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DOE OSTI2021

A Kinematic Perspective on the Formation Process of the Stellar Groups in the Rosette Nebula

Stellar kinematics is a powerful tool for understanding the formation process of stellar associations. Here, we present a kinematic study of the young stellar population in the Rosette nebula using recent Gaia data and high-resolution spectra. We first isolate member candidates using the published mid-infrared photometric data and the list of X-ray sources. A total of 403 stars with similar parallaxes and proper motions are finally selected as members. The spatial distribution of the members shows that this star-forming region is highly substructured. The young open cluster NGC 2244 in the center of the nebula has a pattern of radial expansion and rotation. We discuss its implication on the cluster formation, e.g., monolithic cold collapse or hierarchical assembly. On the other hand, we also investigate three groups located around the border of the H ii bubble. The western group seems to be spatially correlated with the adjacent gas structure, but their kinematics is not associated with that of the gas. The southern group does not show any systematic motion relative to NGC 2244. These two groups might be spontaneously formed in filaments of a turbulent cloud. The eastern group is spatially and kinematically associated with the gas pillar receding away from NGC 2244. This group might be formed by feedback from massive stars in NGC 2244. Our results suggest that the stellar population in the Rosette Nebula may form through three different processes: the expansion of stellar clusters, hierarchical star formation in turbulent clouds, and feedback-driven star formation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Gas fraction and depletion time of massive star-forming galaxies at z ∼ 3.2: no change in global star formation process out to z > 3

The observed evolution of the gas fraction and its associated depletion time in main-sequence (MS) galaxies provides insights on how star formation proceeds over cosmic time. We report ALMA detections of the rest-frame ∼300 μm continuum observed at 240 GHz for 45 massive (〈log(M{sub ⋆}(M{sub ⊙}))〉=10.7), normal star-forming (〈log(sSFR(yr{sup −1}))〉=−8.6), i.e., MS, galaxies at z≈3.2 in the COSMOS field. From an empirical calibration between cold neutral, i.e., molecular and atomic, gas mass M{sub gas} and monochromatic (rest-frame) infrared luminosity, the gas mass for this sample is derived. Combined with stellar mass M{sub ⋆} and star formation rate (SFR) estimates (from MagPhys fits) we obtain a median gas fraction of μ{sub gas}=M{sub gas}/M{sub ⋆}=1.65{sub −0.19}{sup +0.18} and a median gas depletion time t{sub depl.}(Gyr)=M{sub gas/SFR}=0.68{sub −0.08}{sup +0.07}; correction for the location on the MS will only slightly change the values. The reported uncertainties are the 1σ error on the median. Our results are fully consistent with the expected flattening of the redshift evolution from the 2-SFM (2 star formation mode) framework which empirically prescribes the evolution assuming a universal, log-linear relation between SFR and gas mass coupled to the redshift evolution of the specific star formation rate (sSFR) of MS galaxies. While t{sub depl.} shows only a mild dependence on location within the MS, a clear trend of increasing μ{sub gas} across the MS is observed (as known from previous studies). Further, we comment on trends within the MS and (in)consistencies with other studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Demonstrating diversity in star-formation histories with the CSI survey

