Deep Hubble Space Telescope Research Articles

Surface Brightness Evolution of Galaxies in the CANDELS GOODS Fields up to z ∼ 6: High-z Galaxies Are Unique or Remain Undetected

Abstract We investigate the rest-frame ultraviolet (UV, λ ∼ 2000 Å) surface brightness (SB) evolution of galaxies up to z ∼ 6 using a variety of deep Hubble Space Telescope (HST) imaging. UV SB is a measure of the density of emission from mostly young stars and correlates with an unknown combination of star formation rate, initial mass function, cold gas mass density, dust attenuation, and the size evolution of galaxies. In addition to physical effects, the SB is, unlike magnitude, a more direct way in which a galaxy’s detectability is determined. We find a very strong evolution in the intrinsic SB distribution that declines as , decreasing by 4–5 mag arcsec−2 between z = 6 and z = 1. This change is much larger than expected in terms of the evolution in UV luminosity, sizes, or dust extinction, and we demonstrate that this evolution is “unnatural” and due to selection biases. We also find no strong correlation between mass and UV SB. Thus, deep HST imaging is unable to discover all of the most massive galaxies in the distant universe. Through simulations we show that only ∼15% of galaxies that we can detect at z = 2 would be detected at high z. We furthermore explore possible origins of high-SB galaxies at high z by investigating the relationship between intrinsic SB and star formation rates. We conclude that ultra–high-SB galaxies are produced by very gas-rich dense galaxies that are in a unique phase of evolution, possibly produced by mergers. Analogs of such galaxies do not exist in the relatively nearby universe.

The Elusive Distance Gradient in the Ultrafaint Dwarf Galaxy Hercules: A Combined Hubble Space Telescope and Gaia View

Abstract The ultrafaint dwarf galaxy Hercules has an extremely elongated morphology with both photometric overdensities and kinematic members at large radii, suggesting that it may be tidally disrupting due to a previous close encounter with the Milky Way. To explain its observational peculiarities, we present a deep Hubble Space Telescope (HST) imaging study of Hercules and its surrounding regions and investigate its tidal history through a careful search for a distance gradient along its stretched body. Our off-center HST data clearly resolve a main sequence, showing that the stellar extension seen along the major axis of Hercules is genuine, not a clump of background galaxies. Utilizing Gaia DR2 data, we clean the region around the Hercules of field contamination, and find four new plausible member stars, all of which are located on the outskirts of the dwarf galaxy. We update the distance to Hercules, and find 130.6 ± 6.1 kpc ( ) for the main body, which is consistent with earlier estimates in the literature. While we find no conclusive evidence for a distance gradient, our work demonstrates that constraining a distance gradient in such a faint system is not trivial, and the possible thickness of the dwarf along the line of sight and field contamination make it harder to make decisive conclusions even with these high-precision data. Future studies coupled with tailored theoretical models are needed to understand the true nature of Hercules and of tidal distortion observables in ultrafaint galaxies in general.

