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We present uniformly measured resolved stellar photometry and star formation histories (SFHs) for 36 nearby (≲400 kpc) ultra-faint dwarf galaxies (UFDs; −7.1 ≤M_V≤ +0.0) from new and archival Hubble Space Telescope (HST) imaging. We measure homogeneous distances to all systems via isochrone fitting and find good agreement (≤2%) for the 18 UFDs that have literature RR Lyrae distances. From the ensemble of SFHs, we find (i) an average quenching time (here defined as the lookback time by which 80% of the stellar mass formed,τ₈₀) of 12.48  ±  0.18 Gyr ago ( $z=4.6_{-0.5}^{+0.6}$), which is compatible with reionization-based quenching scenarios; and (ii) modest evidence of a delay (≲800 Myr) in quenching times of UFDs thought to be satellites of the LMC or on their first infall, relative to long-term Galactic satellites, which is consistent with previous findings. We show that robust SFH measurement via the ancient main-sequence turnoff (MSTO) requires a minimum effective luminosity (i.e., luminosity within the observed field of view) ofM_V≤ −2.5, which corresponds to ∼100 stars around the MSTO. We also find that increasing the signal-to-noise ratio above ∼100 at the MSTO does not improve SFH precision, which remains dominated by stochastic effects associated with the number of available stars. A main challenge driving the precision of UFD SFHs is the limitations in the accuracy of foreground dust maps. We make all photometry catalogs public as the first data release of a larger HST archival program targeting all dwarf galaxies within ∼1.3 Mpc. 

Understanding the detailed properties of stars in the infrared is of critical importance with new missions focusing on the infrared (JWST, Roman). The ground-based wide-field infrared instrument, FourStar, on the Magellan telescope contains a number of medium band filters that are optimized for photometric redshifts, but also provide nearly continuous spectral coverage for stars. These filters, however, lack absolute calibration. In this project, we adapted a technique used for galaxies to be used for stars. We used multi-band photometry to fit a likely SED model to each star and then generated synthetic photometry from the SED model to produce the magnitude of the star in a given filter. Statistically, we then determined the zero point in each filter in each image. This will be used to calibrate a large survey aiming to better understand near-infrared properties of stars for application to the extragalactic distance scale (with JWST and Roman). 

Understanding the detailed properties of stars in the infrared is of critical importance with new missions focusing on the infrared (JWST, Roman). The ground-based wide-field infrared instrument, FourStar, on the Magellan telescope contains a number of medium band filters that are optimized for photometric redshifts, but also provide nearly continuous spectral coverage for stars. These filters, however, lack absolute calibration. In this project, we adapted a technique used for galaxies to be used for stars. We used multi-band photometry to fit a likely SED model to each star and then generated synthetic photometry from the SED model to produce the magnitude of the star in a given filter. Statistically, we then determined the zero point in each filter in each image. This will be used to calibrate a large survey aiming to better understand near-infrared properties of stars for application to the extragalactic distance scale (with JWST and Roman). 

Abstract From >1000 orbits of HST imaging, we present deep homogeneous resolved star color–magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies (−6 ≥M_V≥ −17) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs, we find: (i) The median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within 1.8 Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. (ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. (iii) ~50% of our M31 satellites show prominent ancient star formation (>12 Gyr ago) followed by delayed quenching (8–10 Gyr ago), which is not commonly observed among the MW satellites. (iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts shows that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST. 

Abstract Using resolved optical stellar photometry from the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region survey, we measured the star formation history near the position of 85 supernova remnants (SNRs) in M33. We constrained the progenitor masses for 60 of these SNRs, finding that the remaining 25 remnants had no local star formation in the last 56 Myr, consistent with core-collapse supernovae, making them potential Type Ia candidates. We then infer a progenitor mass distribution from the age distribution, assuming single star evolution. We find that the progenitor mass distribution is consistent with being drawn from a power law with an index of − 2.9 − 1.0 + 1.2 . Additionally, we infer a minimum progenitor mass of 7.1 − 0.2 + 0.1 M ⊙ from this sample, consistent with several previous studies, providing further evidence that stars with ages older than the lifetimes of single 8 M ⊙ stars are producing supernovae. 

