Burst Of Star Formation Research Articles

Star-forming and gas-rich brightest cluster galaxies at z ∼ 0.4 in the Kilo-Degree Survey

Brightest cluster galaxies (BCGs) are typically massive ellipticals at the centers of clusters. They are believed to experience strong environmental processing, and their mass assembly and star formation history are still debated. We have selected three star-forming BCGs in the equatorial field of the Kilo-Degree Survey (KiDS). They are KiDS 0920 (z = 0.3216), KiDS 1220 (z = 0.3886), and KiDS 1444 (z = 0.4417). We have observed them with the IRAM 30 m telescope in the first three CO transitions. We remarkably detected all BCGs at high signal-to-noise ratio, S/N ≃ (3.8 − 10.2), for a total of seven detected lines out of eight, corresponding to a success rate of 88%. This allows us to double the number of distant BCGs with clear detections in at least two CO lines. We then combined our observations with available stellar, star formation, and dust properties of the BCGs and compared them with a sample of ∼100 distant cluster galaxies with observations in CO. Our analysis yields large molecular gas reservoirs MH2 ≃ (0.5 − 1.4)×1011 M⊙, high excitation ratios r31 = LCO(3→2)′/LCO(1→0)′ ≃ (0.1 − 0.3), long depletion times τdep ≃ (2 − 4) Gyr, and high MH2/Mdust ≃ (170 − 300) for the three targeted BCGs. The excitation ratio r31 of intermediate-z BCGs, including RX1532 and M1932 from previous studies, appears to be well correlated with the star formation rate and efficiency, which suggests that excited gas is found only in highly star-forming and cool-core BCGs. By performing color-magnitude plots and a red-sequence modeling, we find that recent bursts of star formation are needed to explain the fact that the BCGs are measurably bluer than photometrically selected cluster members. To explain the global observed phenomenology, we suggest that a substantial amount of the molecular gas has been accreted by the KiDS BCGs but still not efficiently converted into stars. KiDS 1220 also shows a double-horn emission in CO(3→2), which implies a low gas concentration. The modeling of the spectrum yields an extended molecular gas reservoir of ∼9 kpc, which is reminiscent of the mature extended-disk phase observed in some local BCGs.

Comparison of star formation histories of AGN and non-AGN galaxies

We used active galactic nuclei (AGNs) with X-ray luminosities, LX, 2 − 10 keV ∼ 1042.5 − 44 erg s−1, from the COSMOS-Legacy survey that lie within the UltraVISTA region and cross-matched them with the LEGA-C catalogue. The latter provides measurements of the calcium break, Dn4000, and Hδ Balmer line that allow us to study the stellar populations of AGNs and compare them with a galaxy reference catalogue. Our samples consist of 69 AGNs and 2176 non-AGN systems, within 0.6 < z < 1.3, that satisfy the same photometric selection criteria. We constructed the spectral energy distributions (SEDs) of both population and used the CIGALE code to investigate the effect of the two indices in the SED fitting process. Our analysis shows that the inclusion of Dn4000 and Hδ allows CIGALE to better constrain the ages of the stellar populations. Furthermore, we find an increase of the estimated stellar masses by ∼0.2 dex, in particular for systems with young stars (Dn4000 < 1.5), when the two indices are included in the SED fitting. We then compare the Dn4000 and Hδ of AGNs with sources in the reference catalogue, accounting for the different stellar mass of the two populations. Our analysis reveals that low-to-moderate LX AGNs tend to reside in galaxies with older stellar populations and are less likely to have experienced a recent star formation burst compared to galaxies in the control sample. Finally, we compared the two populations as a function of their morphology (bulge-dominated, BD, versus non-BD) and compactness (mass-to-size ratio). A similar fraction of AGN and non-AGN systems are classified as non-BD (∼70%). Our analysis shows that BD AGN tend to have younger stellar populations compared to BD non-AGN systems. On the other hand, non-BD AGNs have, on average, older stellar populations and are less likely to have experienced a burst compared to non-BD sources in the reference sample. Furthermore, AGNs tend to prefer more compact systems compared to non-AGNs.