Discovery of molecular gas fueling galaxy growth in a protocluster at z = 1.7

Based on ALMA Band 3 observations of the CO(2→1) line transition, we report the discovery of three new gas-rich (MH2 ∼ 1.5 − 4.8 × 1010 M⊙) galaxies in an overdense region at z = 1.7 that already contains eight spectroscopically confirmed members. This leads to a total of 11 confirmed overdensity members within a projected distance of ∼1.15 Mpc and in a redshift range of Δz = 0.012. Under simple assumptions, we estimate that the system has a total mass of ≥3 − 6 × 1013 M⊙, and show that it will likely evolve into a ≳1014 M⊙ cluster at z = 0. The overdensity includes a powerful Compton-thick Fanaroff-Riley type II (FRII) radio galaxy, around which we discovered a large molecular gas reservoir (MH2 ∼ 2 × 1011 M⊙). We fit the FRII resolved CO emission with a 2D Gaussian model with a major (minor) axis of ∼27 (∼17) kpc, which is a factor of ∼3 larger than the optical rest-frame emission. Under the assumption of a simple edge-on disk morphology, we find that the galaxy interstellar medium produces a column density toward the nucleus of ∼5.5 × 1023 cm−2. A dense interstellar medium like this may then contribute significantly to the total nuclear obscuration measured in the X-rays (NH, X ∼ 1.5 × 1024 cm−2) in addition to a small, paresec-scale absorber around the central engine. The velocity map of this source unveils a rotational motion of the gas that is perpendicular to the radio jets. All ALMA sources have a dust-reddened counterpart in deep Hubble Space Telescope images (bands i, z, H), while we do not detect any molecular gas reservoir around the known UV-bright, star-forming members discovered by MUSE. This highlights the capability of ALMA of tracing gas-rich members of the overdensity. For the MUSE sources, we derive 3σ upper limits to the molecular gas mass of MH2 ≤ 2.8 − 4.8 × 1010 M⊙. We derive star formation rates in the range ∼5 − 100 M⊙ yr−1 for the three new ALMA sources. The FRII is located at the center of the projected spatial distribution of the structure members, and its velocity offset from the peak of the redshift distribution is well within the velocity dispersion of the structure. All this, coupled with the large amount of gas around the FRII, its stellar mass of ∼3 × 1011 M⊙, star formation rate of ∼200 − 600 M⊙ yr−1, and powerful radio-to-X-ray emission, suggests that this source is the likely progenitor of the future brightest cluster galaxy.

REQUIEM-2D Methodology: Spatially Resolved Stellar Populations of Massive Lensed Quiescent Galaxies from Hubble Space Telescope 2D Grism Spectroscopy

Abstract We present a novel Bayesian methodology to jointly model photometry and deep Hubble Space Telescope 2D grism spectroscopy of high-redshift galaxies. Our requiem2d code measures both unresolved and resolved stellar populations, ages, and star formation histories (SFHs) for the ongoing REsolving QUIEscent Magnified (REQUIEM) Galaxies Survey, which targets strong gravitationally lensed quiescent galaxies at z ∼ 2. We test the accuracy of requiem2d using a simulated sample of massive galaxies at z ∼ 2 from the Illustris cosmological simulation and find that we recover the general trends in SFH and median stellar ages. We further present a pilot study for the REQUIEM Galaxies Survey: MRG-S0851, a quintuply imaged, massive ( ) red galaxy at z = 1.883 ± 0.001. With an estimated gravitational magnification of , we sample the stellar populations on 0.6 kpc physical size bins. The global mass-weighted median age is constrained to be Gyr, and our spatially resolved analysis reveals that MRG-S0851 has a flat age gradient in the inner 3 kpc core after taking into account the subtle effects of dust and metallicity on age measurements, favoring an early formation scenario. The analysis for the full REQUIEM-2D sample will be presented in a forthcoming paper with a beta release of the requiem2d code.

The Kinematics of Massive Quiescent Galaxies at 1.4 < z < 2.1: Dark Matter Fractions, IMF Variation, and the Relation to Local Early-type Galaxies*

Abstract We study the dynamical properties of massive quiescent galaxies at 1.4 < z < 2.1 using deep Hubble Space Telescope WFC3/F160W imaging and a combination of literature stellar velocity dispersion measurements and new near-infrared spectra obtained using the K-band Multi Object Spectrograph (KMOS) on the ESO Very Large Telescope. We use these data to show that the typical dynamical-to-stellar mass ratio has increased by ∼0.2 dex from z = 2 to the present day, and we investigate this evolution in the context of possible changes in the stellar initial mass function (IMF) and/or fraction of dark matter contained within the galaxy effective radius, f DM[<r e]. Comparing our high-redshift sample to their likely descendants at low redshift, we find that f DM[<r e] has increased by a factor of more than 4 since z ≈ 1.8, from f DM[<r e] = 6.6% ± 1.0% to ∼24%. The observed increase appears robust to changes in the methods used to estimate dynamical masses or match progenitors and descendants. We quantify possible variation of the stellar IMF through the offset parameter α, defined as the ratio of dynamical mass in stars to the stellar mass estimated using a Chabrier IMF. We demonstrate that the correlation between stellar velocity dispersion and α reported among quiescent galaxies at low redshift is already in place at z = 2, and we argue that subsequent evolution through (mostly minor) merging should act to preserve this relation while contributing significantly to galaxies’ overall growth in size and stellar mass.

