Massive Galaxies Research Articles

Stellar angular momentum of intermediate-redshift galaxies in MUSE surveys

We quantify the stellar rotation of galaxies by computing the λR parameter, a proxy for the stellar angular momentum in a sample of 106 intermediate-redshift galaxies (0.1 < z < 0.8). The sample is located in the CANDELS/GOODS-S and CANDELS/COSMOS regions, and it was observed by various MUSE surveys. We created spatially resolved stellar velocity and velocity dispersion maps using a full-spectrum fitting technique, covering spatially ∼2Re for the galaxies. The sample spans stellar masses from ∼107.5 M⊙ to 1011.8 M⊙ with star formation rates (SFRs) from log10(SFR)≈ − 3 M⊙ yr−1 to ≈1.7 M⊙ yr−1 over a range of 6 Gyr in cosmic time. We studied how the atmospheric seeing, introduced by the instrumental point spread function (PSF), affects the measured spin parameter, and we applied corrections when pertinent. Through the analysis of the λR − ϵ diagram, we note that the fraction of round and massive galaxies increases with redshift. We did not measure any galaxy with λR < 0.1 in the sample, and we found only one potential (but uncertain) low-mass slow rotator at z ∼ 0.3, more similar to the z = 0 low-mass slow rotators characterized by counter-rotation than to massive ellipticals. Moreover, we do not see an evident evolution or trend in the stellar angular momentum with redshift. We characterized the galaxy environment using two different indicators: a local estimator based on the Voronoi tesselation method, and a global estimator derived by the use of the friends-of-friends (FoF) algorithm. We find no correlation between the environment and λR given that we are not probing dense regions or massive galaxy structures. We also analysed the kinematic maps of the sample finding that about 40% of the galaxies are consistent with being regular rotators (RRs), having rotating stellar discs with flat velocity dispersion maps, while ∼20% have complex velocity maps and can be identified as non-regular rotators in spite of their λR values. For the remaining galaxies the classification is uncertain. As we lack galaxies with λR < 0.1 in the sample, we are not able to identify when galaxies lose their angular momentum and become slow rotators within the surveyed environments, area, and redshift range.

MHONGOOSE: A MeerKAT nearby galaxy H I survey

The MHONGOOSE (MeerKAT H I Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (H I) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low H I column densities. The H I column density sensitivity (3σ over 16 km s−1) ranges from ∼5 × 1017 cm−2 at 90″ resolution to ∼4 × 1019 cm−2 at the highest resolution of 7″. The H I mass sensitivity (3σ over 50 km s−1) is ∼5.5 × 105 M⊙ at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km s−1. One of the main science goals of the survey is the detection of cold accreting gas in the outskirts of the sample galaxies. The sample was selected to cover a range in H I masses from 107 M⊙ to almost 1011 M⊙ in order to optimally sample possible accretion scenarios and environments. The distance to the sample galaxies ranges from 3 to 23 Mpc. In this paper, we present the sample selection, survey design, and observation and reduction procedures. We compared the integrated H I fluxes based on the MeerKAT data with those derived from single-dish measurement and find good agreement, indicating that our MeerKAT observations are recovering all flux. We present H I moment maps of the entire sample based on the first ten percent of the survey data, and find that a comparison of the zeroth- and second-moment values shows a clear separation in the physical properties of the H I between areas with star formation and areas without related to the formation of a cold neutral medium. Finally, we give an overview of the H I-detected companion and satellite galaxies in the 30 fields, five of which have not previously been cataloged. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

Rapidly growing primordial black holes as seeds of the massive high-redshift JWST Galaxies

Rapidly growing primordial black holes as seeds of the massive high-redshift JWST Galaxies

A hidden population of active galactic nuclei can explain the overabundance of luminous z > 10 objects observed by JWST

