errors introduced by the observational data reduction or inaccuracies in the models. We find that a majority of PSB regions (69 per cent at &amp;gt;1σ significance) increased in stellar metallicity during the recent starburst, with an average increase of 0.8 dex and a standard deviation of 0.4 dex. A much smaller fraction of PSBs are found to have remained constant (22 per cent) or declined in metallicity (9 per cent, average decrease 0.4 dex, standard deviation 0.3 dex). The pre-burst metallicities of the PSB galaxies are in good agreement with the mass–metallicity (MZ) relation of local star-forming galaxies. These results are consistent with hydrodynamic simulations, which suggest that mergers between gas-rich galaxies are the primary formation mechanism of local PSBs, and rapid metal recycling during the starburst outweighs the impact of dilution by any gas inflows. The final mass-weighted metallicities of the PSB galaxies are consistent with the MZ relation of local passive galaxies. Our results suggest that rapid quenching following a merger-driven starburst is entirely consistent with the observed gap between the stellar mass–metallicity relations of local star-forming and passive galaxies.

LATIS: The Stellar Mass–Metallicity Relation of Star-forming Galaxies at z ∼ 2.5

We present the stellar mass–stellar metallicity relation for 3491 star-forming galaxies at 2 ≲ z ≲ 3 using rest-frame far-ultraviolet spectra from the Lyα Tomography IMACS Survey (LATIS). We fit stellar population synthesis models from the Binary Population And Spectral Synthesis code (v2.2.1) to medium-resolution (R ∼ 1000) and high signal-to-noise (>30 per 100 km s−1 over the wavelength range 1221–1800 Å) composite spectra of galaxies in bins of stellar mass to determine their stellar metallicity, primarily tracing Fe/H. We find a strong correlation between stellar mass and stellar metallicity, with stellar metallicity monotonically increasing with stellar mass at low masses and flattening at high masses (M * ≳ 1010.3 M ⊙). Additionally, we compare our stellar metallicity measurements with the gas-phase oxygen abundance of galaxies at similar redshift and estimate the average [α/Fe] ∼ 0.6. Such high α-enhancement indicates that high-redshift galaxies have not yet undergone significant iron enrichment through Type Ia supernovae. Moreover, we utilize an analytic chemical evolution model to constrain the mass loading parameter of galactic winds as a function of stellar mass. We find that as the stellar mass increases, the mass loading parameter decreases. The parameter then flattens or reaches a turning point at around M * ∼ 1010.5 M ⊙. Our findings may signal the onset of black-hole-driven outflows at z ∼ 2.5 for galaxies with M * ≳ 1010.5 M ⊙.

The star formation histories of quiescent ultra-diffuse galaxies and their dependence on environment and globular cluster richness

ABSTRACT We derive the stellar population parameters of 11 quiescent ultra-diffuse galaxies (UDGs) from Keck/KCWI data. We supplement these with 14 literature UDGs, creating the largest spectroscopic sample of UDGs to date (25). We find a strong relationship between their α-enhancement and their star formation histories: UDGs that formed on very short time-scales have elevated [Mg/Fe] abundance ratios, whereas those forming over extended periods present lower values. Those forming earlier and faster are overall found in high-density environments, being mostly early infalls into the cluster. No other strong trends are found with infall times. We analyse the stellar mass–metallicity, age–metallicity, and [Mg/Fe]–metallicity relations of the UDGs, comparing them to other types of low mass galaxies. Overall, UDGs scatter around the established stellar mass–metallicity relations of classical dwarfs. We find that GC-rich UDGs have intermediate-to-old ages, but previously reported trends of galaxy metallicity and GC richness are not reproduced with this spectroscopic sample due to the existence of GC-rich UDGs with elevated metallicities. In addition, we also find that a small fraction of UDGs could be ‘failed-galaxies’, supported by their GC richness, high alpha-abundance, fast formation time-scales and that they follow the mass–metallicity relation of z ∼2 galaxies. Finally, we also compare our observations to simulated UDGs. We caution that there is not a single simulation that can produce the diverse UDG properties simultaneously, in particular the low metallicity failed galaxy like UDGs.

