Increase In Stellar Mass Research Articles

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 ⊙.

FOREVER22: the first bright galaxies with Population III stars at redshifts z ≃ 10–20 and comparisons with JWST data

ABSTRACT We study the formation of the first galaxies in overdense regions modelled by the FORmation and EVolution of galaxies in Extremely overdense Regions motivated by SSA22 (FOREVER22) simulation project. Our simulations successfully reproduce the star formation rates and the MUV–Mstar relations of candidate galaxies at z ∼ 10–14 observed by the JWST. We suggest that the observed galaxies are hosted by dark matter haloes with Mh ≳ 1010 M⊙ and are in short-period starburst phases. On the other hand, even simulated massive galaxies in overdense regions cannot reproduce the intense star formation rates and the large stellar masses of observed candidates at z ∼ 16. Also, we show that the contribution of Population III stars to the ultraviolet (UV) flux decreases as the stellar mass increases and it is a few per cent for galaxies with Mstar ∼ 107 M⊙. Therefore, a part of the observed flux by JWST could be the light from Population III stars. Our simulations suggest that the UV flux can be dominated by Population III stars and the UV slope shows β ≲ −3 if future observations would reach galaxies with Mstars ∼ 105 M⊙ at z ∼ 20 of which the mass fraction of Population III stars can be greater than 10 per cent.

First light and reionization epoch simulations (FLARES) X iii: the lyman-continuum emission of high-redshift galaxies

ABSTRACT The history of reionization is highly dependent on the ionizing properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionizing properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionization Epoch Simulations (Flares) to study the Lyman-continuum (LyC, i.e. hydrogen-ionizing) emission of massive ($M_*\gt 10^8\, \mathrm{M_\odot }$) galaxies at redshifts z = 5 − 10. We find that the specific ionizing emissivity (i.e. intrinsic ionizing emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. Flares predicts a median ionizing photon production efficiency (i.e. intrinsic ionizing emissivity per unit intrinsic far-UV luminosity) of $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})=25.40^{+0.16}_{-0.17}$, with values spanning the range $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})=25-25.75$. This is within the range of many observational estimates, but below some of the extremes observed. We compare the production efficiency with observable properties, and find a weak negative correlation with the UV-continuum slope, and a positive correlation with the [O iii] equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionizing emissivity per unit dust-attenuated far-UV luminosity), and find a median of $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})\sim 25.5$. Within our sample of $M_*\gt 10^8\, \mathrm{M_\odot }$ galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionizing emissivity. Active galactic nuclei (AGN) emission accounts for 10 − 20 per cent of the total emissivity at a given redshift, and extends the LyC luminosity function by ∼0.5 dex.

A Direct Measurement of Galaxy Major and Minor Merger Rates and Stellar Mass Accretion Histories at Z < 3 Using Galaxy Pairs in the REFINE Survey

We measure the role of major and minor mergers in forming the stellar masses of galaxies over redshifts 0 < z < 3 using a combination of ∼3.25 deg2 of the deepest ground-based near-infrared imaging taken to date (Ultra Deep Survey, Ultra-VISTA, and VIDEO) as part of the collated REFINE survey. We measure the pair fraction and merger fractions for galaxy mergers of different mass ratios, and quantify the merger rate with newly measured timescales derived from the Illustris simulation as a function of redshift and merger mass ratio. For a M * > 1011 M ⊙ selection, we find that over 0 < z < 3 major mergers with mass ratios greater than 1:4 occur times on average, while minor mergers down to ratios of 1:10 occur on average times per galaxy. We also quantify the role of major and minor mergers in galaxy formation, whereby the increase in mass due to major mergers is while minor mergers account for an increase of using a M * > 1011 M ⊙ selection. We thus find that major mergers add more stellar mass to galaxies than minor mergers over this epoch. Overall, mergers will more than double the mass of massive galaxies over this epoch when selecting by stellar mass. We however find a lower increase in stellar mass when selecting by a constant number density. Finally, we compare our results to simulations, finding that minor mergers are overpredicted in Illustris and in semi-analytical models, suggesting a mismatch between observations and theory in this fundamental aspect of galaxy assembly.