Quasar Sightline and Galaxy Evolution (QSAGE) survey – II. Galaxy overdensities around UV luminous quasars at z = 1–2

ABSTRACT We demonstrate that the UV brightest quasars at z = 1–2 live in overdense environments. This is based on an analysis of deep Hubble Space Telescope WFC3 G141 grism spectroscopy of the galaxies along the lines-of-sight to UV luminous quasars in the redshift range z = 1–2. This constitutes some of the deepest grism spectroscopy performed by WFC3, with four roll angles spread over a year of observations to mitigate the effect of overlapping spectra. Of the 12 quasar fields studied, 8 display evidence for a galaxy overdensity at the redshift of the quasar. One of the overdensities, PG0117 + 213 at z = 1.50, has potentially 36 spectroscopically confirmed members, consisting of 19 with secure redshifts and 17 with single-line redshifts, within a cylinder of radius ∼700 kpc. Its halo mass is estimated to be log (M/M⊙) = 14.7. This demonstrates that spectroscopic and narrow-band observations around distant UV bright quasars may be an excellent route for discovering protoclusters. Our findings agree with previous hints from statistical observations of the quasar population and theoretical works, as feedback regulated black hole growth predicts a correlation between quasar luminosity and halo mass. We also present the high signal-to-noise rest-frame optical spectral and photometric properties of the quasars themselves.

A study of stellar population of intermediate-age globular cluster NGC 2249

The Large Magellanic Cloud (LMC) star cluster NGC 2249 has been found to exhibit an extended main-sequence turn-off (eMSTO) in the color-magnitude diagram (CMD) derived from the data of the Hubble Space Telescope (HST). Such CMDs can potentially be explained via a spread of stellar age or rotation rate of single-aged stars, leading to a confusion of its stellar population type. In this work, we use different combinations of age spread (A), rotation (R), and binarity (B) to study the stellar population of this cluster, via a deep HST CMD. Because binaries are commonly observed in star clusters, three models of this study take binaries into account. We finally derive the best-fit stellar population parameters, i.e., metallicity (Z = 0.008), age (0.7–1.0Gyr), reddening (E(B-I) = 0.52–0.55 mag), distance modulus (18.54–18.57mag), binary fraction (50%), rotating star fraction (0–50%), and star formation history mode for NGC 2249. We find that both age spread (A + B or A + R + B) and rotating stars (R + B) can reproduce the main CMD features well, while rotation alone does not reproduce the detailed eMSTO morphology. This suggests that an age spread of 200Myrs is needed to form the eMSTO of cluster NGC 2249, even if stellar rotation is taken into account. Dynamical simulation could be useful for revealing the origin of the eMSTO.

The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description

ABSTRACT We present an overview and description of the e-MERGE Survey (e-MERLIN Galaxy Evolution Survey) Data Release 1 (DR1), a large program of high-resolution 1.5-GHz radio observations of the GOODS-N field comprising ∼140 h of observations with enhanced-Multi-Element Remotely Linked Interferometer Network (e-MERLIN) and ∼40 h with the Very Large Array (VLA). We combine the long baselines of e-MERLIN (providing high angular resolution) with the relatively closely packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5-GHz radio survey with the sensitivity (${\sim}1.5\, \mu$ Jy beam−1), angular resolution (0.2–0.7 arcsec) and field-of-view (∼15 × 15 arcmin2) to detect and spatially resolve star-forming galaxies and active galactic nucleus (AGN) at $z$ ≳ 1. The goal of e-MERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1-mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over an ∼20-yr baseline and the development of new point spread function/primary beam models to seamlessly merge e-MERLIN and VLA data in the uv plane. We present early science results, including measurements of the luminosities and/or linear sizes of ∼500 galaxies selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of re-MERGE/rHST ∼ 1.02 ± 0.03, suggesting that in most high-redshift galaxies, the ∼GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers that will explore the ∼kpc-scale radio properties of star-forming galaxies and AGN in the GOODS-N field observed by e-MERGE DR1.