We present coarse but robust star-formation histories (SFHs) derived from spectrophotometric data of the Carnegie-Spitzer-IMACS Survey, for 22,494 galaxies at 0.3<z<0.9 with stellar masses of 10{sup 9} M {sub ⊙} to 10{sup 12} M {sub ⊙}. Our study moves beyond “average” SFHs and distribution functions of specific star-formation rates (sSFRs) to individually measured SFHs for tens of thousands of galaxies. By comparing star-formation rates (SFRs) with timescales of 10{sup 10},10{sup 9}, and 10{sup 8} years, we find a wide diversity of SFHs: “old galaxies” that formed most or all of their stars early, galaxies that formed stars with declining or constant SFRs over a Hubble time, and genuinely “young galaxies” that formed most of their stars since z = 1. This sequence is one of decreasing stellar mass, but remarkably, each type is found over a mass range of a factor of 10. Conversely, galaxies at any given mass follow a wide range of SFHs, leading us to conclude that (1) halo mass does not uniquely determine SFHs, (2) there is no “typical” evolutionary track, and (3) “abundance matching” has limitations as a tool for inferring physics. Our observations imply that SFHs are set at an early epoch, and that—for most galaxies—the decline and cessation of star formation occurs over a Hubble time, without distinct “quenching” events. SFH diversity is inconsistent with models where galaxy mass, at any given epoch, grows simply along relations between SFR and stellar mass, but is consistent with a two-parameter lognormal form, lending credence to this model from a new and independent perspective.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Modeling for Stellar Feedback in Galaxy Formation Simulations

Various heuristic approaches to model unresolved supernova (SN) feedback in galaxy formation simulations exist to reproduce the formation of spiral galaxies and the overall inefficient conversion of gas into stars. Some models, however, require resolution-dependent scalings. We present a subresolution model representing the three major phases of supernova blast wave evolution—free expansion, energy-conserving Sedov–Taylor, and momentum-conserving snowplow—with energy scalings adopted from high-resolution interstellar-medium simulations in both uniform and multiphase media. We allow for the effects of significantly enhanced SN remnant propagation in a multiphase medium with the cooling radius scaling with the hot volume fraction, f{sub hot}, as (1−f{sub hot}){sup −4/5}. We also include winds from young massive stars and AGB stars, Strömgren sphere gas heating by massive stars, and a mechanism that limits gas cooling that is driven by radiative recombination of dense H ii regions. We present initial tests for isolated Milky Way-like systems simulated with the Gadget-based code SPHgal with improved SPH prescription. Compared to pure thermal SN input, the model significantly suppresses star formation at early epochs, with star formation extended both in time and space in better accord with observations. Compared to models with pure thermal SN feedback, the age at which half the stellar mass is assembled increases by a factor of 2.4, and the mass-loading parameter and gas outflow rate from the galactic disk increase by a factor of 2. Simulation results are converged for a variation of two orders of magnitude in particle mass in the range (1.3–130) × 10{sup 4} solar masses.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Insights from Synthetic Star-forming Regions. III. Calibration of Measurement and Techniques of Star Formation Rates

Through an extensive set of realistic synthetic observations (produced in Paper I), we assess in this part of the paper series (Paper III) how the choice of observational techniques affects the measurement of star formation rates (SFRs) in star-forming regions. We test the accuracy of commonly used techniques and construct new methods to extract the SFR, so that these findings can be applied to measure the SFR in real regions throughout the Milky Way. We investigate diffuse infrared SFR tracers such as those using 24 μ m, 70 μ m and total infrared emission, which have been previously calibrated for global galaxy scales. We set up a toy model of a galaxy and show that the infrared emission is consistent with the intrinsic SFR using extra-galactic calibrated laws (although the consistency does not prove their reliability). For local scales, we show that these techniques produce completely unreliable results for single star-forming regions, which are governed by different characteristic timescales. We show how calibration of these techniques can be improved for single star-forming regions by adjusting the characteristic timescale and the scaling factor and give suggestions of new calibrations of the diffuse star formation tracers. We show that star-forming regions that are dominated by high-mass stellar feedback experience a rapid drop in infrared emission once high-mass stellar feedback is turned on, which implies different characteristic timescales. Moreover, we explore the measured SFRs calculated directly from the observed young stellar population. We find that the measured point sources follow the evolutionary pace of star formation more directly than diffuse star formation tracers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Complex Organic Molecules Formation in Space Through Gas Phase Reactions: A Theoretical Approach