Radial velocities and stellar population properties of 56 MATLAS dwarf galaxies observed with MUSE

Dwarf galaxies have been extensively studied in the Local Group, in nearby groups, and selected clusters, giving us a robust picture of their global stellar and dynamical properties, such as their circular velocity, stellar mass, surface brightness, age, and metallicity in particular locations in the Universe. Intense study of these properties has revealed correlations between them, called the scaling relations, including the well-known universal stellar mass-metallicity relation. However, since dwarfs play a role in a vast range of different environments, much can be learned about galaxy formation and evolution through extending the study of these objects to various locations. We present MUSE spectroscopy of a sample of 56 dwarf galaxies as a follow-up to the MATLAS survey in low- to moderate-density environments beyond the Local Volume. The dwarfs have stellar masses in the range of M*/M⊙ = 106.1–109.4 and show a distance range of D = 14–148 Mpc, the majority of which (75%) are located in the range targeted by the MATLAS survey (10–45 Mpc). We thus report a 75% success rate for the semi-automatic identification of dwarf galaxies (79% for dwarf ellipticals) in the MATLAS survey on the subsample presented here. Using pPXF full spectrum fitting, we determine their line-of-sight velocity and can match the majority of them with their massive host galaxy. Due to the observational setup of the MATLAS survey, the dwarfs are located in the vicinity of massive galaxies. Therefore, we are able to confirm their association through recessional velocity measurements. Close inspection of their spectra reveals that ∼30% show clear emission lines, and thus star formation activity. We estimate their stellar population properties (age and metallicity) and compare our results with other works investigating Local Volume and cluster dwarf galaxies. We find that the dwarf galaxies presented in this work show a systematic offset from the universal stellar mass-metallicity relation toward lower metallicities at the same stellar mass. A similar deviation is present in other works in the stellar mass range probed in this work and might be attributed to the use of different methodologies for deriving the metallicity.

The star formation history and the nature of the mass–metallicity relation of passive galaxies at 1.0 &amp;lt;z&amp;lt; 1.4 from VANDELS

ABSTRACTWe derived stellar ages and metallicities [Z/H] for ∼70 passive early-type galaxies (ETGs) selected from VANDELS survey over the redshift range 1.0 &amp;lt; z &amp;lt; 1.4 and stellar mass range 10 &amp;lt; log(M*/M⊙) &amp;lt; 11.6. We find significant systematics in their estimates depending on models and wavelength ranges considered. Using the full-spectrum fitting technique, we find that both [Z/H] and age increase with mass as for local ETGs. Age and metallicity sensitive spectral indices independently confirm these trends. According to EMILES models, for 67 per cent of the galaxies we find [Z/H] &amp;gt; 0.0, a percentage which rises to ∼90 per cent for log(M*/M⊙) &amp;gt; 11 where the mean metallicity is [Z/H] = 0.17 ± 0.1. A comparison with homogeneous measurements at similar and lower redshift does not show any metallicity evolution over the redshift range 0.0 &amp;lt; z &amp;lt; 1.4. The derived star formation (SF) histories show that the stellar mass fraction formed at early epoch increases with the mass of the galaxy. Galaxies with log(M*/M⊙) &amp;gt; 11.0 host stellar populations with [Z/H] &amp;gt; 0.05, formed over short time-scales (Δt50 &amp;lt; 1 Gyr) at early epochs (tform &amp;lt; 2 Gyr), implying high star formation rates (SFR &amp;gt; 100 M⊙ yr−1) in high-mass density regions (log(Σ1kpc) &amp;gt; 10 M⊙/kpc2). This sharp picture tends to blur at lower masses: log(M*/M⊙) ∼ 10.6 galaxies can host either old stars with [Z/H] &amp;lt; 0.0 or younger stars with [Z/H] &amp;gt; 0.0, depending on the duration (Δt50) of the SF. The relations between galaxy mass, age, and metallicities are therefore largely set up ab initio as part of the galaxy formation process. Mass, SFR, and SF time-scale all contribute to shape up the stellar mass–metallicity relation with the mass that modulates metals retention.