Evolution of X-Ray Activity in

Measuring the evolution of X-ray emission from pre-main-sequence (PMS) stars gives insight into two issues: the response of magnetic dynamo processes to changes in the interior structure, and the effects of high-energy radiation on protoplanetary disks and primordial planetary atmospheres. We present a sample of 6003 stars with ages 7–25 Myr in 10 nearby open clusters from Chandra X-ray and Gaia-EDR3 surveys. Combined with previous results in large samples of younger (≲5 Myr) stars in MYStIX and SFiNCs star-forming regions, mass-stratified activity-age relations are derived for the early phases of stellar evolution. X-ray luminosity (L X ) is constant during the first few Myr, possibly due to the presence of extended X-ray coronas insensitive to temporal changes in stellar size. L X then decays during the 7–25 Myr period, more rapidly as stellar mass increases. This decay is interpreted as decreasing efficiency of the α 2 dynamo as radiative cores grow and a solar-type αΩ dynamo emerges. For more massive 3.5–7 M ⊙ fully radiative stars, the X-ray emission plummets—indicating the lack of an effective magnetic dynamo. The findings provide improved measurements of high-energy radiation effects on circumstellar material, first for the protoplanetary disk and then for the atmospheres of young planets. The observed X-ray luminosities can be so high that an inner Earth-mass rocky, unmagnetized planet around a solar-mass PMS star might lose its primary and secondary atmospheres within a few (several) million years. PMS X-ray emission may thus have a significant impact on the evolution of early-planetary atmospheres and the conditions promoting the rise of habitability.

Stellar and black hole assembly inz&amp;lt; 0.3 infrared-luminous mergers: intermittent starbursts versus super-Eddington accretion

ABSTRACTWe study stellar and black hole mass assembly in a sample of 42 infrared-luminous galaxy mergers at z &amp;lt; 0.3 by combining results from radiative transfer modelling with archival measures of molecular gas and black hole mass. The ratios of stellar mass, molecular gas mass, and black hole mass to each other are consistent with those of massive gas-rich galaxies at z &amp;lt; 0.3. The advanced mergers may show increased black hole mass to stellar mass ratios, consistent with the transition from active galactic nucleus (AGN) to ellipticals and implying substantial black hole mass growth over the course of the merger. Star formation rates are enhanced relative to the local main sequence, by factors of ∼100 in the starburst and ∼1.8 in the host, respectively. The starburst star formation rates appear distinct to star formation in the main sequence at all redshifts up to at least z ∼ 5. Starbursts may prefer late-stage mergers, but are observed at any merger stage. We do not find evidence that the starbursts in these low-redshift systems substantially increase the total stellar mass, with a soft upper limit on the stellar mass increase from starburst activity of about a factor of two. In contrast, 12 objects show evidence for super-Eddington accretion, associated with late-stage mergers, suggesting that many AGN in infrared-luminous mergers go through a super-Eddington phase. The super-Eddington phase may increase black hole mass by up to an order of magnitude at an accretion efficiency of $42\pm 33{{\ \rm per\ cent}}$ over a period of 44 ± 22 Myr. Our results imply that super-Eddington accretion is an important black hole growth channel in infrared-luminous galaxies at all redshifts.

Star-dust geometry main determinant of dust attenuation in galaxies

ABSTRACT Analysing a large representative sample of local galaxies (8707), we find that the variation in the shape of their dust attenuation curves is driven primarily by their structure, i.e. distribution of stars (and dust) within them. The attenuation curve for spheroid dominated galaxies, as compared to the disc-dominated ones, is nearly twice as steep. Both structural types cover distinct ranges of attenuation slope values. Similar findings are reflected in the case of star-forming and passive galaxies. Spheroids and passive galaxies witness minimal attenuation in the optical compared to UV wavelengths, underlining the lack of dusty birth-clouds that define complex star-dust geometry. The distinction in the attenuation properties of spheroids and discs is maintained in each stellar mass range, emphasising that structure is the primal cause of variation. However, within a structural group, the attenuation curve becomes shallower with both the increase in total stellar mass and optical depth of the galaxy. Overall, with the extinction curve fixed to be the same for all galaxies, the star-dust geometry emerges to be the prime determinant of the variation in their attenuation properties.