Two modes of LyC escape from bursty star formation: implications for [C ii] deficits and the sources of reionization

ABSTRACT We use the SPHINX20 cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR10) or 100 Myr (SFR100). Both have $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ and experienced an extreme burst of star formation, but Bursty Leakers have SFR10 > SFR100, while Remnant Leakers have SFR10 < SFR100. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H ii region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H ii region densities. Both types of leaker exhibit [C ii]158μm deficits on the [C ii]–SFR100 relation, while only Bursty Leakers show deficits when SFR10 is used. We predict that [C ii] luminosity and SFR indicators such as H α and M$_{\rm 1500\mathring{\rm A}}$ can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C ii] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with $f_{\rm esc}\lt 5{{\ \rm per\ cent}}$ becomes significant at the tail-end of reionization.

Chemical evolution of 26Al and 60Fe in the Milky Way

ABSTRACT We present theoretical mass estimates of 26Al and 60Fe throughout the Galaxy, performed with a numerical chemical evolution model including detailed nucleosynthesis prescriptions for stable and radioactive nuclides. We compared the results for several sets of stellar yields taken from the literature, for massive, low and intermediate mass stars, nova systems (only for 26Al) and supernovae Type Ia. We then computed the total masses of 26Al and 60Fe in the Galaxy. We studied the bulge and the disc of the Galaxy in a Galactocentric radius range of 0–22 kpc. We assumed that the bulge region (within 2 kpc) evolved quickly suffering a strong star formation burst, while the disc formed more slowly and inside–out. We compared our results with the 26Al mass observed by the gamma-ray surveys COMPTEL and INTEGRAL to select the best model. Concerning 60Fe, we do not have any observed mass value so we just performed a theoretical prediction for future observations. In conclusion, low, intermediate mass stars, and Type Ia supernovae contribute negligibly to the two isotopes, while massive stars are the dominant source. The nova contribution is, however, necessary to reproduce the observations of 26Al. Our best model predicts 2.12 M⊙ of 26Al, in agreement with observations, while for 60Fe our best mass estimate is ∼1.05 M⊙. We also predicted the present injection rate of 26Al and 60Fe in the Galaxy and compared it with previous results, and we found a larger present time injection rate along the disc.

Distance and age of the massive stellar cluster Westerlund 1 – II. The eclipsing binary W36

ABSTRACT Westerlund 1 (Wd 1) is one of the most relevant star clusters in the Milky Way to study massive star formation, although it is still poorly known. Here, we used photometric and spectroscopic data to model the eclipsing binary W36, showing that its spectral type is O6.5 III + O9.5 IV, hotter and more luminous than thought before. Its distance dW36 = 4.03 ± 0.25 kpc agrees, within the errors, with three recent Gaia-EDR3-based distances reported in Paper I, Beasor & Davies, and by Negueruela’s group. However, they follow different approaches to fix the zero-points for red sources such as those in Wd 1, and to select the best approach, we used an accurate modelling of W36. The weighted mean distance of our parallax (Paper I) and binary distances results in dwd1 = 4.05 ± 0.20 kpc, with an unprecedented accuracy of 5 per cent. We adopted isochrones based on the Geneva code with supersolar abundances to infer the age of W36B as 6.4 ± 0.7 Myr. This object seems to be part of the prolific star formation burst represented by OB giants and supergiants that occurred at 7.1 ± 0.5 Myr ago, which coincides with the recently published PMS isochrone with age 7.2 Myr. Other BA-type luminous evolved stars and yellow hypergiants spread in the age range of 8–11 Myr. The four red supergiants discussed in paper I represent the oldest population of the cluster with an age of 10.7 ± 1 Myr. The multiple episodes of star formation in Wd 1 are reminiscent of that reported for the R136/30 Dor LMC cluster.