Nature of the galaxies on top of quasars producing Mg ii absorption

ABSTRACT Quasar–galaxy pairs at small separations are important probes of gas flows in the disc–halo interface in galaxies. We study host galaxies of 198 Mg ii absorbers at 0.39 ≤ zabs ≤ 1.05 that show detectable nebular emission lines in the Sloan Digital Sky Survey (SDSS) spectra. We report measurements of impact parameter (5.9 ≤ D [kpc] ≤ 16.9) and absolute B-band magnitude (−18.7 ≤ MB ≤ −22.3 mag) of host galaxies of 74 of these absorbers using multiband images from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey, more than doubling the number of known host galaxies with D ≤ 17 kpc. This has allowed us to quantify the relationship between Mg ii rest equivalent width (W2796) and D, with best-fitting parameters of W2796 (D = 0) = 3.44 ± 0.20 Å and an exponential scale length of 21.6$^{+2.41}_{-1.97}\, \mathrm{ kpc}$. We find a significant anticorrelation between MB and D, and MB and W2796, consistent with the brighter galaxies producing stronger Mg ii absorption. We use stacked images to detect average emissions from galaxies in the full sample. Using these images and stacked spectra, we derive the mean stellar mass (9.4 ≤ log(M*/M⊙) ≤ 9.8), star formation rate (2.3 ≤ SFR [M⊙ yr−1] ≤ 4.5), age (2.5–4 Gyr), metallicity (12 + log(O/H) ∼ 8.3), and ionization parameter (log q [cm s−1] ∼ 7.7) for these galaxies. The average M* found is less than that of Mg ii absorbers studied in the literature. The average SFR and metallicity inferred are consistent with that expected in the main sequence and the known stellar mass–metallicity relation, respectively. High spatial resolution follow-up spectroscopic and imaging observations of this sample are imperative for probing gas flows close to the star-forming regions of high-z galaxies.

First Light And Reionization Epoch Simulations (FLARES) VII: The star formation and metal enrichment histories of galaxies in the early Universe

ABSTRACT The star formation and metal enrichment histories of galaxies – at any epoch – constitute one of the key properties of galaxies, and their measurement is a core aim of observational extragalactic astronomy. The lack of deep rest-frame optical coverage at high redshift has made robust constraints elusive, but this is now changing thanks to JWST. In preparation for the constraints provided by JWST, we explore the star formation and metal enrichment histories of galaxies at z = 5–13 using the First Light And Reionization Epoch Simulations (flares) suite. Built on the EAGLE model, the unique strategy of flares allows us to simulate galaxies with a wide range of stellar masses (and luminosities) and environments. While we predict significant redshift evolution of average ages and specific star formation rates, our core result is mostly a flat relationship of age and specific star formation rate with stellar mass. We also find that galaxies in this epoch predominantly have strongly rising star formation histories, albeit with the normalization dropping with redshift and stellar mass. In terms of chemical enrichment, we predict a strong stellar mass–metallicity relation present at z = 10 and beyond alongside significant α-enhancement. Finally, we find no large-scale environmental dependence of the relationship between age, specific star formation rate, or metallicity with stellar mass.