Abstract We present multiwavelength characterization of 65 high-mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point-source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region catalog, which covers the inner disk of M33 at near-IR, optical, and near-UV wavelengths. We perform spectral energy distribution fitting on multiband photometry for each point-source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main-sequence stars, suggesting that the HMXB population of M33 is dominated by Be X-ray binaries (Be-XRBs), as is seen in other Local Group galaxies. We use spatially resolved recent star formation history maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at ∼10 and ∼40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be-XRBs, respectively. We measure an HMXB production rate of 107–136 HMXBs/(M_⊙yr⁻¹) over the last 50 Myr and 150–199 HMXBs/(M_⊙yr⁻¹) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in black hole HMXBs and main-sequence companion stars in neutron star HMXBs. 

Abstract We construct a catalog of star clusters from Hubble Space Telescope images of the inner disk of the Triangulum Galaxy (M33) using image classifications collected by the Local Group Cluster Search, a citizen science project hosted on the Zooniverse platform. We identify 1214 star clusters within the Hubble Space Telescope imaging footprint of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. Comparing this catalog to existing compilations in the literature, 68% of the clusters are newly identified. The final catalog includes multiband aperture photometry and fits for cluster properties via integrated light spectral energy distribution fitting. The cluster catalog’s 50% completeness limit is ∼1500 M ☉ at an age of 100 Myr, as derived from comprehensive synthetic cluster tests. 

Abstract We measure the star cluster mass function (CMF) for the Local Group galaxy M33. We use the catalog of stellar clusters selected from the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region survey. We analyze 711 clusters in M33 with $7.0<\log \left(\mathrm{Age}/\mathrm{yr}\right)<8.5$, and log(M/M_⊙) > 3.0 as determined from color–magnitude diagram fits to individual stars. The M33 CMF is best described by a Schechter function with power-law slopeα= − ${2.06}_{-0.13}^{+0.14}$, and truncation mass log(M_c/M_⊙) $={4.24}_{-0.13}^{+0.16}$. The data show strong evidence for a high-mass truncation, thus strongly favoring a Schechter function fit over a pure power law. M33's truncation mass is consistent with the previously identified linear trend betweenM_c, and star formation rate surface density, Σ_SFR. We also explore the effect that individual cluster mass uncertainties have on derived mass function parameters, and find evidence to suggest that large cluster mass uncertainties have the potential to bias the truncation mass of fitted mass functions at the 1σlevel. 

Abstract We measure the spatially resolved recent star formation history (SFH) of M33 using optical images taken with the Hubble Space Telescope as part of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. The area covered by the observations used in this analysis covers a de-projected area of ∼38 kpc²and extends to ∼3.5 and ∼2 kpc from the center of M33 along the major and semimajor axes, respectively. We divide the PHATTER optical survey into 2005 regions that measure 24 arcsec, ∼100 pc, on a side and fit color–magnitude diagrams for each region individually to measure the spatially resolved SFH of M33 within the PHATTER footprint. There are significant fluctuations in the SFH on small spatial scales and also galaxy-wide scales that we measure back to about 630 Myr ago. We observe a more flocculent spiral structure in stellar populations younger than about 80 Myr, while the structure of the older stellar populations is dominated by two spiral arms. We also observe a bar in the center of M33, which dominates at ages older than about 80 Myr. Finally, we find that the mean star formation rate (SFR) over the last 100 Myr within the PHATTER footprint is 0.32 ± 0.02 M_⊙yr⁻¹. We measure a current SFR (over the last 10 Myr) of 0.20 ± 0.03 M_⊙yr⁻¹. This SFR is slightly higher than previous measurements from broadband estimates, when scaled to account for the fraction of the D25 area covered by the PHATTER survey footprint.