The MOSDEF survey: a new view of a remarkable z = 1.89 merger

ABSTRACT We present a detailed study of a galaxy merger taking place at z = 1.89 in the GOODS-S field. Here, we analyse Keck/MOSFIRE spectroscopic observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey along with multiwavelength photometry assembled by the 3D-HST survey. The combined data set is modelled to infer the past star formation histories (SFHs) of both merging galaxies. They are found to be massive, with log10(M*/M⊙) > 11, with a close mass ratio satisfying the typical major-merger definition. Additionally, in the context of delayed-τ models, GOODS-S 43114, and GOODS-S 43683 have similar SFHs and low star formation rates (log10(SFR(SED)/${\rm M}_{\odot }\,\rm {yr}^{-1}$) < 1.0) compared to their past averages. The best-fitting model SEDs show elevated H δA values for both galaxies, indicating that their stellar spectra are dominated by A-type stars, and that star formation peaked ∼0.5−1 Gyr ago and has recently declined. Additionally, based on SED fitting both merging galaxies turned on and shut off star formation within a few hundred Myr of each other, suggesting that their bursts of star formation may be linked. Combining the SFHs and H δA results with recent galaxy merger simulations, we infer that these galaxies have recently completed their first pericentric passage and are moving apart. Finally, the relatively low second velocity moment of GOODS-S 43114, given its stellar mass suggests a disc-like structure. However, including the geometry of the galaxy in the modelling does not completely resolve the discrepancy between the dynamical and stellar masses. Future work is needed to resolve this inconsistency in mass.

Towards a fully consistent Milky Way disk model

Context. The semi-analytic Just-Jahreiß (JJ) model of the Galactic disk is a flexible tool for stellar population synthesis with a fine age resolution of 25 Myr. The model has recently been calibrated in the solar neighbourhood against the Gaia DR2 stars. We have identified two star-formation bursts within the last ∼4 Gyr of the local star-formation rate (SFR) history. Aims. In this work we present a generalised version of the JJ model that incorporates our findings for the solar neighbourhood and is applicable to a wide range of galactocentric distances, 4 kpc ≲R ≲ 14 kpc. Methods. The JJ model includes the four flattened and two spheroidal components of the Milky Way, describing it as an axisymmetric system. The thin and thick disks, as well as atomic and molecular gas layers, are assumed to have exponential radial surface density profiles. Spherical stellar halo and dark matter in the form of a cored isothermal sphere are also added to the model. The overall thickness of the thin disk is assumed to be constant at all radii, though model realisations with a flaring disk can also be tested. The adopted radial variation in the thin-disk SFR reflects the inside-out disk growth scenario. Motivated by our findings for the solar neighbourhood, we allow a smooth power-law SFR continuum to be modified by an arbitrary number of Gaussian peaks. Additionally, the vertical kinematics of the stellar populations associated with these episodes of star-formation excess is allowed to differ from the kinematics prescribed by the age-velocity dispersion relation for the thin-disk populations of the same age. Results. We present a public code of the JJ model complemented by the three sets of isochrones generated by the stellar tracks and isochrones with the PAdova and TRieste Stellar Evolution Code (PARSEC), the Modules and Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST), and a Bag of Stellar Tracks and Isochrones (BaSTI). Assuming a plausible set of parameters, we take the first step towards calibrating the JJ model at non-solar radii. Using metallicity distributions of the red clump giants from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), we constrain the radial variation of the JJ-model age-metallicity relation and propose a new analytic form for the age-metallicity relation function. Conclusions. The generalised JJ model is a publicly available tool for studying different stellar populations across the Milky Way disk. With its fine age resolution and flexibility, it can be particularly useful for reconstructing the thin-disk SFR, as a variety of different SFR shapes can be constructed within its framework.

Characterizing Extreme Emission Line Galaxies. II. A Self-consistent Model of Their Ionizing Spectrum* *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NASA 5-26555.