Spatially resolved properties of supernova host galaxies in SDSS-IV MaNGA

We crossmatch galaxies from Mapping Nearby Galaxies at Apache Point Observatory with the Open Supernova Catalog, obtaining a total of 132 SNe within MaNGA bundle. These 132 SNe can be classified into 67 Type Ia and 65 Type CC. We study the global and local properties of supernova host galaxies statistically. Type Ia SNe are distributed in both star-forming galaxies and quiescent galaxies, while Type CC SNe are all distributed along the star-forming main sequence. As the stellar mass increases, the Type Ia/CC number ratio increases. We find: (1) there is no obvious difference in the interaction possibilities and environments between Type Ia SN hosts and a control sample of galaxies with similar stellar mass and SFR distributions, except that Type Ia SNe tend to appear in galaxies which are more bulge-dominated than their controls. For Type CC SNe, there is no difference between their hosts and the control galaxies in galaxy morphology, interaction possibilities as well as environments; (2) compared to galaxy centers, the SN locations have smaller velocity dispersion, lower metallicity, and younger stellar population. This is a natural result of radius gradients for all these parameters. The SN location and its symmetrical position relative to the galaxy center, as well as regions with similar effective radii have very similar [Mg/Fe], gas-phase metallicity, gas velocity dispersion and stellar population age.

Exploring the Stellar Rotation of Early-type Stars in the LAMOST Medium-resolution Survey. II. Statistics

Abstract Angular momentum is a key property regulating star formation and evolution. However, the physics driving the distribution of the stellar rotation rates of early-type main-sequence stars is as yet poorly understood. Using our catalog of 40,034 early-type stars with homogeneous v sin i parameters, we review the statistical properties of their stellar rotation rates. We discuss the importance of possible contaminants, including binaries and chemically peculiar stars. Upon correction for projection effects and rectification of the error distribution, we derive the distributions of our sample’s equatorial rotation velocities, which show a clear dependence on stellar mass. Stars with masses less than 2.5 M ⊙ exhibit a unimodal distribution, with the peak velocity ratio increasing as stellar mass increases. A bimodal rotation distribution, composed of two branches of slowly and rapidly rotating stars, emerges for more massive stars (M &gt; 2.5 M ⊙). For stars more massive than 3.0 M ⊙, the gap between the bifurcated branches becomes prominent. For the first time, we find that metal-poor ([M/H] &lt; −0.2 dex) stars only exhibit a single branch of slow rotators, while metal-rich ([M/H] &gt; 0.2 dex) stars clearly show two branches. The difference could be attributed to unexpectedly high spin-down rates and/or in part strong magnetic fields in the metal-poor subsample.

Implications of a spatially resolved main sequence for the size evolution of star-forming galaxies

ABSTRACT Two currently debated problems in galaxy evolution, the fundamentally local or global nature of the main sequence of star formation and the evolution of the mass–size relation of star-forming galaxies (SFGs), are shown to be intimately related to each other. As a preliminary step, a growth function g is defined, which quantifies the differential change in half-mass radius per unit increase in stellar mass (g = d log R1/2/d log M⋆) due to star formation. A general derivation shows that g = KΔ(sSFR)/sSFR, meaning that g is proportional to the relative difference in specific star formation rate between the outer and the inner half of a galaxy, with K a dimensionless structural factor for which handy expressions are provided. As an application, it is shown that galaxies obeying a fundamentally local main sequence also obey, to a good approximation, g ≃ γn, where γ is the slope of the normalized local main sequence ($\mathrm{ sSFR} \,\, \propto \,\, \Sigma _\star ^{-\gamma }$) and n is the Sersic index. An exact expression is also provided. Quantitatively, a fundamentally local main sequence is consistent with SFGs growing along a stationary mass–size relation, but inconsistent with the continuation at z = 0 of evolutionary laws derived at higher z. This demonstrates that either the main sequence is not fundamentally local, or the mass–size relation of SFGs has converged to an equilibrium state at some finite time in the past, or both.