Mass–metallicity and star formation rate in galaxies: A complex relation tuned to stellar age

Context. In this work we study the stellar mass–metallicity relation (MZR) of an extended sample of star-forming galaxies in the local Universe and its possible dependence on the star formation rate (SFR). Aims. We selected a sample of approximately 195 000 Sloan Digital Sky Survey (SDSS) star-forming galaxies up to z = 0.22 with the aim of analysing the behaviour of the MZR with respect to SFR whilst taking into account the age of their stellar populations. Methods. For the first time, with this sample, we obtained aperture corrected oxygen and nitrogen-to-oxygen abundances (O/H and N/O, respectively) and SFR using the empirical prescriptions from the Calar Alto Legacy Integral Field Area (CALIFA) survey. To perform this study we also make use of the stellar mass of the galaxies and the parameter Dn(4000) as a proxy for the age of the stellar population. Results. We derive a robust MZR locus, which is found to be fully consistent with the ‘anchoring’ points of a selected set of well-studied nearby galaxies for which the chemical abundance has been derived using the direct method. We observe a complex relation between MZR and SFR across the whole range of galaxy mass and metallicity, where the slope changes seen in the O/H–SFR plane present a pattern that seems to be tuned to the stellar age of the galaxies, and therefore stellar age has to be taken into account in the stellar mass–metallicity–SFR relation. Conclusions. In order to provide an answer to the question of whether or not the MZR depends on the SFR, it is essential to take into account the age of the stellar populations of galaxies. A strong dependence of the MZR on SFR is observed mainly for star-forming galaxies with strong SFR values and low Dn(4000). The youngest galaxies of our SDSS sample show the highest SFR measured for their stellar mass.

Exploring metallicity-dependent rates of Type Ia supernovae and their impact on galaxy formation

ABSTRACT Type Ia supernovae are critical for feedback and elemental enrichment in galaxies. Recent surveys like the All-Sky Automated Survey for Supernova (ASAS-SN) and the Dark Energy Survey (DES) find that the specific supernova Ia rate at z ∼ 0 may be ≲20–50× higher in lower mass galaxies than at Milky Way-mass. Independently, observations show that the close-binary fraction of solar-type Milky Way stars is higher at lower metallicity. Motivated by these observations, we use the FIRE-2 cosmological zoom-in simulations to explore the impact of metallicity-dependent rate models on galaxies of $M_* \sim 10^7\!-\!10^{11}\, \rm {M}_{\odot }$. First, we benchmark our simulated star formation histories against observations, and show that the assumed stellar mass functions play a major role in determining the degree of tension between observations and metallicity-independent rate models, potentially causing ASAS-SN and DES observations to agree more than might appear. Models in which the supernova Ia rate increases with decreasing metallicity ($\propto Z^{-0.5 \,\, \rm {to} \,\, -1}$) provide significantly better agreement with observations. Encouragingly, these rate increases (≳10× in low-mass galaxies) do not significantly impact galaxy masses and morphologies, which remain largely unaffected except for our most extreme models. We explore implications for both [Fe/H] and [$\alpha /\rm {Fe}$] enrichment; metallicity-dependent rate models can improve agreement with the observed stellar mass–metallicity relations in low-mass galaxies. Our results demonstrate that a range of metallicity-dependent rate models are viable for galaxy formation and motivate future work.

The environmental dependence of the stellar and gas-phase mass–metallicity relation at 2 &lt; z &lt; 4

