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The tip of the red giant branch (TRGB) is a standardizable candle, identifiable as the discontinuity at the bright extreme of the red giant branch (RGB) stars in color–magnitude diagram space. The TRGB-based distance method has been used to measure distances to galaxies out to D ≤ 20 Mpc with the Hubble Space Telescope F814W filter, and is an important rung in the distance ladder to measure the Hubble constant, H 0. In the infrared (IR), the TRGB apparent magnitude ranges from 1–2 mag brighter than in the optical. Now with the James Webb Space Telescope (JWST), the feasible distance range of the TRGB method can reach ∼50 Mpc. However, the IR TRGB luminosity depends to varying degrees on stellar metallicity/age. Here we standardize the TRGB luminosity using stellar colors as a proxy for metallicity/age to derive color-based corrections for the JWST Near-Infrared Camera short-wavelength filters F090W, F115W, and F150W, and the long-wavelength filters F277W, F356W, and F444W. We provide recommended filters for distance measurements depending on the requisite precision. For science requiring high precision (≤1% in distance), we recommend measuring the TRGB in F090W versus F090W − F150W or F115W versus F115W − F277W with the caveat that even with JWST, long integration times will be necessary at farther distances. If lower precision (>1.5% in distance) can be tolerated, or if shorter integration times are desirable, we recommend measuring the TRGB in either F115W or F150W. We do not recommend F444W for precision TRGB measurements due to its lower angular resolution.

The tip of the red giant branch (TRGB) based distance method in the I band is one of the most efficient and precise techniques for measuring distances to nearby galaxies (D ≲ 15 Mpc). The TRGB in the near-infrared (NIR) is 1–2 mag brighter relative to the I band, and has the potential to expand the range over which distance measurements to nearby galaxies are feasible. Using Hubble Space Telescope (HST) imaging of 12 fields in eight nearby galaxies, we determine color-based corrections and zero-points of the TRGB in the Wide Field Camera 3 IR (WFC3/IR) F110W and F160W filters. First, we measure TRGB distances in the I band equivalent Advanced Camera System (ACS) F814W filter from resolved stellar populations with the HST. The TRGB in the ACS F814W filter is used for our distance anchor and to place the WFC3/IR magnitudes on an absolute scale. We then determine the color dependence (a proxy for metallicity/age) and zero-point of the NIR TRGB from photometry of WFC3/IR fields that overlap with the ACS fields. The new calibration is accurate to ∼1% in distance relative to the F814W TRGB. Validating the accuracy of the calibrations, we find that the distance modulus for each field using the NIR TRGB calibration agrees with the distance modulus of the same field as determined from the F814W TRGB. This is a JWST preparatory program, and the work done here will directly inform our approach to calibrating the TRGB in JWST NIRCam and NIRISS photometric filters.

We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.

Isolated, low-mass galaxies provide the opportunity to assess the impact of reionization on their star formation histories (SFHs) without the ambiguity of environmental processes associated with massive host galaxies. There are very few isolated, low-mass galaxies that are close enough to determine their SFHs from resolved star photometry reaching below the oldest main-sequence turnoff. The James Webb Space Telescope (JWST) has increased the volume for which this is possible, and here we report on JWST observations of the low-mass, isolated galaxy Leo P. From NIRCam imaging in F090W, F150W, and F277W, we derive an SFH that shows early star formation followed by a pause subsequent to the Epoch of Reionization, which is then later followed by a reignition of star formation. This is very similar to the SFHs from previous studies of other dwarf galaxies in the “transition zone” between quenched very-low-mass galaxies and the more massive galaxies that show no evidence of the impact of reionization on their SFHs; this pattern is rarely produced in simulations of SFHs. The lifetime SFH reveals that Leo P’s stellar mass at the Epoch of Reionization was in the range that is normally associated with being totally quenched. The extended pause in star formation from z ∼ 5 to 1 has important implications for the contribution of low-mass galaxies to the ultraviolet photon budget at intermediate redshifts. We also demonstrate that, due to higher sensitivity and angular resolution, observing in two NIRCam short-wavelength filters is superior to observing in a combination of a short- and a long-wavelength filter.

The proximity of the Magellanic Clouds provides the opportunity to study interacting dwarf galaxies near a massive host, and spatial trends in their stellar population properties in particular, with a unique level of detail. The Scylla pure parallel program has obtained deep (80% complete to >1 mag below the ancient main-sequence turnoff), homogeneous two-filter Hubble Space Telescope imaging sampling the inner star-forming disk of the Large Magellanic Cloud (LMC), the perfect complement to shallower, contiguous ground-based surveys. We harness this imaging together with extant archival data and fit lifetime star formation histories (SFHs) to resolved color–magnitude diagrams of 111 individual fields, using three different stellar evolutionary libraries. We validate per-field recovered distances and extinctions, as well as the combined global LMC age–metallicity relation and SFH against independent estimates. We find that the present-day radial age gradient reverses from an inside-out gradient in the inner disk to an outside-in gradient beyond ∼2 disk scale lengths, supported by ground-based measurements. The gradients become relatively flatter at earlier look-back times, while the location of the inversion remains constant over an order of magnitude in look-back time, from ∼1 to 10 Gyr. This suggests at least one mechanism that predates the recent intense LMC–Small Magellanic Cloud interaction. We compare observed radial age trends to other late-type galaxies at fixed stellar mass and discuss similarities and differences in the context of potential drivers, implying strong radial migration in the LMC.