Rotational Velocities of Am and Non-chemical-peculiar Stars Based on Kepler and LAMOST DR5

Abstract Rotation is a critical physical process operating in the formation of Am stars. There is a strong correlation between low-velocity rotation and chemical peculiarity. However, the existence of many non-chemical-peculiar slow rotators challenges the understanding of Am stars. The purpose of our work is to search for low-velocity rotating non-chemical-peculiar A-type stars and Am stars and to make a comparative analysis. In this paper, we pick out a sample from the LAMOST–Kepler project, including 21 Am stars, 125 non-chemical-peculiar slow rotators, and 53 non-chemical-peculiar fast rotators. We calculate the rotational frequencies through a periodic change of light curves caused by inhomogeneous stellar surfaces and then obtain the rotational velocities. For slow rotators, the age of Am stars is statistically younger than that of non-chemical-peculiar stars in the same temperature bin. In the comparison of the period, the average amplitude, and stellar mass of Am and non-chemical-peculiar stars, we discover that there is no difference in the photometric variability between Am and non-chemical-peculiar stars, which implies similar inhomogeneities on the surfaces. The average amplitude of non-chemical-peculiar stars has a downward trend with the increase of effective temperature and stellar mass, which is consistent with the theoretical prediction caused by weak dynamo-generated magnetic fields in A-type stars. In addition, we confirm four non-chemical-peculiar stars that have flares by checking field-of-view images, pixel images, and pixel-level light curves.

The evolution of the UV luminosity and stellar mass functions of Lyman-α emitters from z ∼ 2 to z ∼ 6

ABSTRACT We measure the evolution of the rest-frame UV luminosity function (LF) and the stellar mass function (SMF) of Lyman-α (Ly α) emitters (LAEs) from z ∼ 2 to z ∼ 6 by exploring ∼4000 LAEs from the SC4K sample. We find a correlation between Ly α luminosity (LLy α) and rest-frame UV (MUV), with best fit M$_{\rm UV}=-1.6_{-0.3}^{+0.2}\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1})+47_{-11}^{+12}$ and a shallower relation between LLy α and stellar mass (M⋆), with best fit $\log _{10} (\rm M_\star /{\rm M}_\odot)=0.9_{-0.1}^{+0.1}\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1})-28_{-3.8}^{+4.0}$. An increasing LLy α cut predominantly lowers the number density of faint MUV and low M⋆ LAEs. We estimate a proxy for the full UV LFs and SMFs of LAEs with simple assumptions of the faint end slope. For the UV LF, we find a brightening of the characteristic UV luminosity (M$_{\rm UV}^*$) with increasing redshift and a decrease of the characteristic number density (Φ*). For the SMF, we measure a characteristic stellar mass (${\rm M_\star ^*/{\rm M}_\odot }$) increase with increasing redshift, and a Φ* decline. However, if we apply a uniform luminosity cut of $\log _{10} (\rm L_{Ly\,\alpha }/erg\, s^{-1}) \ge 43.0$, we find much milder to no evolution in the UV and SMF of LAEs. The UV luminosity density (ρUV) of the full sample of LAEs shows moderate evolution and the stellar mass density (ρM) decreases, with both being always lower than the total ρUV and ρM of more typical galaxies but slowly approaching them with increasing redshift. Overall, our results indicate that both ρUV and ρM of LAEs slowly approach the measurements of continuum-selected galaxies at z &amp;gt; 6, which suggests a key role of LAEs in the epoch of reionization.

Pulsation-driven Mass Loss from Massive Stars behind Stellar Mergers in Metal-poor Dense Clusters

Abstract The recent discovery of high-redshift (z &gt; 6) supermassive black holes (SMBH) favors the formation of massive seed BHs in protogalaxies. One possible scenario is the formation of massive stars via runaway stellar collisions in a dense cluster, leaving behind massive BHs without significant mass loss. We study the pulsational instability of massive stars with the zero-age main-sequence (ZAMS) mass and metallicity , and discuss whether or not pulsation-driven mass loss prevents massive BH formation. In the MS phase, the pulsational instability excited by the ϵ-mechanism grows in . As the stellar mass and metallicity increase, the mass-loss rate increases to . In the red supergiant (RSG) phase, the instability is excited by the κ-mechanism operating in the hydrogen ionization zone and grows more rapidly in . The RSG mass-loss rate is almost independent of metallicity and distributes in the range of . Conducting stellar structure calculations including feedback due to pulsation-driven winds, we find that the stellar models of can leave behind remnant BHs more massive than . We conclude that massive merger products can seed monster SMBHs observed at z &gt; 6.