Chemistry in the interstellar medium (ISM) is capable of producing complex organic molecules (COMs) of great importance to astrobiology. Gas phase and grain surface chemistry almost certainly both contribute to COM formation. Amino acids as building blocks of proteins are some of the most interesting COMs. The simplest one, glycine, has been characterized in meteorites and comets and, its conclusive detection in the ISM seems to be highly plausible. In this work, we analyze the gas phase reaction of glycine and CH{sub 5}{sup +} to establish the role of this process in the formation of alanine or other COMs in the ISM. Formation of protonated α- and β-alanine in spite of being exothermic processes is not viable under interstellar conditions because the different paths leading to these isomers present net activation energies. Nevertheless, glycine can evolve to protonated 1-imide-2, 2-propanediol, protonated amino acetone, protonated hydroxyacetone, and protonated propionic acid. However, formation of acetic acid and protonated methylamine is also a favorable process and therefore will be a competitive channel with the evolution of glycine to COMs.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Triggering the Formation of Direct Collapse Black Holes by Their Congeners

Direct collapse black holes (DCBHs) are excellent candidates for seeds of supermassive black holes observed at z ≳ 6. The formation of a DCBH requires a strong external radiation field to suppress H{sub 2} formation and cooling in a collapsing gas cloud. Such a strong field is not easily achieved by first stars or normal star-forming galaxies. Here we investigate a scenario in which a previously formed DCBH can provide the necessary radiation field for the formation of additional ones. Using a one-zone model and simulated DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas initially having column density N {sub H}, we derive the critical field intensity, J{sub LW}{sup crit}, to suppress H{sub 2} formation and cooling. For the SED model with N{sub H}=1.3×10{sup 25} cm{sup −2}, 8.0 × 10{sup 24} cm{sup −2}, and 5.0 × 10{sup 24} cm{sup −2}, we obtain J{sub LW}{sup crit}≈22, 35, and 54, all much smaller than the critical field intensity for normal star-forming galaxies J{sub LW}{sup crit}≳1000 X-ray photons from previously formed DCBHs build up a high-z X-ray background (XRB) that may boost the J{sub LW}{sup crit}. However, we find that in the three SED models, J{sub LW}{sup crit} only increases to ≈80, 170, and 390, even when ρ-dot {sub ∙} reaches the maximum value allowed by the present-day XRB level (0.22, 0.034, 0.006 M {sub ⊙} yr{sup −1} Mpc{sup −3}), which is still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase J{sub LW}{sup crit}, we conclude that our investigation supports a scenario in which DCBHs may be more abundant than predicted by models only including galaxies as external radiation sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Shock Dynamics in Stellar Outbursts. I. Shock Formation

Wave-driven outflows and non-disruptive explosions have been implicated in pre-supernova outbursts, supernova impostors, luminous blue variable eruptions, and some narrow-line and superluminous supernovae. To model these events, we investigate the dynamics of stars set in motion by strong acoustic pulses and wave trains, focusing on nonlinear wave propagation, shock formation, and an early phase of the development of a weak shock. We identify the shock formation radius, showing that a heuristic estimate based on crossing characteristics matches an exact expansion around the wave front and verifying both with numerical experiments. Our general analytical condition for shock formation applies to one-dimensional motions within any static environment, including both eruptions and implosions. We also consider the early phase of shock energy dissipation. We find that waves of super-Eddington acoustic luminosity always create shocks, rather than damping by radiative diffusion. Therefore, shock formation is integral to super-Eddington outbursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Non-potential Field Formation in the X-shaped Quadrupole Magnetic Field Configuration