Observations of high-redshift galaxies (z > 5) have shown that these galaxies have extreme emission lines with equivalent widths much larger than their local star-forming counterparts. Extreme emission line galaxies (EELGs) in the nearby universe are likely analogs to galaxies during the Epoch of Reionization and provide nearby laboratories to understand the physical processes important to the early universe. We use Hubble Space Telescope/Cosmic Origins Spectrograph and Large Binocular Telescope/Multi-Object Double Spectrographs spectra to study two nearby EELGs, J104457 and J141851. The far-UV spectra indicate that these two galaxies contain stellar populations with ages ≲10 Myr and metallicities ≤0.15 Z ⊙. We use photoionization modeling to compare emission lines from models of single-age bursts of star formation to observed emission lines and find that the single-age bursts do not reproduce high-ionization lines including [O iii] or very-high-ionization lines like He ii or O iv]. Photoionization modeling using the stellar populations fit from the UV continuum similarly is not capable of reproducing the very-high-energy emission lines. We add a blackbody to the stellar populations fit from the UV continuum to model the necessary high-energy photons to reproduce the very-high-ionization lines of He ii and O iv]. We find that we need a blackbody of 80,000 K and ∼45%–55% of the luminosity from the blackbody and young stellar population to reproduce the very-high-ionization lines while simultaneously reproducing the low-, intermediate-, and high-ionization emission lines. Our self-consistent model of the ionizing spectra of two nearby EELGs indicates the presence of a previously unaccounted-for source of hard ionizing photons in reionization analogs.

The survey of planetary nebulae in Andromeda (M31)

Context. The Andromeda (M31) galaxy presents evidence of recent substantial mass accretion. This is unlike what has happened in the Milky Way, which has experienced a rather quiescent evolution. Aims. We use oxygen and argon abundances for planetary nebulae (PNe) with low internal extinction (progenitor ages of > 4.5 Gyr) and high extinction (progenitor ages < 2.5 Gyr), as well as those of the HII regions, to constrain the chemical enrichment and star formation efficiency in the thin and thicker discs of M31. Methods. The argon element is produced in larger fractions by Type Ia supernovae compared to oxygen. We find that the mean log(O/Ar) values of PNe as a function of their argon abundances, 12 + log(Ar/H), trace the interstellar medium (ISM) conditions at the time of birth of the M31 disc PN progenitors. Thus, the chemical enrichment and star formation efficiency information encoded in the [α/Fe] versus [Fe/H] distribution of stars is also imprinted in the oxygen-to-argon abundance ratio log(O/Ar) versus argon abundance for the nebular emissions of the different stellar evolution phases. We propose using the log(O/Ar) versus (12 + log(Ar/H)) distribution of PNe with different ages to constrain the star formation histories of the parent stellar populations in the thin and thicker M31 discs. Results. For the inner M31 disc (RGC < 14 kpc), the chemical evolution model that reproduces the mean log(O/Ar) values as a function of argon abundance for the high- and low-extinction PNe requires a second infall of metal-poorer gas during a gas-rich (wet) satellite merger. This wet merger triggered the burst of star formation seen by the PHAT survey in the M31 disc, ∼3 Gyr ago. A strong starburst is ongoing in the intermediate radial range (14 ≤ RGC ≤ 18 kpc). In the outer M31 disc (RGC > 18 kpc), the log(O/Ar) versus argon abundance distribution of the younger high-extinction PNe indicates that they too were formed in a burst, though mostly from the metal-poorer gas. Present-day HII regions show a range of oxygen-to-argon ratios, which is indicative of spatial variations and consistent with a present-day rainfall of metal-poorer gas onto the disc with different degrees of mixing with the previously enriched ISM. Conclusions. We implement the use of the log(O/Ar) versus argon abundance distribution for emission nebulae as a complement to the [α/Fe] versus [Fe/H] diagram for stars, and use it to constrain the star formation efficiency in the thin and thicker discs of M31. Diagrams for M31 PNe in different age ranges reveal that a secondary infall of gas affected the chemical evolution of the M31 thin disc. In M31, the thin disc is younger and less radially extended, formed stars at a higher star formation efficiency, and had a faster chemical enrichment timescale than the more extended thicker disc. Both the thin and thicker discs in M31 reach similar high argon abundances (12 + log(Ar/H)) ≃ 6.7. The chemical and structural properties of the thin and thicker discs in M31 are thus remarkably different from those determined for the Milky Way thin and thick discs.