In the local universe, galaxies in clusters typically show different physical and chemical properties compared to more isolated systems. Understanding how this difference originates, and whether it is already in place at high redshift, is still a matter of debate. Thanks to uniquely deep optical spectra available from the VANDELS survey, we investigate environmental effects on the stellar mass–metallicity relation (MZR) for a sample of nearly 1000 star-forming galaxies in the redshift range 2 &lt; z &lt; 4. We complement our dataset with the MOSFIRE follow-up of 21 galaxies to study the environmental dependence of the gas-phase MZR. Robust stellar and gas-phase metallicities are derived from well-calibrated photospheric absorptions features, respectively at 1501 and 1719 Å in the stacked spectra, and from optical emission lines ([OII]λλ3726−3729, [OIII]λ5007, and Hβ) in individual systems. We characterize the environment through multiple criteria by using the local galaxy density maps derived in the VANDELS fields to identify overdense structures and protoclusters of varying sizes. We find that environmental effects are weak at redshifts 2 &lt; z &lt; 4, and they are more important around the densest overdensity structures and protoclusters, where galaxies have a lower stellar metallicity (by ∼0.2 dex) and a lower gas-phase metallicity (by 0.1 dex) compared to the field, with a significance of 1σ and 2σ, respectively. Crucially, this downward offset cannot be explained by a selection effect due to a higher star formation rate, a fainter UV continuum, or different dust attenuations and stellar ages for galaxies in overdense enviroments with respect to the field. In spite of the still low signal-to-noise ratio of our results, we consider possible explanations of this environmental dependence. We propose a combination of increased mergers and high-speed encounters, more efficient AGN feedback in dense cores, and cold gas inflows from the cosmic web as viable physical mechanisms diluting the metal content of the cold gas reservoirs of overdense galaxies or expelling their metals to the intergalactic medium, even though additional studies are needed to determine the most significant scenario. Finally, some tensions remain between observations and both semi-analytic models and hydrodynamical simulations, which predict no significant metallicity offset as a function of host halo mass, suggesting that an explicit implementation of environmental processes in dense protocluster cores is needed.

Chemical Evolution History of MaNGA Galaxies

We show the results of a study using the spectral synthesis technique study for the full MaNGA sample showing their chemical enrichment history (ChEH) as well as the evolution of the stellar mass–metallicity relation (MZR) over cosmic time. We find that the more massive galaxies became enriched first and the lower-mass galaxies did so later, producing a change in the MZR that becomes shallower in time. Separating the sample into morphology and star-forming status bins, some particularly interesting results appear: The mass dependence of the MZR becomes less relevant for later morphological types, to the extent that it inverts for Sd/Irr galaxies, suggesting that morphology is at least as important a factor as mass in the chemical evolution. The MZR for the full sample shows a flattening at the high-mass end and another in the low-mass range, but the former only appears for retired galaxies, while the latter only appears for star-forming galaxies. We also find that the average metallicity gradient is currently negative for all mass bins, but for low-mass galaxies, it was inverted at some point in the past, before which all galaxies had a positive gradient. We also compare how diverse the ChEHs are in the different bins we considered, as well as what primarily drives the diversity: By how much galaxies become enriched, or how quickly they do so.

Asymmetric Star Formation Triggered by Gas Inflow in a Barred Lenticular Galaxy PGC 34107

Abstract Comparing the inactive and gas-poor normal lenticular galaxies (S0s) in the local universe, we study a barred star-forming S0 galaxy, PGC 34107, which has been observed by the Centro Astronómico Hispano Alemán 3.5 m telescope and the Northern Extended Millimeter Array. The spatially resolved ionized gas and molecular gas traced by 12CO(1–0), hereafter CO(1–0), show similar distribution and kinematics to the stellar component with an off-center star-forming region, ∼380 pc away from the center. The resolved kinematics of molecular CO(1–0) emission reveals that there is a blueshifted (redshifted) velocity component on the receding (approaching) side of the galaxy along the stellar bar. This might provide plausible evidence of noncircular motion, such as the bar-induced molecular gas inflow. The velocity of the molecular gas inflow decreases when approaching toward the peak of the off-center star formation in the north, which might be associated with the inner Lindblad resonance. In addition to CO(1–0), we also detect the isotopic line of 13CO(1–0). For most Hα, CO(1–0) and 13CO(1–0) emissions are concentrated on this northern star-forming region. We find that PGC 34107 follows the local stellar mass–metallicity relation, star-forming main sequence, and the Kennicutt–Schmidt law. The resolved and integrated molecular gas main sequence suggests that there is a higher gas fraction in the central region of the galaxy, which supports a scenario that the bar-induced gas reservoir provides the raw material, and subsequently triggers the central star formation.