The first wave of observations with JWST has revealed a striking overabundance of luminous galaxies at early times (z > 10) compared to models of galaxies calibrated to pre-JWST data. Early observations have also uncovered a large population of supermassive black holes (SMBHs) at z > 6. Because many of the high-z objects appear extended, the contribution of active galactic nuclei (AGNs) to the total luminosity has been assumed to be negligible. In this work, we use a semi-empirical model for assigning AGNs to galaxies to show that active galaxies can boost the stellar luminosity function (LF) enough to solve the overabundance problem while simultaneously remaining consistent with the observed morphologies of high-z sources. We construct a model for the composite AGN+galaxy LF by connecting dark matter halo masses to galaxy and SMBH masses and luminosities, accounting for dispersion in the mapping between host galaxy and SMBH mass and luminosity. By calibrating the model parameters — which characterize the M ∙ -M* relation — to a compilation of z > 10 JWST UVLF data, we show that AGN emission can account for the excess luminosity under a variety of scenarios, including one where 10% of galaxies host BHs of comparable luminosities to their stellar components. Using a sample of simulated objects and real observations, we demonstrate that such low-luminosity AGNs can be `hidden' in their host galaxies and be missed in common morphological analyses. We find that for this explanation to be viable, our model requires a population of BHs that are overmassive (M ∙ /M* ~ 10-2) with respect to their host galaxies compared to the local relation and are more consistent with the observed relation at z = 4-8. We explore the implications of this model for BH seed properties and comment on observational diagnostics necessary to further investigate this explanation.

JWST/CEERS sheds light on dusty star-forming galaxies: Forming bulges, lopsidedness, and outside-in quenching at cosmic noon

Context. We investigate the morphology and resolved physical properties of a sample of 22 IR-selected dusty star-forming galaxies at cosmic noon using the James Webb Space Telescope NIRCam images obtained in the EGS field for the CEERS survey. The exceptional resolution of the NIRCam images allowed us to spatially resolve these galaxies up to 4.4 μm and identify their bulge or core even when very extinguished by dust. Aims. The goal of this study is to obtain a better understanding of the formation and evolution of FIR-bright galaxies by spatially resolving their properties using JWST in order to look through the dust and bridge the gap between the compact FIR sources and the larger optical star-forming galaxies. Methods. Based on red-green-blue images from the F115W, F200W, and F444W filters, we divided each galaxy into several uniformly colored regions, fit their respective SEDs, and measured physical properties. After classifying each region as star forming or quiescent, we assigned galaxies to three classes depending on whether active star formation is located in the core, in the disk, or in both. Results. (i) We find that the galaxies at a higher redshift tend to have a fragmented disk with a low core mass fraction. They are at an early stage of bulge formation. When moving toward a lower redshift, the core mass fraction increases, and the bulge growth is associated with a stabilization of the disk, which translates into less patches and clumps. The NIRCam data clearly point toward bulge formation in preexisting disks. (ii) Lopsidedness is a very common feature of DSFGs. It has been wrongly overlooked for a long time and could have a major impact on the evolution of DSFGs. (iii) Twenty-three percent of the galaxies have a star-forming core embedded in a quiescent disk. They seem to be undergoing outside-in quenching, often facilitated by their strong lopsidedness inducing instabilities. (iv) We show that half of our galaxies with star formation concentrated in their core are good sub-millimeter galaxy near-IR counterpart candidates, demonstrating that compact SMGs are usually surrounded by a larger, less obscured disk. (v) Finally, we found surprising evidence for clump-like substructures being quiescent or residing in quiescent regions. Conclusions. This work demonstrates the major impact JWST/NIRCam has on understanding the complexity of the evolution of distant massive galaxies regarding bulge formation and quenching mechanisms.

The star-forming and ionizing properties of dwarf z ~ 6–9 galaxies in JADES: insights on bursty star formation and ionized bubble growth