Dust depletion of metals from local to distant galaxies

Large fractions of metals are missing from the observable gas-phase in the interstellar medium (ISM) because they are incorporated into dust grains. This phenomenon is called dust depletion. It is important to study the depletion of metals into dust grains in the ISM to investigate the origin and evolution of metals and cosmic dust. We characterize the dust depletion of several metals from the Milky Way to distant galaxies. We collected measurements of ISM metal column densities from absorption-line spectroscopy in the literature, and in addition, we determined Ti and Ni column densities from a sample of 70 damped Lyman-α absorbers (DLAs) toward quasars that were observed at high spectral resolution with the Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). We used relative ISM abundances to estimate the dust depletion of 18 metals (C, P, O, Cl, Kr, S, Ge, Mg, Si, Cu, Co, Mn, Cr, Ni, Al, Ti, Zn, and Fe) for different environments (the Milky Way, the Magellanic Clouds, and DLAs toward quasars and towards gamma-ray bursts). We observed overall linear relations between the depletion of each metal and the overall strength of the dust depletion, which we traced with the observed [Zn/Fe]. The slope of these dust depletion sequences correlates with the condensation temperature of the various elements, that is, the more refractory elements show steeper depletion sequences. In the neutral ISM of the Magellanic Clouds, small deviations from linearity are observed as an overabundance of the α-elements Ti, Mg, S, and an underabundance of Mn, including for metal-rich systems. The Ti, Mg, and Mn deviations completely disappear when we assume that all systems in our sample of OB stars observed toward the Magellanic Clouds have an α-element enhancement and Mn underabundance, regardless of their metallicity. This may imply that the Magellanic Clouds have recently been enriched in α-elements, potentially through recent bursts of star formation. We also observe an S overabundance in all local galaxies, which is an effect of ionization due to the contribution of their H ii regions to the measured S ii column densities. The observed strong correlations of the depletion sequences of the metals all the way from low-metallicity quasi-stellar object DLAs to the Milky Way suggest that cosmic dust has a common origin, regardless of the star formation history, which, in contrast, varies significantly between these different galaxies. This supports the importance of grain growth in the ISM as a significant process of dust production.

The merger fraction of post-starburst galaxies in UNIONS

ABSTRACT Post-starburst galaxies (PSBs) are defined as having experienced a recent burst of star formation, followed by a prompt truncation in further activity. Identifying the mechanism(s) causing a galaxy to experience a post-starburst phase therefore provides integral insight into the causes of rapid quenching. Galaxy mergers have long been proposed as a possible post-starburst trigger. Effectively testing this hypothesis requires a large spectroscopic galaxy survey to identify the rare PSBs as well as high-quality imaging and robust morphology metrics to identify mergers. We bring together these critical elements by selecting PSBs from the overlap of the Sloan Digital Sky Survey and the Canada–France Imaging Survey and applying a suite of classification methods: non-parametric morphology metrics such as asymmetry and Gini-M20, a convolutional neural network trained to identify post-merger galaxies, and visual classification. This work is therefore the largest and most comprehensive assessment of the merger fraction of PSBs to date. We find that the merger fraction of PSBs ranges from 19 per cent to 42 per cent depending on the merger identification method and details of the PSB sample selection. These merger fractions represent an excess of 3–46× relative to non-PSB control samples. Our results demonstrate that mergers play a significant role in generating PSBs, but that other mechanisms are also required. However, applying our merger identification metrics to known post-mergers in the IllustrisTNG simulation shows that 70 per cent of recent post-mergers (≲200 Myr) would not be detected. Thus, we cannot exclude the possibility that nearly all PSBs have undergone a merger in their recent past.