The VANDELS survey: Global properties of CIII]λ1908 Å emitting star-forming galaxies at z ∼ 3

Context. Strong nebular emission is ubiquitous in galaxies that contribute to cosmic reionization at redshift z ≳ 6. High-ionization UV metal lines, such as CIII]λ1908 Å, show high equivalent widths (EW) in these early galaxies, suggesting harder radiation fields at low metallicity than low-z galaxies of similar stellar mass. Understanding the physical properties driving the observed UV nebular line emission at high-z requires large and very deep spectroscopic surveys, which are now only accessible out to z ∼ 4. Aims. We study the mean properties of a large representative sample of 217 galaxies showing CIII] emission at 2 &lt; z &lt; 4, selected from a parent sample of ∼750 main-sequence star-forming galaxies in the VANDELS survey. These CIII] emitters have a broad range of UV luminosities, allowing for a detailed stacking analysis to characterize their stellar mass, star formation rate (SFR), and metallicity as a function of the UV emission line ratios, EWs, and the carbon-to-oxygen (C/O) abundance ratio. Methods. Stacking provides unprecedented high signal-to-noise (S/N) spectra for CIII] emitters over more than three decades in luminosity, stellar mass, and SFR. This enables a full spectral fitting to derive stellar metallicities for each stack. Moreover, we use diagnostics based on photoionization models and UV line ratios to constrain the ionization sources of the galaxies and derive the C/O abundance. Results. Reliable CIII] detections (S/N ≥ 3) represent ∼30% of the parent sample. However, stacked spectra of non-detections (S/N &lt; 3) show weak (EW ≲ 2 Å) CIII] emission, suggesting that this line is common in normal star-forming galaxies at z ∼ 3. On the other hand, extreme CIII] emitters (EW(CIII]) ≳ 8 Å) are exceedingly rare (∼3%) in VANDELS. The UV line ratios of the sample suggest no ionization source other than massive stars. Stacks with larger EW(CIII]) show larger EW(Lyα) and lower metallicity, but not all CIII] emitters are Lyα emitters. The stellar metallicities of CIII] emitters are not significantly different from that of the parent sample, increasing from ∼10% to ∼40% solar for stellar masses log(M⋆/M⊙) ∼ 9−10.5. The stellar mass-metallicity relation of the CIII] emitters is consistent with previous works, exhibiting a strong evolution from z = 0 to z ∼ 3. The C/O abundances of the sample range between 35%−150% solar, with a noticeable increase with FUV luminosity and a smooth decrease with the CIII] EW. Here, we discuss the CIII] emitters in the C/O–Fe/H and the C/O–O/H planes and we find that they follow stellar and nebular abundance trends consistent with those of Milky Way halo and thick-disk stars and local HII galaxies, respectively. A qualitative agreement is also found with chemical evolution models, which suggests that CIII] emitters at z ∼ 3 are experiencing an active phase of chemical enrichment. Conclusions. Our results provide new insights into the nature of UV line emitters at z ∼ 2 − 4, paving the way for future studies at higher z using the James Webb Space Telescope.

The Stellar Mass versus Stellar Metallicity Relation of Star-forming Galaxies at 1.6 ≤ z ≤ 3.0 and Implications for the Evolution of the α-enhancement

Abstract We measure the relationship between stellar mass and stellar metallicity for 1336 star-forming galaxies at 1.6 ≤ z ≤ 3.0 using rest-frame far-ultraviolet spectra from the zCOSMOS-deep survey. High signal-to-noise ratio composite spectra containing stellar absorption features are fit with stellar population synthesis model spectra of a range of stellar metallicity. We find stellar metallicities, which mostly reflect instantaneous iron abundances, scaling as [ Fe / H ] = − ( 0.81 ± 0.01 ) + ( 0.32 + 0.03 ) log ( M * / 10 10 M ⊙ ) across the stellar mass range of 109 ≲ M */M ⊙ ≲ 1011. The instantaneous oxygen-to-iron ratio (α-enhancement) inferred using the gas-phase mass–metallicity relation is on average found to be O / Fe ≈ 0.47 , being higher than the local O / Fe ≈ 0 . The observed changes in [O/Fe] and [Fe/H] are reproduced in simple gas-regulator models with steady star formation histories. Our models show that the [O/Fe] is determined almost entirely by the instantaneous specific star formation rate alone while being independent of the mass and the characteristic of the gas regulation. We also find that the locations of ∼ 1010 M ⊙ galaxies at z ∼ 2 in the [O/Fe]–metallicity planes are in remarkable agreement with the sequence of low-metallicity thick-disk stars in our own Galaxy. This manifests a beautiful concordance between the results of Galactic archeology and observations of high-redshift Milky Way progenitors. There remains, however, a question of how and when the old metal-rich, low α/Fe stars seen in the bulge had formed by z ∼ 2 because such a stellar population is not seen in our data and is difficult to explain in the context of our models.