ABSTRACT Reionization is thought to be driven by faint star-forming galaxies, but characterizing this population has long remained very challenging. Here, we utilize deep nine-band JADES (JWST Advanced Deep Extragalactic Survey)/NIRCam (Near-Infrared Camera) imaging to study the star-forming and ionizing properties of 756 $z\sim 6-9$ galaxies, including hundreds of very ultraviolet (UV)-faint objects ($M_\mathrm{UV}\gt -18$). The faintest ($m\sim 30$) galaxies in our sample typically have stellar masses of $M_\ast \sim (1-3)\times 10^7\ \mathrm{ M}_\odot$ and young light-weighted ages ($\sim$50 Myr), though some show strong Balmer breaks implying much older ages ($\sim$500 Myr). We find no evidence for extremely massive galaxies ($\gt 3\times 10^{10}\ \mathrm{ M}_\odot$) in our sample. We infer a strong (factor $\gt $2) decline in the typical [O iii]$+$H $\beta$ equivalent widths (EWs) towards very faint $z\sim 6-9$ galaxies, yet a weak UV luminosity dependence on the H $\alpha$ EWs at $z\sim 6$. We demonstrate that these EW trends can be explained if fainter galaxies have systematically lower metallicities as well as more recently declining star formation histories relative to the most UV-luminous galaxies. Our data provide evidence that the brightest galaxies are frequently experiencing a recent strong upturn in star formation rate. We also discuss how the EW trends may be influenced by a strong correlation between $M_\mathrm{UV}$ and Lyman continuum escape fraction. This alternative explanation has dramatically different implications for the contribution of galaxies along the luminosity function to cosmic reionization. Finally, we quantify the photometric overdensities around two $z\,\gt\,7$ strong Ly $\alpha$ emitters. One Ly $\alpha$ emitter lies close to a strong photometric overdensity, while the other shows no significant nearby overdensity, perhaps implying that not all strong $z\,\gt\, 7$ Ly $\alpha$ emitters reside in large ionized bubbles.

3D-DASH: The Evolution of Size, Shape, and Intrinsic Scatter in Populations of Young and Old Quiescent Galaxies at 0.5 < z < 3

We present a study of the growth of the quiescent galaxy population between 0.5 < z < 3 by tracing the number density and structural evolution of a sample of 4518 old and 583 young quiescent galaxies with log(M ⋆/M ⊙) > 10.4, selected from the COSMOS2020 catalog with complementary Hubble Space Telescope F160W imaging from the 3D-DASH survey. Among the quiescent population at z ∼ 2, roughly 50% are recently quenched galaxies; these young quiescent galaxies become increasingly rare toward lower redshift, supporting the idea that the peak epoch of massive galaxy quenching occurred at z > 2. Our data show that while the effective half-light radius of quiescent galaxies generally increases with time, young quiescent galaxies are significantly smaller than their older counterparts at the same redshift. In this work we investigate the connection between this size difference and other structural properties, including axis ratio, color gradients, stellar mass, and the intrinsic scatter in effective radius. We demonstrate that the size difference is driven by the most massive subpopulation (log(M ⋆/M ⊙) > 11) and does not persist when restricting the sample to intermediate-mass galaxies (10.4 < log(M ⋆/M ⊙) < 11). Interestingly, the intrinsic scatter in physical size shows a strong coevolution over the investigated time period and peaks around z ∼ 2 for both populations, only diverging at z < 1. Taken together, and assuming we are not missing a significant population of lower surface brightness galaxies, while the formation and quenching mechanisms that dominate at higher redshifts yield compact remnants, multiple evolutionary pathways may explain the diverse morphologies of galaxies that quench at z < 1.

The robustness in identifying and quantifying high-redshift bars using JWST observations

Understanding the methodological reliability in identifying and quantifying high-redshift bars is essential for studying their evolution with the James Webb Space Telescope (JWST). We used nearby spiral galaxies to generate simulated images at various resolutions and signal-to-noise ratios, and obtained the simulated galaxy images observed in the Cosmic Evolution Early Release Science (CEERS) survey. Through a comparison of measurements before and after image degradation, we show that the bar measurements for massive galaxies remain robust against noise. While the measurement of the bar position angle remains unaffected by resolution, the measured bar ellipticity is significantly underestimated in low-resolution images. The size measurement is barely affected on average as long as the intrinsic bar size abar, true &gt; 2 × FWHM. To address these effects, correction functions are derived. We also find that bar detections remain effective at ∼100% when the abar, true/FWHM is above 2, below which the rate drops sharply, quantitatively validating the effectiveness of using abar, true &gt; 2 × FWHM as a bar detection threshold. We analyzed a set of simulated CEERS images and took into account observational effects and plausible galaxy (and bar-size) evolution models. We show that a significant (and misleading) reduction in the detected bar fraction with increasing redshift would apparently result even if the true bar fraction remained constant. Our results underscore the importance of disentangling the true bar fraction evolution from resolution effects and bar size growth.