Some types of solar flares are observed in X-shaped quadrupolar field configuration. To understand the magnetic energy storage in such a region, we studied non-potential field formation in an X-shaped quadrupolar field region formed in the active region NOAA 11967, which produced three X-shaped M-class flares on 2014 February 2. Nonlinear force-free field modeling was applied to a time series of vector magnetic field maps from the Solar Optical Telescope on board Hinode and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory . Our analysis of the temporal three-dimensional magnetic field evolution shows that the sufficient free energy had already been stored more than 10 hr before the occurrence of the first M-class flare and that the storage was observed in a localized region. In this localized region, quasi-separatrix layers (QSLs) started to develop gradually from 9 hr before the first M-class flare. One of the flare ribbons that appeared in the first M-class flare was co-spatial with the location of the QSLs, suggesting that the formation of the QSLs is important in the process of energy release. These QSLs do not appear in the potential field calculation, indicating that they were created by the non-potential field. The formation of the QSLs was associated with the transverse photospheric motion of the pre-emerged flux and the emergence of a new flux. This observation indicates that the occurrence of the flares requires the formation of QSLs in the non-potential field in which free magnetic energy is stored in advance.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Formation and Atmosphere of Complex Organic Molecules of the HH 212 Protostellar Disk

HH 212 is a nearby (400 pc) Class 0 protostellar system recently found to host a “hamburger”-shaped dusty disk with a radius of ∼60 au, deeply embedded in an infalling-rotating flattened envelope. We have spatially resolved this envelope-disk system with the Atacama Large Millimeter/submillimeter Array at up to ∼16 au (0.″04) resolution. The envelope is detected in HCO{sup +} J = 4–3 down to the dusty disk. Complex organic molecules (COMs) and doubly deuterated formaldehyde (D{sub 2}CO) are detected above and below the dusty disk within ∼40 au of the central protostar. The COMs are methanol (CH{sub 3}OH), deuterated methanol (CH{sub 2}DOH), methyl mercaptan (CH{sub 3}SH), and formamide (NH{sub 2}CHO, a prebiotic precursor). We have modeled the gas kinematics in HCO{sup +} and COMs and found a centrifugal barrier (CB) at a radius of ∼44 au, within which a Keplerian rotating disk is formed. This indicates that HCO{sup +} traces the infalling-rotating envelope down to the CB and COMs trace the atmosphere of a Keplerian rotating disk within the CB. The COMs are spatially resolved for the first time, both radially and vertically, in the atmosphere of a disk in the earliest, Class 0 phase of star formation. Our spatially resolved observations of COMs favor their formation in the disk rather than a rapidly infalling (warm) inner envelope. The abundances and spatial distributions of the COMs provide strong constraints on models of their formation and transport in low-mass star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Star Formation at z = 2.481 in the Lensed Galaxy SDSS J1110+6459. II. What is Missed at the Normal Resolution of the Hubble Space Telescope?

For lensed galaxy SGAS J111020.0+645950.8 at redshift z = 2.481, which is magnified by a factor of 28 ± 8, we analyze the morphology of star formation, as traced by rest-frame ultraviolet emission, in both the highly magnified source plane and simulations of how this galaxy would appear without lensing magnification. Were this galaxy not lensed, but rather drawn from a Hubble Space Telescope deep field, we would conclude that almost all its star formation arises from an exponential disk (Sérsic index of 1.0 ± 0.4) with an effective radius of r{sub e}=2.7±0.3 kpc measured from two-dimensional fitting to F606W using Galfit, and r{sub e}=1.9±0.1 kpc measured by fitting a radial profile to F606W elliptical isophotes. At the normal spatial resolution of the deep fields, there is no sign of clumpy star formation within SGAS J111020.0+645950.8. However, the enhanced spatial resolution enabled by gravitational lensing tells a very different story; much of the star formation arises in two dozen clumps with sizes of r = 30–50 pc spread across the 7 kpc length of the galaxy. The color and spatial distribution of the diffuse component suggests that still-smaller clumps are unresolved. Despite this clumpy, messy morphology, the radial profile is still well-characterized by an exponential profile. In this lensed galaxy, stars are forming in complexes with sizes well below 100 pc; such sizes are wholly unexplored by surveys of galaxy evolution at 1<z<3.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Supermassive Black Holes as the Regulators of Star Formation in Central Galaxies