L-Galaxies 2020: the formation and chemical evolution of stellar haloes in Milky Way analogues and galaxy clusters

ABSTRACT We present an analysis of the formation and chemical evolution of stellar haloes around (a) Milky Way analogue (MWA) galaxies and (b) galaxy clusters in the L-Galaxies 2020 semi-analytic model of galaxy evolution. Observed stellar halo properties are better reproduced when assuming a gradual stripping model for the removal of cold gas and stars from satellites, compared to an instantaneous stripping model. The slope of the stellar mass–metallicity relation for MWA stellar haloes is in good agreement with that observed in the local Universe . This extends the good agreement between L-Galaxies 2020 and metallicity observations from the gas and stars inside galaxies to those outside. Halo stars contribute on average only ∼0.1 per cent of the total circumgalactic medium enrichment by z = 0 in MWAs, ejecting predominantly carbon produced by asymptotic giant branch stars. Around a quarter of MWAs have a single ‘significant progenitor’ with a mean mass of ∼$2.3\times {}10^{9}\, \rm {M}_{\odot }$, similar to that measured for Gaia Enceladus. For galaxy clusters, L-Galaxies 2020 shows good correspondence with observations of stellar halo mass fractions, but slightly overpredicts stellar masses. Assuming a Navarro–Frenk–White profile for the stellar halo mass distribution provides the best agreement. On average, the intracluster stellar component is responsible for 5.4 per cent of the total intracluster medium iron enrichment, exceeding the contribution from the brightest cluster galaxy by z = 0. We find that considering gradual stripping of satellites and realistic radial profiles is crucial for accurately modelling stellar halo formation on all scales in semi-analytic models.

NGC 147 Corroborates the Break in the Stellar Mass–Stellar Metallicity Relation for Galaxies

Abstract The stellar mass–stellar metallicity relation (MZR) is an essential approach to probing the chemical evolution of galaxies. It reflects the balance between galactic feedback and gravitational potential as a function of stellar mass. However, the current MZR of local dwarf satellite galaxies (M * ≲ 108 M ⊙; measured from resolved stellar spectroscopy) may not be reconcilable with that of more massive galaxies (M * ≳ 109.5 M ⊙; measured from integrated-light spectroscopy). Such a discrepancy may result from a systematic difference between the two methods, or it may indicate a break in the MZR around 109 M ⊙. To address this question, we measured the stellar metallicity of NGC 147 from integrated light using the Palomar Cosmic Web Imager. We compared the stellar metallicity estimates from integrated light with measurements from resolved stellar spectroscopy and found them to be consistent within 0.1 dex. On the other hand, the high-mass MZR overpredicts the metallicity by 0.6 dex at the mass of NGC 147. Therefore, our results tentatively suggest that the discrepancy between the low-mass MZR and high-mass MZR should not be attributed to a systematic difference in techniques. Instead, real physical processes cause the transition in the MZR. In addition, we discovered a positive age gradient in the innermost region and a negative metallicity gradient from the resolved stars at larger radii, suggesting a possible outside-in formation of NGC 147.

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 (&amp;lt;0.1 dex) and older (&amp;lt;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 (&amp;lt;0.03 dex) and age (&amp;lt;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 (&amp;lt;0.05 and &amp;lt;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.