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The stellar halos of galaxies, primarily formed through the accretion and merger of smaller objects, are an important tool for understanding the hierarchical mass assembly of galaxies. However, the inner regions of stellar halos in disk galaxies are predicted to have an in situ component that is expected to be prominent along the major axis. Kinematic information is crucial to disentangle the contribution of the in situ component from the accreted stellar halos. The low surface brightness of stellar halos makes it inaccessible with traditional integrated light spectroscopy. In this work, we used a novel technique to study the kinematics of the stellar halo of the edge-on galaxy NGC 4945. We couple new deep Multi Unit Spectroscopic Explorer spectroscopic observations with existingHubbleSpace Telescope imaging data to spectroscopically measure the line-of-sight (LOS) heliocentric velocity and velocity dispersion in two fields at a galactocentric distance of 12.2 kpc (outer disk field) and 34.6 kpc (stellar halo field) along the NGC 4945 major axis, by stacking individual spectra of red giant branch and asymptotic giant branch stars. We obtained a LOS velocity and dispersion of 673 ± 11 km s⁻¹and 73 ± 14 km s⁻¹, respectively, for the outer disk field. This is consistent with the mean HI velocity of the disk at that distance. For the halo field, we obtained a LOS velocity and dispersion of 519 ± 12 km s⁻¹and 42 ± 22 km s⁻¹. The halo fields’ velocity measurement is within ∼40 km s⁻¹from the systemic LOS velocity of NGC 4945, which is 563 km s⁻¹, suggesting that its stellar halo at 34.6 kpc along the major axis is counter-rotating and its origins are likely to be the result of accretion. This provides the first-ever kinematic measurement of the stellar halo of a Milky Way-mass galaxy outside the Local Group from its resolved stellar population. Thus, we have established a powerful technique for measuring the velocity field for the stellar halos of nearby galaxies. 

Abstract We report the discovery of three faint and ultrafaint dwarf galaxies—Sculptor A, Sculptor B, and Sculptor C—in the direction of NGC 300 (D= 2.0 Mpc), a Large Magellanic Cloud–mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or Higas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D= 2.04 ${}_{-0.13}^{+0.10}$Mpc;M_V=  −9.1 ± 0.1 mag orL_V= (3.7 ${}_{-0.3}^{+0.4}$) × 10⁵L_⊙) is consistent with being a satellite of NGC 300. Sculptor A (D= 1.35 ${}_{-0.08}^{+0.22}$Mpc;M_V= −6.9 ± 0.3 mag orL_V= (5 ${}_{-1}^{+1}$) × 10⁴L_⊙) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultrafaint Tucana B in terms of its physical properties and environment. Sculptor B (D= 2.48 ${}_{-0.24}^{+0.21}$Mpc;M_V= −8.1 ± 0.3 mag orL_V= (1.5 ${}_{-0.4}^{+0.5}$) × 10⁵L_⊙) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳2–4r_virfrom NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram-pressure stripping, reionization, and internal feedback in influencing the star formation history of the faintest stellar systems. 

Abstract Mergers of and interactions between galaxies imprint a wide diversity of morphological, dynamical, and chemical characteristics in stellar halos and tidal streams. Measuring these characteristics elucidates aspects of the progenitors of the galaxies we observe today. The M81 group is the perfect galaxy group to understand the past, present, and future of a group of galaxies in the process of merging. Here, we measure the end of star formation (t₉₀) and metallicity ([M/H]) of the stellar halo of M82 and the eastern tidal stream of NGC 3077 to: (1) test the idea that M82 possesses a genuine stellar halo, formed before any interaction with M81; (2) determine if NGC 3077's tidal disruption is related to the star formation history in its tails; and (3) create a timeline of the assembly history of the central trio in the M81 group. We argue that M82 possesses a genuine, metal-poor ([M/H] ∼ −1.62 dex) stellar halo, formed from the merger of a small satellite galaxy roughly 6.6 Gyr ago. We also find that the stars present in NGC 3077's tails formed before tidal disruption with M81, and possess a roughly uniform metallicity as shown in S. Okamoto et al., implying that NGC 3077's progenitor had significant population gradients. Finally, we present a timeline of the central trio’s merger/interaction history. 

ABSTRACT The star formation histories (SFHs) of galactic stellar haloes offer crucial insights into the merger history of the galaxy and the effects of those mergers on their hosts. Such measurements have revealed that while the Milky Way’s most important merger was 8–10 Gyr ago, M31’s largest merger was more recent, within the last few Gyr. Unfortunately, the required halo SFH measurements are extremely observationally expensive outside of the Local Group. Here, we use asymptotic giant branch (AGB) stars brighter than the tip of the red giant branch (RGB) to constrain stellar halo SFHs. Both stellar population models and archival data sets show that the AGB/RGB ratio constrains the time before which 90 per cent of the stars formed, t90. We find AGB stars in the haloes of three highly inclined roughly Milky Way-mass galaxies with resolved star measurements from the Hubble Space Telescope; this population is most prominent in the stellar haloes of NGC 253 and NGC 891, suggesting that their stellar haloes contain stars born at relatively late times, with inferred t90 ∼ 6 ± 1.5 Gyr. This ratio also varies from region to region, tending towards higher values along the major axis and in tidal streams or shells. By combining our measurements with previous constraints, we find a tentative anticorrelation between halo age and stellar halo mass, a trend that exists in models of galaxy formation but has never been elucidated before, i.e. the largest stellar haloes of Milky Way-mass galaxies were assembled more recently. 