We present the relationship between the black hole mass, stellar mass, and star formation rate (SFR) of a diverse group of 91 galaxies with dynamically measured black hole masses. For our sample of galaxies with a variety of morphologies and other galactic properties, we find that the specific SFR is a smoothly decreasing function of the ratio between black hole mass and stellar mass, or what we call the specific black hole mass. In order to explain this relation, we propose a physical framework where the gradual suppression of a galaxy’s star formation activity results from the adjustment to an increase in specific black hole mass, and accordingly, an increase in the amount of heating. From this framework, it follows that at least some galaxies with intermediate specific black hole masses are in a steady state of partial quiescence with intermediate specific SFRs, implying that both transitioning and steady-state galaxies live within this region that is known as the “green valley.” With respect to galaxy formation models, our results present an important diagnostic with which to test various prescriptions of black hole feedback and its effects on star formation activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Methanol Formation via Oxygen Insertion Chemistry in Ices

We present experimental constraints on the insertion of oxygen atoms into methane to form methanol in astrophysical ice analogs. In gas-phase and theoretical studies this process has previously been demonstrated to have a very low or nonexistent energy barrier, but the energetics and mechanisms have not yet been characterized in the solid state. We use a deuterium UV lamp filtered by a sapphire window to selectively dissociate O{sub 2} within a mixture of O{sub 2}:CH{sub 4} and observe efficient production of CH{sub 3}OH via O({sup 1}D) insertion. CH{sub 3}OH growth curves are fit with a kinetic model, and we observe no temperature dependence of the reaction rate constant at temperatures below the oxygen desorption temperature of 25 K. Through an analysis of side products we determine the branching ratio of ice-phase oxygen insertion into CH{sub 4}: ∼65% of insertions lead to CH{sub 3}OH, with the remainder leading instead to H{sub 2}CO formation. There is no evidence for CH{sub 3} or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity. CH{sub 3}OH formation from O{sub 2} and CH{sub 4} diluted in a CO-dominated ice similarly shows no temperature dependence, consistent with expectations that insertion proceeds with a small or nonexistent barrier. Oxygen insertion chemistry in ices should therefore be efficient under low-temperature ISM-like conditions and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Cloud-scale ISM Structure and Star Formation in M51

We compare the structure of molecular gas at 40 pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370 pc and 1.1 kpc resolution elements, and within each we estimate the molecular gas depletion time (τ{sub Dep}{sup mol}), the star-formation efficiency per free-fall time (ϵ{sub ff}), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Σ, line width, σ, and b≡Σ/σ{sup 2}∝α{sub vir}{sup −1}, a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star-formation efficiency per free-fall time, ϵ{sub ff}(〈Σ{sub 40pc}〉)∼0.3%--0.36%, lower than adopted in many models and found for local clouds. Furthermore, the efficiency per free-fall time anti-correlates with both Σ and σ, in some tension with turbulent star-formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is b≡Σ/σ{sup 2}, tracing the strength of self-gravity, with τ{sub Dep}{sup mol}∝b{sup −0.9}. The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low τ{sub Dep}{sup mol}). The different regions of the galaxy mostly overlap in τ{sub Dep}{sup mol} as a function of b, so that low b explains the surprisingly high τ{sub Dep}{sup mol} found toward the inner spiral arms found by Meidt et al. (2013).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Star Formation Activity of Barred Spiral Galaxies

We study the star formation activity of nearby galaxies with bars using a sample of late-type galaxies at 0.02⩽z⩽0.05489 and M{sub r}<−19.5 from the Sloan Digital Sky Survey. We compare the physical properties of strongly and weakly barred galaxies with those of non-barred galaxies that have stellar mass and redshift distributions similar to barred galaxies. We find that the star formation activity of strongly barred galaxies probed by starburstiness, g−r, NUV−r, and mid-infrared [3.4]–[12] colors is, on average, lower than that of non-barred galaxies. However, weakly barred galaxies do not show such a difference between barred and non-barred galaxies. The amounts of atomic and molecular gas in strongly barred galaxies are smaller than those in non-barred galaxies, and the gas metallicity is higher in strongly barred galaxies than in non-barred galaxies. The gas properties of weakly barred galaxies again show no difference from those of non-barred galaxies. We stack the optical spectra of barred and non-barred galaxies in several mass bins and fit to the stacked spectra with a spectral fitting code, STARLIGHT. We find no significant difference in stellar populations between barred and non-barred galaxies for both strongly and weakly barred galaxies. Our results are consistent with the idea that the star formation activity of barred galaxies was enhanced in the past along with significant gas consumption, and is currently lower than or similar to that of non-barred galaxies. The past star formation enhancement depends on the strength of bars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Formation of Formaldehyde on Interstellar Carbonaceous Grain Analogs by O/H Atom Addition