Evidence of deep mixing in IRS 7, a cool massive supergiant member of the Galactic nuclear star cluster

ABSTRACT The centre of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the nuclear star cluster (NSC), IRS 7, are presented for 19F, 12C, 13C, 14N, 16O, 17O, and Fe from a local thermodynamic equilibrium analysis based on spherical modelling and radiative transfer with a 25-M⊙ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both 12C (∼–0.8 dex) and 16O (∼–0.4 dex), while exhibiting an extremely enhanced 14N abundance (∼+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The 19F abundance is also heavily depleted by ∼1 dex relative to the baseline fluorine of the NSC, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abundances of the minor isotopes 13C and 17O are also derived, with ratios of 12C/13C ∼ 5.3 and 16O/17O ∼ 525. The derived abundances for IRS 7, in conjunction with previous abundance results for massive stars in the NSC, are compared with rotating and non-rotating models of massive stars and it is found that the IRS 7 abundances overall follow the behaviour predicted by stellar models. The depleted fluorine abundance in IRS 7 illustrates, for the first time, the potential of using the 19F abundance as a mixing probe in luminous red giants.

Arp 58 and Arp 68: two M 51-type systems

ABSTRACT We study two M 51-type systems Arp 68 and Arp 58, which strongly differ by their stellar masses, gas content, and environment. Long-slit spectral observations obtained at the Russian 6-m telescope were used to trace the distributions of a line-of-sight (LOS) velocity and a gas-phase oxygen abundance along the spectral cuts. Two systems are compared by their observed properties. We found a very strong large-scale non-circular motion of gas in both systems and a kpc-size saw-edged velocity profile along the tidal spiral arm of Arp 68, probably caused by the gas outflow due to the stellar feedback. A deep decrease of LOS velocity is also found in the ‘hinge’ region in Arp 58, where the inner spiral arm transforms into the tidal spiral arm, which was predicted earlier for M 51-type galaxies. Local sites of star formation and the satellites are compared with the evolutionary models at the colour–colour diagrams. Unlike the spiral galaxy Arp 58, the main galaxy in Arp 68 system is experiencing an ongoing burst of star formation. Gas-phase metallicity estimates show that Arp 58 has a higher metal abundance and reveals a shallow negative radial gradient of the gas-phase oxygen abundance. The emission gas in Arp 68 has noticeably lower metallicity than it is expected for a given luminosity of this galaxy, which may be connected with its space position in the local void.

Detection of companion galaxies around hot dust-obscured hyper-luminous galaxy W0410-0913

The phase transition between galaxies and quasars is often identified with the rare population of hyper-luminous, hot dust-obscured galaxies. Galaxy formation models predict these systems to grow via mergers, that can deliver large amounts of gas toward their centers, induce intense bursts of star formation and feed their supermassive black holes. Here we report the detection of 24 galaxies emitting Lyman-α emission on projected physical scales of about 400 kpc around the hyper-luminous hot dust-obscured galaxy W0410-0913, at redshift z = 3.631, using Very Large Telescope observations. While this indicates that W0410-0913 evolves in a very dense environment, we do not find clear signs of mergers that could sustain its growth. Data suggest that if mergers occurred, as models expect, these would involve less massive satellites, with only a moderate impact on the internal interstellar medium of W0410-0913, which is sustained by a rotationally-supported fast-rotating molecular disk, as Atacama Large Millimeter Array observations suggest.

Recovering the Star Formation Histories of Recently Quenched Galaxies: The Impact of Model and Prior Choices