Mergers trigger active galactic nuclei out to z ∼ 0.6

Aims. The fueling and feedback of active galactic nuclei (AGNs) are important for understanding the co-evolution between black holes and host galaxies. Mergers are thought to have the capability to bring gas inward and ignite nuclear activity, especially for more powerful AGNs. However, there is still significant ongoing debate on whether mergers can trigger AGNs and, if they do, whether mergers are a significant triggering mechanism. Methods. We selected a low-redshift (0.005 &lt; z &lt; 0.1) sample from the Sloan Digital Sky Survey and a high-redshift (0 &lt; z &lt; 0.6) sample from the Galaxy And Mass Assembly survey. We took advantage of the convolutional neural network technique to identify mergers. We used mid-infrared (MIR) color cut and optical emission line diagnostics to classify AGNs. We also included low excitation radio galaxies (LERGs) to investigate the connection between mergers and low accretion rate AGNs. Results. We find that AGNs are more likely to be found in mergers than non-mergers, with an AGN excess up to 1.81 ± 0.16, suggesting that mergers can trigger AGNs. We also find that the fraction of mergers in AGNs is higher than that in non-AGN controls, for both MIR and optically selected AGNs, as well as LERGs, with values between 16.40 ± 0.5% and 39.23 ± 2.10%, implying a non-negligible to potentially significant role of mergers in triggering AGNs. This merger fraction in AGNs increases as stellar mass increases, which supports the idea that mergers are more important for triggering AGNs in more massive galaxies. In terms of merger fraction as a function of AGN power we find a positive trend for MIR selected AGNs and a complex trend for optically selected AGNs, which we interpret under an evolutionary scenario proposed by previous studies. In addition, obscured MIR selected AGNs are more likely to be hosted in mergers than unobscured MIR selected AGNs.

Evidence for a Young Stellar Population in Nearby Type 1 Active Galaxies

Abstract To understand the physical origin of the close connection between supermassive black holes (BHs) and their host galaxies, it is vital to investigate star formation properties in active galaxies. Using a large data set of nearby type 1 active galactic nuclei (AGNs) with detailed structural decomposition based on high-resolution optical images obtained with the Hubble Space Telescope, we study the correlation between BH mass and bulge luminosity and the (Kormendy) relation between bulge effective radius and surface brightness. In both relations, the bulges of type 1 AGNs tend to be more luminous than those of inactive galaxies with the same BH mass or the same bulge size. This suggests that the central regions of AGN host galaxies have characteristically lower mass-to-light ratios than inactive galaxies, most likely due to the presence of a younger stellar population in active systems. In addition, the degree of luminosity excess appears to be proportional to the accretion rate of the AGN, revealing a physical connection between stellar growth and BH growth. Adopting a simple toy model for the increase of stellar mass and BH mass, we show that the fraction of young stellar population flattens out toward high accretion rates, possibly reflecting the influence of AGN-driven feedback.

Refuelled and shielded – the early evolution of tidal dwarf galaxies

We present, for the first time, numerical simulations of young tidal dwarf galaxies (TDGs), including a self-consistent treatment of the tidal arm in which they are embedded. Thereby, we do not rely on idealised initial conditions, as the initial data of the presented simulation emerge from a galaxy interaction simulation. By comparing models which are either embedded in or isolated form the tidal arm, we demonstrate its importance on the evolution of TDGs, as additional source of gas which can be accreted and is available for subsequent conversion into stars. During the initial collapse of the proto-TDG, with a duration of a few 100 Myr, the evolution of the embedded and isolated TDGs are indistinguishable. Significant differences appear after the collapse has halted and the further evolution is dominated by the possible accretion of material form the surroundings of the TDGs. The inclusion of the tidal arm in the simulation of TDGs results in roughly a doubling of the gas mass ($M_\mathrm{gas}$) and gas fraction ($f_\mathrm{gas}$), an increase in stellar mass by a factor of 1.5 and a $\sim3$ times higher star formation rate (SFR) compared to the isolated case. Moreover, we perform a parametric study on the influence of different environmental effects, i.e. the tidal field and ram pressure. Due to the orbit of the chosen initial conditions, no clear impact of the environmental effects on the evolution of TDG candidates can be found.