A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy NGC 1052-DF4 from Deep Hubble Space Telescope Data

Abstract Previous studies have shown that the large, diffuse galaxies NGC 1052-DF2 and NGC 1052-DF4 both have populations of unusually luminous globular clusters as well as a very low dark matter content. Here we present newly obtained deep Hubble Space Telescope Advanced Camera for Surveys imaging of one of these galaxies, NGC 1052-DF4. We use these data to measure the distance of the galaxy from the location of the tip of the red giant branch (TRGB). We find a rapid increase in the number of detected stars fainter than m F814W ∼ 27.3, which we identify as the onset of the red giant branch. Using a forward modeling approach that takes the photometric uncertainties into account, we find a TRGB magnitude of . The inferred distance, including the uncertainty in the absolute calibration, is D TRGB = 20.0 ± 1.6 Mpc. The TRGB distance of NGC 1052-DF4 is consistent with the previously determined surface brightness fluctuation distance of D SBF = 18.7 ± 1.7 Mpc to NGC 1052-DF2 and is consistent with the distance of the bright elliptical galaxy NGC 1052. We conclude that the unusual properties of these galaxies cannot be explained by distance errors.

The Satellite Luminosity Function of M101 into the Ultra-faint Dwarf Galaxy Regime

Abstract We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101–Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et al. Previous distance estimates using the surface brightness fluctuation technique have suggested that these four dwarf candidates are the only remaining viable M101 satellites identified in ground-based imaging out to the virial radius of M101 (D ≈ 250 kpc). Advanced Camera for Surveys imaging of all four dwarf candidates shows no associated resolved stellar populations, indicating that they are thus background galaxies. We confirm this by generating simulated HST color–magnitude diagrams of similar brightness dwarfs at the distance of M101. Our targets would have displayed clear, resolved red giant branches with dozens of stars if they had been associated with M101. With this information, we construct a satellite luminosity function for M101, which is 90% complete to M V = −7.7 mag and 50% complete to M V = −7.4 mag, that extends into the ultra-faint dwarf galaxy regime. The M101 system is remarkably poor in satellites in comparison to the Milky Way and M31, with only eight satellites down to an absolute magnitude of M V = −7.7 mag, compared to the 14 and 26 seen in the Milky Way and M31, respectively. Further observations of Milky Way analogs are needed to understand the halo-to-halo scatter in their faint satellite systems, and connect them with expectations from cosmological simulations.

Constraining the Mass of the Emerging Galaxy Cluster SpARCS1049+56 at z = 1.71 with Infrared Weak Lensing

Abstract In the hierarchical structure formation model of the universe, galaxy clusters are assembled through a series of mergers. Accordingly, it is expected that galaxy clusters in the early universe are actively forming and dynamically young. Located at a high redshift of z = 1.71, SpARCS1049+56 offers a unique look into the galaxy cluster formation process. This cluster has been shown to be rich in cluster galaxies and to have intense star formation. Its high redshift pushes a weak-lensing analysis beyond the regime of the optical spectrum into that of the infrared. Equipped with deep Hubble Space Telescope Wide Field Camera 3 UVIS and IR observations, we present a weak-lensing characterization of SpARCS1049+56. As few IR weak-lensing studies have been performed, we discuss the details of point-spread function modeling and galaxy shape measurement for an IR weak-lensing procedure and the systematics that come with the territory. It will be critical to understand these systematics in future weak-lensing studies in the IR with the next-generation space telescopes such as the James Webb Space Telescope, Euclid, and WFIRST. Through a careful analysis, the mass distribution of this young galaxy cluster is mapped and the convergence peak is detected at a 3.3σ level. The weak-lensing mass of the cluster is estimated to be 3.5 ± 1.2 × 1014 M ⊙ and is consistent with the mass derived from a mass–richness scaling relation. This mass is extreme for a cluster at such a high redshift and suggests that SpARCS1049+56 is rare in the standard ΛCDM universe.