UVCANDELS: The Role of Dust on the Stellar Mass–Size Relation of Disk Galaxies at 0.5 ≤ z ≤ 3.0

We use the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields (UVCANDELS) to measure half-light radii in the rest-frame far-UV for ∼16,000 disk-like galaxies over 0.5 ≤ z ≤ 3. We compare these results to rest-frame optical sizes that we measure in a self-consistent way and find that the stellar mass–size relation of disk galaxies is steeper in the rest-frame UV than in the optical across our entire redshift range. We show that this is mainly driven by massive galaxies (≳1010 M ⊙), which we find to also be among the most dusty. Our results are consistent with the literature and have commonly been interpreted as evidence of inside-out growth wherein galaxies form their central structures first. However, they could also suggest that the centers of massive galaxies are more heavily attenuated than their outskirts. We distinguish between these scenarios by modeling and selecting galaxies at z = 2 from the VELA simulation suite in a way that is consistent with UVCANDELS. We show that the effects of dust alone can account for the size differences we measure at z = 2. This indicates that, at different wavelengths, size differences and the different slopes of the stellar mass–size relation do not constitute evidence for inside-out growth.

Two distinct molecular cloud populations detected in massive galaxies

ABSTRACT We present new ALMA observations of CO, CN, CS, HCN, and HCO$^{+}$ absorption seen against the bright and compact radio continuum sources of eight galaxies. Combined with archival observations, they reveal two distinct populations of molecular clouds, which we identify by combining CO emission and absorption profiles to unambiguously reveal each cloud’s direction of motion and likely location. In galaxy discs, we see clouds with low velocity dispersions, low line-of-sight velocities, and a lack of any systemic inflow or outflow. In galactic cores, we find high velocity dispersion clouds inflowing at up to 550 km s−1. This provides observational evidence in favour of cold accretion on to galactic centres, which likely contributes to the fuelling of active galactic nuclei. We also see a wide range in the CO(2-1)/CO(1-0) ratios of the absorption lines. This is likely the combined effect of hierarchical substructure within the molecular clouds and continuum sources which vary in size with frequency.

Galaxy Ages with Redshift z = 2–4: Stellar Population Synthesis for Candidates in FourStar Galaxy Evolution Survey

Observations of large amounts of massive galaxies with relatively old populations found at high redshifts are challenging galaxy formation scenarios within the standard cosmology. Precise determinations of the average age of these galaxies would be useful for the discussion of this problem. Here we carry out a better constraint of the age of 200 V-shaped spectral energy distribution (SED) nonactive galactic nucleus galaxies at redshifts 2 < z < 4 of the catalog of the FourStar Galaxy Evolution Survey, identified by the V shape in their SED with a Lyman and a Balmer break. The SED fitting includes a main stellar population in addition to a residual younger population and extinction. The galaxies are younger at a higher redshift on average. However, for the galaxies with z > 2.5, we do not see a significant evolution of their average age, with all average ages of the galaxies mostly remaining between 1 and 2 Gyr. Our research finds that most massive galaxies (∼1010 M ⊙) are older (typically > ∼1 Gyr old) and formed earlier than less massive galaxies in our sample.

The Galaxy–Galaxy Strong Lensing Cross Section and the Internal Distribution of Matter in ΛCDM Substructure