An understanding of possible scenarios for the formation of astrophysically relevant molecules, particularly complex organic molecules, will bring us one step closer to the understanding of our astrochemical heritage. In this context, formaldehyde is an important molecule as a precursor of methanol, which in turn is a starting point for the formation of more complex organic species. In the present experiments, for the first time, following the synthesis of CO, formaldehyde has been produced on the surface of interstellar grain analogs, hydrogenated fullerene-like carbon grains, by O and H atom bombardment. The formation of H{sub 2}CO is an indication for a possible methanol formation route in such systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

A Fundamental Test for Galaxy Formation Models: Matching the Lyman- α Absorption Profiles of Galactic Halos Over Three Decades in Distance

Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflow from the IGM and outflows from supernovae and/or AGN feedback. The average Lyα absorption profile around galactic halos represents a powerful tool to probe their gaseous environments. We compare predictions from Illustris and Nyx hydrodynamical simulations with the observed absorption around foreground quasars, damped Lyα systems, and Lyman-break galaxies. We show how large-scale BOSS and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance 20 kpc ≲ b ≲ 20 Mpc. Far from galaxies, ≲ 2 Mpc, the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the ΛCDM model successfully describes the ambient IGM and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations, are present on scales 20 kpc ≲ b ≲ 20 Mpc, illustrating the challenges of accurately modeling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as ~2 Mpc, indicating that the "sphere of influence" of galaxies could extend to approximately ~7 times the halo virial radius. Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. We demonstrate that the Lyα absorption profile is primarily sensitive to the underlying temperature-density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

The Demographics, Stellar Populations, and Star Formation Histories of Fast Radio Burst Host Galaxies: Implications for the Progenitors

We present a comprehensive catalog of observations and stellar population properties for 23 highly secure host galaxies of fast radio bursts (FRBs). Our sample comprises 6 repeating FRBs and 17 apparent nonrepeaters. We present 82 new photometric and 8 new spectroscopic observations of these hosts. Using stellar population synthesis modeling and employing nonparametric star formation histories (SFHs), we find that FRB hosts have a median stellar mass of ≈10 9.9 M ⊙ , mass-weighted age ≈5.1 Gyr, and ongoing star formation rate ≈1.3 M ⊙ yr -1 but span wide ranges in all properties. Classifying the hosts by degree of star formation, we find that 87% (20 of 23 hosts) are star-forming, two are transitioning, and one is quiescent. The majority trace the star-forming main sequence of galaxies, but at least three FRBs in our sample originate in less-active environments (two nonrepeaters and one repeater). Across all modeled properties, we find no statistically significant distinction between the hosts of repeaters and nonrepeaters. However, the hosts of repeating FRBs generally extend to lower stellar masses, and the hosts of nonrepeaters arise in more optically luminous galaxies. While four of the galaxies with the clearest and most prolonged rises in their SFHs all host repeating FRBs, demonstrating heightened star formation activity in the last ≲100 Myr, one nonrepeating host shows this SFH as well. Our results support progenitor models with short delay channels (i.e., magnetars formed via core-collapse supernova) for most FRBs, but the presence of some FRBs in less-active environments suggests a fraction form through more delayed channels.

79 ASTRONOMY AND ASTROPHYSICS↗