Constraining Type Iax supernova progenitor systems with stellar population age dating

ABSTRACT Type Iax supernovae (SNe Iax) are the most common class of peculiar SNe. While they are thought to be thermonuclear white-dwarf (WD) SNe, SNe Iax are observationally similar to, but distinct from SNe Ia. Unlike SNe Ia, where roughly 30 per cent occur in early-type galaxies, only one SN Iax has been discovered in an early-type galaxy, suggesting a relatively short delay time and a distinct progenitor system. Furthermore, one SN Iax progenitor system has been detected in pre-explosion images with its properties consistent with either of two models: a short-lived (<100 Myr) progenitor system consisting of a WD primary and a He-star companion, or a singular Wolf–Rayet progenitor star. Using deep Hubble Space Telescope images of nine nearby SN Iax host galaxies, we measure the properties of stars within 200 pc of the SN position. The ages of local stars, some of which formed with the SN progenitor system, can constrain the time between star formation and SN, known as the delay time. We compare the local stellar properties to synthetic photometry of single-stellar populations, fitting to a range of possible delay times for each SN. With this sample, we uniquely constrain the delay-time distribution for SNe Iax, with a median and 1σ confidence interval delay time of $63_{- 15}^{+ 58} \times 10^{6}$ yr. The measured delay-time distribution provides an excellent constraint on the progenitor system for the class, indicating a preference for a WD progenitor system over a Wolf–Rayet progenitor star.

Deep Hubble Space Telescope Imaging on the Extended Lyα Emission of a QSO at z = 2.19 with a Damped Lyman Alpha System as a Natural Coronagraph

Abstract Recent observations suggest that proximate damped Lyα (PDLA) systems can be used to study the host galaxies of quasi-stellar objects (QSOs), because the PDLAs can block the bright point-spread function from central QSOs. Using six orbits of narrowband imaging with Hubble Space Telescope (HST)/WFC3, we present the first high-resolution narrowband image of the Lyα emission in the PDLA trough of the QSO SDSS J115432.67–021537.9. We detect one major component and one minor component in the narrowband imaging. Combining the /WFC3 imaging with deep Magellan/MagE spectra, we measure that the Lyα flux = , which is among the luminous (≈2.7 ) Lyα emitters at 2.19. The -based star formation rate is ∼7 . These observational results favor that the star formation from the host galaxy could be the main mechanism powering the Lyα emission. This new method sheds light on the study of the kinematic structure and the spatial distribution of the extended Lyα emitting regions around the QSO host.

The M101 Satellite Luminosity Function and the Halo–Halo Scatter among Local Volume Hosts

Abstract We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys HST photometry for two of these objects showed resolved stellar populations and tip of the red giant branch derived distances (D ∼ 7 Mpc) consistent with M101 group membership. The remaining 17 were found to have no resolved stellar populations, meaning they are either part of the background NGC 5485 group or are distant low surface brightness (LSB) galaxies. It is noteworthy that many LSB objects that had previously been assumed to be M101 group members based on projection have been shown to be background objects, indicating the need for future diffuse dwarf surveys to be very careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function of M101 based on the presence of these new objects down to M V = −8.2. M101 is a sparsely populated system with only nine satellites down to M V ≈ −8, as compared with 26 for M31 and 24.5 ± 7.7 for the median host in the Local Volume. This makes M101 by far the sparsest group probed to this depth, although M94 is even sparser to the depth at which it has been examined (M V = −9.1). M101 and M94 share several properties that mark them as unusual compared with the other Local Volume galaxies examined: they have a very sparse satellite population but also have high star-forming fractions among these satellites; such properties are also found in the galaxies examined as part of the Satellites around Galactic Analogs survey. We suggest that these properties appear to be tied to the wider galactic environment, with more isolated galaxies showing sparse satellite populations that are more likely to have had recent star formation, while those in dense environments have more satellites that tend to have no ongoing star formation. Overall, our results show a level of halo-to-halo scatter between galaxies of similar mass that is larger than is predicted in the lambda cold dark matter model.