Strong gravitational lensing offers a powerful probe of the detailed distribution of matter in lenses, while magnifying and bringing faint background sources into view. Observed strong lensing by massive galaxy clusters, which are often in complex dynamical states, has also been used to map their dark matter (DM) substructures on smaller scales. Deep high-resolution imaging has revealed the presence of strong lensing events associated with these substructures, namely galaxy-scale sub-halos. However, an inventory of these observed galaxy–galaxy strong lensing (GGSL) events is noted to be discrepant with state-of-the-art ΛCDM simulations. Cluster sub-halos appear to be over-concentrated compared to their simulated counterparts yielding an order-of-magnitude higher value of GGSL. In this paper, we explore the possibility of resolving this observed discrepancy by redistributing the mass within observed cluster sub-halos in ways that are consistent within the ΛCDM paradigm of structure formation. Lensing mass reconstructions from data provide constraints on the mass enclosed within apertures and are agnostic to the detailed mass profile within them. Therefore, as the detailed density profile within cluster sub-halos currently remains unconstrained by data, we are afforded the freedom to redistribute the enclosed mass. We investigate if rearranging the mass to a more centrally concentrated density profile helps alleviate the GGSL discrepancy. We report that refitting cluster sub-halos to the ubiquitous ΛCDM-motivated Navarro–Frenk–White profile, and further modifying them to include significant baryonic components, does not resolve this tension. A resolution to this persisting GGSL discrepancy may require more careful exploration of alternative DM models.

El Gordo needs El Anzuelo: Probing the structure of cluster members with multi-band extended arcs in JWST data

Gravitational lensing by galaxy clusters involves hundreds of galaxies over a large redshift range and increases the likelihood of rare phenomena (supernovae, microlensing, dark substructures, etc.). Characterizing the mass and light distributions of foreground and background objects often requires a combination of high-resolution data and advanced modeling techniques. We present the detailed analysis of a prominent quintuply imaged dusty star-forming galaxy ($ mainly lensed by three members of the massive galaxy cluster ACT-CL\,J0102$-$4915, also known as d =0.87$). We leverage JWST/NIRCam images, which contain lensing features that were unseen in previous HST images, using a Bayesian, multi-wavelength, differentiable and GPU-accelerated modeling framework that combines (lens modeling) and (field model and inference) software packages. For one of the deflectors, we complement lensing constraints with stellar kinematics measured from VLT/MUSE data. In our lens model, we explicitly include the mass distribution of the cluster, locally corrected by a constant shear field. We find that the two main deflectors (L1 and L2) have logarithmic mass density slopes steeper than isothermal, with $ L1 and $ L2 We argue that such steep density profiles can arise due to tidally truncated mass distributions, which we probe thanks to the cluster lensing boost and the strong asymmetry of the lensing configuration. Moreover, our three-dimensional source model captures most of the surface brightness of the lensed galaxy, revealing a clump with a maximum diameter of $400$ parsecs at the source redshift, visible at wavelengths $ rest mu m. Finally, we caution on using point-like features within extended arcs to constrain galaxy-scale lens models before securing them with extended arc modeling.

The obsidian model: three regimes of black hole feedback

ABSTRACT In theoretical models of galaxy evolution, black hole feedback is a necessary ingredient in order to explain the observed exponential decline in number density of massive galaxies. Most contemporary black hole feedback models in cosmological simulations rely on a constant radiative efficiency (usually $\eta \sim 0.1$) at all black hole accretion rates. We present the obsidian subgrid model, a synthesis model for the spin-dependent radiative efficiencies of three physical accretion rate regimes, i.e. $\eta = \eta (j, \dot{M}_\mathrm{acc})$, for use in large-volume cosmological simulations. The three regimes include: an advection-dominated accretion flow ($\dot{M}_\mathrm{acc}\lt 0.03\, \dot{M}_\mathrm{Edd}$), a quasar-like mode ($0.03 \lt \dot{M}_\mathrm{acc}/ \dot{M}_\mathrm{Edd}\lt 0.3$), and a slim disc mode ($\dot{M}_\mathrm{acc}\gt 0.3\, \dot{M}_\mathrm{Edd}$). Additionally, we include a large-scale powerful jet at low accretion rates. The black hole feedback model we present is a kinetic model that prescribes mass loadings but could be used in thermal models directly using the radiative efficiency. We implement the obsidian model into the simba galaxy evolution model to determine if it is possible to reproduce galaxy populations successfully, and provide a first calibration for further study. Using a $2\times 1024^3$ particle cosmological simulation in a $(150\, \mathrm{cMpc})^3$ volume, we found that the model is successful in reproducing the galaxy stellar mass function, black hole mass–stellar mass relationship, and stellar mass–halo mass relationship. Moving forward, this model opens new avenues for exploration of the impact of black hole feedback on galactic environments.