Accurate models of the star formation histories (SFHs) of recently quenched galaxies can provide constraints on when and how galaxies shut down their star formation. The recent development of nonparametric SFH models promises the flexibility required to make these measurements. However, model and prior choices significantly affect derived SFHs, particularly for post-starburst galaxies (PSBs), which have sharp changes in their recent SFH. In this paper, we create mock PSBs, then use the Prospector SED fitting software to test how well four different SFH models recover key properties. We find that a two-component parametric model performs well for our simple mock galaxies, but is sensitive to model mismatches. The fixed- and flexible-bin nonparametric models included in Prospector are able to rapidly quench a major burst of star formation, but systematically underestimate the post-burst age by up to 200 Myr. We develop a custom SFH model that allows for additional flexibility in the recent SFH. Our flexible nonparametric model is able to constrain post-burst ages with no significant offset and just ∼90 Myr of scatter. Our results suggest that while standard nonparametric models are able to recover first-order quantities of the SFH (mass, SFR, average age), accurately recovering higher-order quantities (burst fraction, quenching time) requires careful consideration of model flexibility. These mock recovery tests are a critical part of future SFH studies. Finally, we show that our new, public SFH model is able to accurately recover the properties of mock star-forming and quiescent galaxies and is suitable for broader use in the SED fitting community. https://github.com/bd-j/prospector

On the accretion of a new group of galaxies on to Virgo – II. The effect of pre-processing on the stellar population content of dEs

ABSTRACT Using MUSE spectra, we investigate how pre-processing and accretion on to a galaxy cluster affect the integrated stellar population properties of dwarf early-type galaxies (dEs). We analyse a sample of nine dEs with stellar masses of $\rm \sim 10^9 \, M_\odot$, which were accreted (∼ 2–3 Gyr ago) on to the Virgo cluster as members of a massive galaxy group. We derive their stellar population properties, namely age, metallicity ([M/H]), and the abundance ratio of α elements ([α/Fe]), by fitting observed spectral indices with a robust, iterative procedure, and infer their star formation history (SFH) by means of full spectral fitting. We find that these nine dEs are more metal-poor (at the 2–3σ level) and significantly more α-enhanced than dEs in the Virgo and Coma clusters with similar stellar mass, clustercentric distance, and infall time. Moreover, for six dEs, we find evidence for a recent episode of star formation during or right after the time of accretion on to Virgo. We interpret the high [α/Fe] of our sample of dEs as the result of the previous exposure of these galaxies to an environment hostile to star formation, and/or the putative short burst of star formation they underwent after infall into Virgo. Our results suggest that the stellar population properties of low-mass galaxies may be the result of the combined effect of pre-processing in galaxy groups and environmental processes (such as ram-pressure triggering star formation) acting during the early phases of accretion on to a cluster.

The Black Hole Mass Function across Cosmic Time. II. Heavy Seeds and (Super)Massive Black Holes

This is the second paper in a series aimed at modeling the black hole (BH) mass function from the stellar to the (super)massive regime. In the present work, we focus on (super)massive BHs and provide an ab initio computation of their mass function across cosmic time. We consider two main mechanisms to grow the central BH that are expected to cooperate in the high-redshift star-forming progenitors of local massive galaxies. The first is the gaseous dynamical friction process, which can cause the migration toward the nuclear regions of stellar mass BHs originated during the intense bursts of star formation in the gas-rich host progenitor galaxy and the buildup of a central heavy BH seed, M • ∼ 103−5 M ⊙, within short timescales of ≲some 107 yr. The second mechanism is the standard Eddington-type gas disk accretion onto the heavy BH seed through which the central BH can become (super)massive, M • ∼ 106−10 M ⊙, within the typical star formation duration, ≲1 Gyr, of the host. We validate our semiempirical approach by reproducing the observed redshift-dependent bolometric AGN luminosity functions and Eddington ratio distributions and the relationship between the star formation and the bolometric luminosity of the accreting central BH. We then derive the relic (super)massive BH mass function at different redshifts via a generalized continuity equation approach and compare it with present observational estimates. Finally, we reconstruct the overall BH mass function from the stellar to the (super)massive regime over more than 10 orders of magnitudes in BH mass.