Star formation at the edge of the Local Group: a rising star formation history in the isolated galaxy WLM

ABSTRACT We present the star formation history (SFH) of the isolated (D ∼ 970 kpc) Local Group dwarf galaxy Wolf–Lundmark–Melotte (WLM) measured from colour–magnitude diagrams (CMDs) constructed from deep Hubble Space Telescope imaging. Our observations include a central ($0.5 \, r_h$) and outer field ($0.7 \, r_h$) that reach below the oldest main-sequence turn-off. WLM has no early dominant episode of star formation: 20 per cent of its stellar mass formed by ∼12.5 Gyr ago ($z$ ∼ 5). It also has an SFR that rises to the present with 50 per cent of the stellar mass within the most recent 5 Gyr ($z$ < 0.7). There is evidence of a strong age gradient: the mean age of the outer field is 5 Gyr older than the inner field despite being only 0.4 kpc apart. Some models suggest such steep gradients are associated with strong stellar feedback and dark-matter core creation. The SFHs of real isolated dwarf galaxies and those from the Feedback in Realistic Environment suite are in good agreement for M⋆($z$ = 0) ∼ 107–109M⊙, but in worse agreement at lower masses ($M_{\star }(z=0) \sim 10^5\!-\!10^7 \, \mathrm{M}_{\odot }$). These differences may be explainable by systematics in the models (e.g. reionization model) and/or observations (HST field placement). We suggest that a coordinated effort to get deep CMDs between HST/JWST (crowded central fields) and WFIRST (wide-area halo coverage) is the optimal path for measuring global SFHs of isolated dwarf galaxies.

The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

Abstract We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at ≈584 days post-merger, which provide a robust optical template. The light curve spans ≈110–362 days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of 〈 β 〉 = − 0.583 ± 0.013 , demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short gamma-ray bursts matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster (GC) of L ≲ 6.7 × 103 L ⊙, or M ≲ 1.3 × 104 M ⊙, at least 4 standard deviations below the peak of the GC mass function of the host galaxy, NGC 4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a GC. As highlighted here, HST (and soon the James Webb Space Telescope) enables critical observations of the optical emission from neutron star merger jets and outflows.

The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch*

Abstract We present a new and independent determination of the local value of the Hubble constant based on a calibration of the tip of the red giant branch (TRGB) applied to Type Ia supernovae (SNe Ia). We find a value of H 0 = 69.8 ± 0.8 (±1.1% stat) ± 1.7 (±2.4% sys) km s−1 Mpc−1. The TRGB method is both precise and accurate and is parallel to but independent of the Cepheid distance scale. Our value sits midway in the range defined by the current Hubble tension. It agrees at the 1.2σ level with that of the Planck Collaboration et al. estimate and at the 1.7σ level with the Hubble Space Telescope (HST) SHoES measurement of H 0 based on the Cepheid distance scale. The TRGB distances have been measured using deep HST Advanced Camera for Surveys imaging of galaxy halos. The zero-point of the TRGB calibration is set with a distance modulus to the Large Magellanic Cloud of 18.477 ± 0.004 (stat) ± 0.020 (sys) mag, based on measurement of 20 late-type detached eclipsing binary stars, combined with an HST parallax calibration of a 3.6 μm Cepheid Leavitt law based on Spitzer observations. We anchor the TRGB distances to galaxies that extend our measurement into the Hubble flow using the recently completed Carnegie Supernova Project I ( CSP-I ) sample containing about 100 well-observed SNe Ia . There are several advantages of halo TRGB distance measurements relative to Cepheid variables; these include low halo reddening, minimal effects of crowding or blending of the photometry, only a shallow (calibrated) sensitivity to metallicity in the I band, and no need for multiple epochs of observations or concerns of different slopes with period. In addition, the host masses of our TRGB host-galaxy sample are higher, on average, than those of the Cepheid sample, better matching the range of host-galaxy masses in the CSP-I distant sample and reducing potential systematic effects in the SNe Ia measurements.