The hot circumgalactic medium in the eROSITA All-Sky Survey II. Scaling relations between X-ray luminosity and galaxies' mass

Understanding how the properties of galaxies relate to the properties of the hot circum-galactic medium (CGM) around them can constrain galaxy evolution models. We aim to measure the scaling relations between the X-ray luminosity of the hot CGM and the fundamental properties (stellar mass and halo mass) of a galaxy. We measured the X-ray luminosity of the hot CGM based on the surface brightness profiles of central galaxy samples measured from Spectrum Roentgen Gamma (SRG)/eROSITA all-sky survey data. We related the X-ray luminosity to the galaxies' stellar and halo mass, and we compared the observed relations to the self-similar model and intrinsic (i.e., not forward-modeled) output of the IllustrisTNG, EAGLE, and SIMBA simulations. The average hot CGM X-ray luminosity ($L_ X,CGM $) correlates with the galaxy's stellar mass ($M_*$). It increases from $(1.6 $ to $(3.4 $, when $ increases from 10.0 to 11.5. A power law describes the correlation as $ X,CGM The hot CGM X-ray luminosity as a function of halo mass is measured within $ 500c )=11.3-13.7$, extending our knowledge of the scaling relation by more than two orders of magnitude. $L_ X,CGM $ increases with $M_ 500c $ from $(3.0 $ at $ 500c )=11.3$ to $(1.3 $ at $ 500c )=13.7$. The relation follows a power law of $ X,CGM 500c Our observations highlight the necessity of non-gravitational processes at the galaxy group scale while suggesting these processes are sub-dominant at the galaxy scale. We show that the outputs of current cosmological galaxy simulations generally align with the observational results uncovered here but with possibly important deviations in selected mass ranges. We explore, at the low mass end, the average scaling relations between the CGM X-ray luminosity and the galaxy's stellar mass or halo mass, which constitutes a new benchmark for galaxy evolution models and feedback processes.

Metallicity distributions of halo stars: do they trace the Galactic accretion history?

The standard cosmological scenario predicts a hierarchical formation for galaxies. Many substructures have been found in the Galactic halo, usually identified as clumps in kinematic spaces, like the energy-angular momentum space ($E-L_z$), under the hypothesis that these quantities should be conserved during the interaction. If these clumps also feature different chemical abundances, such as the metallicity distribution function (MDF), these two arguments together (different kinematic and chemical properties) are often used to motivate their association with distinct and independent merger debris. The aim of this study is to explore to what extent we can couple kinematic characteristics and metallicities of stars in the Galactic halo to reconstruct the accretion history of the Milky Way (MW). In particular, we want to understand whether different clumps in the $E-L_z$ space with different MDFs should be associated with distinct merger debris. We analysed dissipationless, self-consistent, high-resolution N-body simulations of a MW-type galaxy accreting a satellite with a mass ratio of 1:10, with different orbital parameters and different metallicity gradients, which were assigned a posteriori. We confirm that accreted stars from a sim 1:10 mass ratio merger event redistribute in a wide range of $E$ and $L_z$, due to the dynamical friction process, and are thus not associated with a single region. Because satellite stars with different metallicities can be deposited in different regions of the $E-L_z$ space (on average the more metal-rich ones end up more gravitationally bound to the MW), this implies that a single accretion of sim 1:10 can manifest with different MDFs, in different regions of the $E-L_z$ space. Groups of stars with different $E$, $L_z$, and metallicities may be interpreted as originating from different satellite galaxies, but our analysis shows that these interpretations are not physically motivated. In fact, as we show, the coupling of kinematic information with MDFs to reconstruct the accretion history of the MW can bias the reconstructed merger tree towards increasing the number of past accretions and decreasing the masses of the progenitor galaxies.

Constraints on the in situ and ex situ stellar masses in nearby galaxies obtained with artificial intelligence

Constraints on the in situ and ex situ stellar masses in nearby galaxies obtained with artificial intelligence

Wormholes in dwarf and spiral galactic halo regions

In this article, we study solutions which describe wormholes in the halos of dwarf and massive spiral galaxies with different morphologies, masses, sizes and gas fractions by taking observed flat rotation curves as input. We assume Singular Isothermal Sphere (SIS) dark matter density profile. This result confirms the possible existence of wormholes in both dwarf and massive spiral galaxies.