We present a near-complete catalog of the metal-rich population of thermally pulsing asymptotic giant branch (AGB) stars in the northwest quadrant of M31. This metal-rich sample complements the equally complete metal-poor Magellanic Cloud AGB catalogs produced by the SAGE program. Our catalog includes Hubble Space Telescope (HST) wide-band photometry from the Panchromatic Hubble Andromeda Treasury survey, HST medium-band photometry used to chemically classify a subset of the sample, and Spitzer mid- and far-IR photometry that we have used to isolate dust-producing AGB stars. We have detected 346,623 AGB stars; these include 4802 AGB candidates producing considerable dust, and 1356 AGB candidates that lie within clusters with measured ages, and in some cases metallicities. Using the Spitzer data and chemical classifications made with the medium-band data, we have identified both carbon- and oxygen-rich AGB candidates producing significant dust. We have applied color–mass-loss relations based on dusty-AGB stars from the LMC to estimate the dust injection by AGB stars in the PHAT footprint. Applying our color relations to a subset of the chemically classified stars producing the bulk of the dust, we find that ∼97.8% of the dust is oxygen-rich. Using several scenarios for the dust lifetime, we have estimated the contribution of AGB stars to the global dust budget of M31 to be 0.9%–35.5%, which is in line with previous estimates in the Magellanic Clouds. Follow-up observations of the M31 AGB candidates with the JWST will allow us to further constrain stellar and chemical evolutionary models, and the feedback and dust production of metal-rich evolved stars.

JWST Near Infrared Camera (NIRCam) observations at 1.5–4.5 μm have provided broadband and narrowband imaging of the evolving remnant of SN 1987A with unparalleled sensitivity and spatial resolution. Comparing with previous marginally spatially resolved Spitzer Infrared Array Camera (IRAC) observations from 2004 to 2019 confirms that the emission arises from the circumstellar equatorial ring (ER), and the current brightness at 3.6 and 4.5 μm was accurately predicted by extrapolation of the declining brightness tracked by IRAC. Despite the regular light curve, the NIRCam observations clearly reveal that much of this emission is from a newly developing outer portion of the ER. Spots in the outer ER tend to lie at position angles in between the well-known ER hotspots. We show that the bulk of the emission in the field can be represented by five standard spectral energy distributions, each with a distinct origin and spatial distribution. This spectral decomposition provides a powerful technique for distinguishing overlapping emission from the circumstellar medium and the supernova ejecta, excited by the forward and reverse shocks, respectively.

Radial stellar population gradients within dwarf galaxies provide a promising avenue for disentangling the drivers of galaxy evolution, including environment. Within the Local Volume, radial stellar age gradient slopes correlate with interaction history, contrary to model predictions, so dwarfs that are isolated provide a critical control sample. We measure radial stellar age gradients in the relatively isolated gas-rich dwarf irregular Wolf–Lundmark–Melotte Galaxy (WLM), combining JWST NIRCam and NIRISS imaging with six archival Hubble Space Telescope fields over semimajor axis equivalent distances of 0 ≲ RSMA ≲ 4 kpc (≲3 Rhl). Fitting lifetime star formation histories to resolved color–magnitude diagrams, radial age gradients are quantified using τ90 and τ50, the lookback times to form 90% and 50% of the cumulative stellar mass. We find that globally, the outskirts of WLM are older on average, with (δτ90, δτ50)/δRSMA = (0.82 −0.10+0.10 , 1.60 −0.22+0.23 ) Gyr kpc−1 (stat.), in good agreement with simulations. However, we also detect an azimuthal dependence of radial stellar age gradients, finding that stars on the leading edge of WLM (relative to its proper motion) are both younger and have a flatter age gradient compared to the trailing edge. This difference persists over 0.6 ≲ RSMA ≲ 3.2 kpc (∼0.5–2.5 Rhl) and lookback times up to ∼8 Gyr, and is robust to the assumed stellar evolutionary model. Our results are consistent with star formation triggered by ram pressure stripping from a circumgalactic and/or intergalactic medium, suggested by recent H I observations. If confirmed, processes typifying dense environments, such as ram pressure stripping, may be more relevant to the evolution of isolated galaxies than previously thought.

We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only ∼3% Z ⊙, it is among the lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.

We present the star formation histories (SFHs) of 10 metal-poor (≲12% Z⊙), star-forming dwarf galaxies from the Local Ultraviolet to Infrared Treasury survey. The derived SFHs exhibit significant variability, consistent with the irregular star formation expected for dwarf galaxies. Using synthetic near-ultraviolet (UV) and optical color–magnitude diagrams (CMDs) with various yet targeted configurations for dust and input SFHs, we quantitatively demonstrate that simultaneous modeling of the UV and optical CMDs (“UVopt” case) improves the precision of SFH measurements in recent time bins up to ∼1 Gyr, compared to the classical single optical CMD modeling (“Opt-only” case). The UVopt case reduces uncertainties relative to the Opt-only case by ∼4%–8% over the past 10 Myr, ∼8%–20% over 100 Myr, and ∼8%–14% over 1 Gyr, across various dust configurations and input SFHs. Additionally, we demonstrate discrepancies in stellar models for blue core helium-burning (BHeB) stars at the low-metallicity regime. This discrepancy can artificially inflate star formation rate (SFR) estimates in younger age bins by misinterpreting the evolved BHeB stars as reddened upper main-sequence (MS) stars. Incorporating UV data improves BHeB-MS separation and mitigates the limitations of current low-metallicity stellar models. Comparisons of the UVopt SFHs with Hα and far-UV (FUV)-based SFRs reconfirm that Hα is an unreliable tracer over its nominal 10 Myr timescale for low-SFR galaxies, while FUV provides a more reliable tracer but yields SFRFUV values up to twice those of CMD-based 〈SFR〉100 Myr. Our findings underscore the importance of UV data in refining recent SFHs in low-metallicity environments.