A rogues gallery of Andromeda’s dwarf galaxies – II. Precise distances to 17 faint satellites

ABSTRACT We present new horizontal branch (HB) distance measurements to 17 of the faintest known M31 satellites (−6 ≲ MV ≲ −13) based on deep Hubble Space Telescope (HST) imaging. The colour–magnitude diagrams extend ∼1–2 mag below the HB, which provides for well-defined HBs, even for faint galaxies in which the tip of the red giant branch (TRGB) is sparsely populated. We determine distances across the sample to an average precision of 4 per cent (∼30 kpc at 800 kpc). We find that the majority of these galaxies are in good agreement, though slightly farther (0.1–0.2 mag) when compared to recent ground-based TRGB distances. Two galaxies (And IX and And XVII) have discrepant HST and ground-based distances by ∼0.3 mag (∼150 kpc), which may be due to contamination from Milky Way foreground stars and/or M31 halo stars in sparsely populated TRGB regions. We use the new distances to update the luminosities and structural parameters for these 17 M31 satellites. The new distances do not substantially change the spatial configuration of the M31 satellite system. We comment on future prospects for precise and accurate HB distances for faint galaxies in the Local Group and beyond.

The evolution of the UV luminosity function of globular clusters in the E-MOSAICS simulations

We present the evolution of the rest-frame ultraviolet (UV) properties of the globular cluster (GC) populations and their host galaxies formed in the E-MOSAICS suite of cosmological hydrodynamical simulations. We compute the luminosities of all clusters associated with 25 simulated Milky Way-mass galaxies, discussed in previous works, in the rest-frame UV and optical bands by combining instantaneous cluster properties (age, mass, metallicity) with simple stellar population models, from redshifts $z=0$ to 10. Due to the rapid fading of young stellar populations in the UV, most of the simulated galaxies do not host GCs bright enough to be individually identified in deep Hubble Space Telescope (HST) observations, even in highly magnified systems. The median age of the most UV-luminous GCs is $<10$ Myr (assuming no extinction), increasing to $\gtrsim 100$ Myr for red optical filters. We estimate that these GCs typically only contribute a few per cent of the total UV luminosity of their host galaxies at any epoch. We predict that the number density of UV-bright proto-GCs (or cluster clumps) will peak between redshifts $z=1-3$. In the main progenitors of Milky Way-mass galaxies, $10$-$20$ per cent of the galaxies at redshifts $1 \lesssim z \lesssim 3$ have clusters brighter than $M_{\rm UV} < -15$, and less than $10$ per cent at other epochs. The brightest cluster in the galaxy sample at $z>2$ is typically $M_{\rm UV} \sim -16$, consistent with the luminosities of observed compact, high-redshift sources.

Chandra and Hubble Space Telescope observations of dark gamma-ray bursts and their host galaxies

Abstract We present a study of 21 dark gamma-ray burst (GRB) host galaxies, predominantly using X-ray afterglows obtained with the Chandra X-Ray Observatory (CXO) to precisely locate the burst in deep Hubble Space Telescope (HST) imaging of the burst region. The host galaxies are well-detected in F160W in all but one case and in F606W imaging in 60 per cent of cases. We measure magnitudes and perform a morphological analysis of each galaxy. The asymmetry, concentration, and ellipticity of the dark burst hosts are compared against the host galaxies of optically bright GRBs. In agreement with other studies, we find that dark GRB hosts are redder and more luminous than the bulk of the GRB host population. The distribution of projected spatial offsets for dark GRBs from their host galaxy centroids is comparable to that of optically bright bursts. The dark GRB hosts are physically larger, more massive and redder, but are morphologically similar to the hosts of bright GRBs in terms of concentration and asymmetry. Our analysis constrains the fraction of high redshift (z &amp;gt; 5) GRBs in the sample to 14 per cent, implying an upper limit for the whole long-GRB population of ≤4.4 per cent. If dust is the primary cause of afterglow darkening amongst dark GRBs, the measured extinction may require a clumpy dust component in order to explain the observed offset and ellipticity distributions.