Realistic simulated galaxies form [α/Fe]–[Fe/H] knees due to a sustained decline in their star formation rates

ABSTRACT We examine the stellar [$\alpha$/Fe]–[Fe/H] distribution of $\simeq 1000$ present-day galaxies in a high-resolution EAGLE simulation. Roughly half of the galaxies exhibit the canonical distribution, characterized by a sequence of low-metallicity stars with high [$\alpha$/Fe] that transitions at a ‘knee’ to a sequence of declining [$\alpha$/Fe] with increasing metallicity. This population yields a knee metallicity–galaxy–mass relation similar to that observed in Local Group galaxies, both in slope and scatter. However, many simulated galaxies lack a knee or exhibit more complicated distributions. Knees are found only in galaxies with star formation histories (SFHs) featuring a sustained decline from an early peak ($t\simeq 7~{\rm Gyr}$), which enables enrichment by Type Ia supernovae to dominate that due to Type II supernovae (SN II), reducing [$\alpha$/Fe] in the interstellar gas. The simulation thus indicates that, contrary to the common interpretation implied by analytic galactic chemical evolution (GCE) models, knee formation is not a consequence of the onset of enrichment by SN Ia. We use the SFH of a simulated galaxy exhibiting a knee as input to the vice GCE model, finding it yields an $\alpha$-rich plateau enriched only by SN II, but the plateau comprises little stellar mass and the galaxy forms few metal-poor ([Fe/H] $\lesssim$$-$1) stars. This follows from the short constant gas consumption time-scale typically assumed by GCEs, which implies the presence of a readily enriched low-mass gas reservoir. When an initially longer, evolving consumption time-scale is adopted, vice reproduces the simulated galaxy’s track through the [$\alpha$/Fe]–[Fe/H] plane and its metallicity distribution function.

Non-monotonic relations of galaxy star formation, radius, and structure at fixed stellar mass

ABSTRACT We investigate the relation between galaxy structure and star formation rate (SFR) in a sample of $\sim 2.9\times 10^{4}$ central galaxies with $z\lt 0.0674$ and axial ratios $b/a\gt 0.5$. The star-forming main sequence (SFMS) shows a bend around the stellar mass of $M_\ast \le {}M_c=2\times 10^{10}{}{\rm M}_{\odot }$. At $M_\ast \le {}M_c$, the SFMS follows a power-law $\text{SFR}\propto {}M_\ast ^{0.85}$, while at higher masses it flattens. $M_c$ corresponds to a dark matter halo mass of $M_\text{vir}\sim {}10^{11.8}{\rm M}_{\odot }$ where virial shocks occurs. Some galaxy structure (e.g. half-light radius, $R_e$) exhibits a non-monotonic dependence across the SFMS at a fixed $M_\ast$. We find $\text{SFR}\propto {R_e^{-0.28}}$ at fixed $M_\ast$, consistent with the global Kennicutt–Schmidt (KS) law. This finding suggests that galaxy sizes contribute to the scatter of the SFMS. However, at $M_\ast \gt M_c$ the relationship between SFR and $R_e$ diminishes. Low-mass galaxies above the mean of the SFMS have smaller radii, exhibit compact and centrally concentrated profiles resembling green valley (GV) and quiescent galaxies at the same mass, and have higher $M_{\text{H}_2}{/}M_\rm{H\,{\small I}}$. Conversely, those below the SFMS exhibit larger radii, lower densities, have no GV or quiescent counterparts at their mass and have lower $M_{\text{H}_2}/M_\rm{H\,{\small I}}$. The above data suggest two pathways for quenching low-mass galaxies, $M_\ast \le {}M_c$: a fast one that changes the morphology on the SFMS and a slow one that does not. Above $M_c$, galaxies below the SFMS resemble GV and quiescent galaxies structurally, implying that they undergo a structural transformation already within the SFMS. For these massive galaxies, CG are strongly bimodal, with SFMS galaxies exhibiting negative colour gradients, suggesting most star formation occurs in their outskirts, maintaining them within the SFMS.