A Controlled Study of Cold Dust Content in Galaxies from z = 0–2

Abstract At , the formation of new stars is dominated by dusty galaxies whose far-IR emission indicates they contain colder dust than local galaxies of a similar luminosity. We explore the reasons for the evolving IR emission of similar galaxies over cosmic time using (1) local galaxies from GOALS ( ), (2) galaxies at from 5MUSES ( ), and (3) IR luminous galaxies spanning from GOODS and Spitzer xFLS ( ). All samples have Spitzer mid-IR spectra, and Herschel and ground-based submillimeter imaging covering the full IR spectral energy distribution, allowing us to robustly measure , , and for every galaxy. Despite similar infrared luminosities, dusty star-forming galaxies (DSFG) have a factor of 5 higher dust masses and 5 K colder temperatures. The increase in dust mass is linked to an increase in the gas fractions with redshift, and we do not observe a similar increase in stellar mass or star formation efficiency. , a proxy for , is strongly correlated with independently of redshift. We measure merger classification and galaxy size for a subsample, and there is no obvious correlation between these parameters and or . In DSFG, the change in can fully account for the observed colder dust temperatures, suggesting that any change in the spatial extent of the interstellar medium is a second-order effect.

The Mass, Color, and Structural Evolution of Today’s Massive Galaxies Since z ∼ 5

Abstract In this paper, we use stacking analysis to trace the mass growth, color evolution, and structural evolution of present-day massive galaxies ( ) out to z = 5. We utilize the exceptional depth and area of the latest UltraVISTA data release, combined with the depth and unparalleled seeing of CANDELS to gather a large, mass-selected sample of galaxies in the NIR (rest-frame optical to UV). Progenitors of present-day massive galaxies are identified via an evolving cumulative number density selection, which accounts for the effects of merging to correct for the systematic biases introduced using a fixed cumulative number density selection, and find progenitors grow in stellar mass by since z = 5. Using stacking, we analyze the structural parameters of the progenitors and find that most of the stellar mass content in the central regions was in place by , and while galaxies continue to assemble mass at all radii, the outskirts experience the largest fractional increase in stellar mass. However, we find evidence of significant stellar mass build-up at beyond probing an era of significant mass assembly in the interiors of present-day massive galaxies. We also compare mass assembly from progenitors in this study to the EAGLE simulation and find qualitatively similar assembly with z at . We identify as a distinct epoch in the evolution of massive galaxies where progenitors transitioned from growing in mass and size primarily through in situ star formation in disks to a period of efficient growth in r e consistent with the minor merger scenario.

Galaxies in the Illustris simulation as seen by the Sloan Digital Sky Survey – II. Size–luminosity relations and the deficit of bulge-dominated galaxies in Illustris at low mass

The interpretive power of the newest generation of large-volume hydrodynamical simulations of galaxy formation rests upon their ability to reproduce the observed properties of galaxies. In this second paper in a series, we employ bulge+disc decompositions of realistic dust-free galaxy images from the Illustris simulation in a consistent comparison with galaxies from the Sloan Digital Sky Survey (SDSS). Examining the size-luminosity relations of each sample, we find that galaxies in Illustris are roughly twice as large and $0.7$ magnitudes brighter on average than galaxies in the SDSS. The trend of increasing slope and decreasing normalization of size-luminosity as a function of bulge-fraction is qualitatively similar to observations. However, the size-luminosity relations of Illustris galaxies are quantitatively distinguished by higher normalizations and smaller slopes than for real galaxies. We show that this result is linked to a significant deficit of bulge-dominated galaxies in Illustris relative to the SDSS at stellar masses $\log\mathrm{M}_{\star}/\mathrm{M}_{\odot}\lesssim11$. We investigate this deficit by comparing bulge fraction estimates derived from photometry \emph{and} internal kinematics. We show that photometric bulge fractions are systematically lower than the kinematic fractions at low masses, but with increasingly good agreement as the stellar mass increases.

On the OVI abundance in the circumgalactic medium of low-redshift galaxies

We analyze the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around $z\sim 0.2$ galaxies of mass $10^9 M_\odot <M_\bigstar < 10^{11.5} M_\odot$ in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column density profile of $\sim 0.5$ dex with each $0.5$ dex increase in stellar mass. Observed OVI column density profiles exhibit a weaker mass dependence than predicted: the simulated OVI abundance profiles are consistent with those observed for star-forming galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$, but underpredict the observed OVI abundances by $\gtrsim 0.8$ dex for lower-mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass halos, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$ are found to have 0.3-0.8 dex lower OVI column density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating the CGM and ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central galaxy's radiation field may enhance the photoionization of the CGM within $\sim 50$ kpc, further increasing the predicted OVI abundance around star-forming galaxies.