Detection of nitrogen and oxygen in a galaxy at the end of reionization

Abstract We present observations of [N ii] 205 μm, [O iii] 88 μm, and dust emission in a strongly-lensed, submillimeter galaxy (SMG) at z = 6.0, G09.83808, with the Atacama Large Millimeter/submillimeter Array (ALMA). Both [N ii] and [O iii] line emissions are detected at >12σ in the ${0{^{\prime \prime}_{.}}8}$-resolution maps. Lens modeling indicates that the spatial distribution of the dust continuum emission is well characterized by a compact disk with an effective radius of 0.64 ± 0.02 kpc and a high infrared surface brightness of ΣIR = (1.8 ± 0.3) × 1012 L⊙ kpc−2. This result supports that G09.83808 is the progenitor of compact quiescent galaxies at z ∼ 4, where the majority of its stars are expected to be formed through a strong and short burst of star formation. G09.83808 and other lensed SMGs show a decreasing trend in the [N ii] line to infrared luminosity ratio with increasing continuum flux density ratio between 63 and 158 μm, as seen in local luminous infrared galaxies (LIRGs). The decreasing trend can be reproduced by photoionization models with increasing ionization parameters. Furthermore, by combining the [N ii]/[O iii] luminosity ratio with far-infrared continuum flux density ratio in G09.83808, we infer that the gas phase metallicity is already Z ≈ 0.5–0.7 Z⊙. G09.83808 is likely one of the earliest galaxies that has been chemically enriched at the end of reionization.

The Compact Structures of Massive z ∼ 0.7 Post-starburst Galaxies in the Sample

We present structural measurements of 145 spectroscopically selected intermediate-redshift (z ∼ 0.7), massive (M ⋆ ∼ 1011 M ⊙) post-starburst galaxies from the sample measured using wide-depth Hyper Suprime-Cam i-band imaging. This deep imaging allows us to probe the sizes and structures of these galaxies, which we compare to a control sample of star-forming and quiescent galaxies drawn from the LEGA-C Survey. We find that post-starburst galaxies systematically lie ∼0.1 dex below the quiescent mass–size (half-light radius) relation, with a scatter of ∼0.2 dex. This finding is bolstered by nonparametric measures, such as the Gini coefficient and the concentration, which also reveal these galaxies to have more compact light profiles than both quiescent and star-forming populations at similar mass and redshift. The sizes of post-starburst galaxies show either negative or no correlation with the time since quenching, such that more recently quenched galaxies are larger or similarly sized. This empirical finding disfavors the formation of post-starburst galaxies via a purely central burst of star formation that simultaneously shrinks the galaxy and shuts off star formation. We show that the central densities of post-starburst and quiescent galaxies at this epoch are very similar, in contrast with their effective radii. The structural properties of z ∼ 0.7 post-starburst galaxies match those of quiescent galaxies that formed in the early universe, suggesting that rapid quenching in the present epoch is driven by a similar mechanism to the one at high redshift.

Stellar populations of a sample of far-infrared AGN and non-AGN green valley galaxies

ABSTRACT We present a study on the stellar populations and stellar ages of a subsample of far-infrared (FIR) active galactic nucleus (AGN) and non-AGN green valley galaxies at ${0.6\, \lt \, z\, \lt \, 1.0}$ using the data from the Cosmological Evolution Survey field. We used long-slit spectroscopy and derived stellar populations and stellar ages using the stellar population synthesis code ‘starlight’ and analysed the available Lick/IDS indices, such as Dn4000 and HδA. We find that both FIR AGN and non-AGN green valley galaxies are dominated by intermediate stellar populations (67 per cent and 53 per cent, respectively). The median stellar ages for AGNs and non-AGNs are ${\log t\, =\, 8.5}$ and ${\, 8.4\, \rm yr}$, respectively. We found that majority of our sources (62 per cent of AGNs and 66 per cent of non-AGNs) could have experienced bursts and continuous star formation (SF). In addition, most of our FIR AGNs (38 per cent) compared to FIR non-AGNs (27 per cent) might have experienced a burst of SF more than 0.1 Gyr ago. We also found that our FIR AGN and non-AGN green valley galaxies have similar quenching time-scales of ∼ 70 Myr. Therefore, the results obtained here are in line with our previous results where we do not find that our sample of FIR AGNs in the green valley shows signs of negative AGN feedback, as has been suggested previously in optical studies.