ABSTRACT Young stellar objects (YSOs) are the gold standard for tracing star formation in galaxies but have been unobservable beyond the Milky Way and Magellanic Clouds. But that all changed when the JWST was launched, which we use to identify YSOs in the Local Group galaxy M33, marking the first time that individual YSOs have been identified at these large distances. We present Mid-Infrared Instrument (MIRI) imaging mosaics at 5.6 and 21 $$\mu$$m that cover a significant portion of one of M33’s spiral arms that has existing panchromatic imaging from the Hubble Space Telescope and deep Atacama Large Millimeter/submillimeter Array CO measurements. Using these MIRI and Hubble Space Telescope images, we identify point sources using the new dolphot MIRI module. We identify 793 candidate YSOs from cuts based on colour, proximity to giant molecular clouds (GMCs), and visual inspection. Similar to Milky Way GMCs, we find that higher mass GMCs contain more YSOs and YSO emission, which further show YSOs identify star formation better than most tracers that cannot capture this relationship at cloud scales. We find evidence of enhanced star formation efficiency in the southern spiral arm by comparing the YSOs to the molecular gas mass. 

Abstract The dwarf galaxy Triangulum (M33) presents an interesting testbed for studying stellar halo formation: it is sufficiently massive so as to have likely accreted smaller satellites, but also lies within the regime where feedback and other “in situ” formation mechanisms are expected to play a role. In this work, we analyze the line-of-sight kinematics of stars across M33 from the TREX survey, with a view to understanding the origin of its halo. We split our sample into two broad populations of varying age, comprising 2032 “old” red giant branch stars and 671 “intermediate-age” asymptotic giant branch and carbon stars. We find decisive evidence for two distinct kinematic components in both the old and intermediate-age populations: a low-dispersion (∼22 km s⁻¹) disk-like component corotating with M33's Higas and a significantly higher-dispersion component (∼50–60 km s⁻¹) that does not rotate in the same plane as the gas and is thus interpreted as M33's stellar halo. While kinematically similar, the fraction of stars associated with the halo component differs significantly between the two populations: this is consistently ∼10% for the intermediate-age population, but decreases from ∼34% to ∼10% as a function of radius for the old population. We additionally find evidence that the intermediate-age halo population is systematically offset from the systemic velocity of M33 by ∼25 km s⁻¹, with a preferred central LOS velocity of ∼ − 155 km s⁻¹. This is the first detection and characterization of an intermediate-age halo in M33, and suggests in situ formation mechanisms, as well as potentially tidal interactions, have helped shaped it. 

ABSTRACT We use young clusters and giant molecular clouds (GMCs) in the galaxies M33 and M31 to constrain temporal and spatial scales in the star formation process. In M33, we compare the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) catalogue of 1214 clusters with ages measured via colour–magnitude diagram (CMD) fitting to 444 GMCs identified from a new 35 pc resolution Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(2–1) survey. In M31, we compare the Panchromatic Hubble Andromeda Treasury (PHAT) catalogue of 1249 clusters to 251 GMCs measured from a Combined Array for Research in Millimeter-wave Astronomy (CARMA) 12CO(1–0) survey with 20 pc resolution. Through two-point correlation analysis, we find that young clusters have a high probability of being near other young clusters, but correlation between GMCs is suppressed by the cloud identification algorithm. By comparing the positions, we find that younger clusters are closer to GMCs than older clusters. Through cross-correlation analysis of the M33 cluster data, we find that clusters are statistically associated when they are ≤10 Myr old. Utilizing the high precision ages of the clusters, we find that clusters older than ≈18 Myr are uncorrelated with the molecular interstellar medium (ISM). Using the spatial coincidence of the youngest clusters and GMCs in M33, we estimate that clusters spend ≈4–6 Myr inside their parent GMC. Through similar analysis, we find that the GMCs in M33 have a total lifetime of ≈11–15 Myr. We also develop a drift model and show that the above correlations can be explained if the clusters in M33 have a 5–10 km s−1 velocity dispersion relative to the molecular ISM. 

Abstract We investigate whether the considerable diversity in the satellite populations of nearby Milky Way (MW)-mass galaxies is connected with the diversity in their host’s merger histories. Analyzing eight nearby galaxies with extensive observations of their satellite populations and stellar halos, we characterize each galaxy’s merger history using the metric of its most dominant merger,M_⋆,Dom, defined as the greater of either its total accreted stellar mass or most massive current satellite. We find an unexpectedly tight relationship between these galaxies’ number ofM_V< − 9 satellites within 150 kpc (N_Sat) andM_⋆,Dom. This relationship remains even after accounting for differences in galaxy mass. Using the star formation and orbital histories of satellites around the MW and M81, we demonstrate that both likely evolved along theM_⋆,Dom–N_Satrelation during their current dominant mergers with the Large Magellanic Cloud and M82, respectively. We investigate the presence of this relation in galaxy formation models, including using the Feedback In Realistic Environments (FIRE) simulations to directly compare to the observations. We find no relation betweenM_⋆,DomandN_Satin FIRE, and a universally large scatter inN_SatwithM_⋆,Domacross simulations—in direct contrast with the tightness of the empirical relation. This acute difference in the observed and predicted scaling relation between two fundamental galaxy properties signals that current simulations do not sufficiently reproduce diverse merger histories and their effects on satellite populations. Explaining the emergence of this relation is therefore essential for obtaining a complete understanding of galaxy formation. 

Abstract It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby (D∼ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal-poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of ∼2.8 × 10⁸M_⊙, with a median metallicity of [M/H] = −1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging. 

Abstract M64, often called the “Evil Eye” galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a “flyby” interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell ofM_⋆= 1.80 ± 0.54 × 10⁸M_⊙and [M/H] = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 10⁸M_⊙for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite ofM_⋆,prog≃ 5 × 10⁸M_⊙. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.