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1. The TREX Survey: Kinematical Complexity Throughout M33's Stellar Disk and Evidence for a Stellar Halo*

   https://doi.org/10.3847/1538-4357/ac3480  

   Gilbert, Karoline M. ; Quirk, Amanda C. N. ; Guhathakurta, Puragra ; Tollerud, Erik ; Wojno, Jennifer ; Dalcanton, Julianne J. ; Durbin, Meredith J. ; Seth, Anil ; Williams, Benjamin F. ; Fung, Justin T. ; et al ( January 2022 , The Astrophysical Journal)

   We present initial results from a large spectroscopic survey of stars throughout M33's stellar disk. We analyze a sample of 1667 red giant branch (RGB) stars extending to projected distances of ∼11 kpc from M33's center (∼18 kpc, or ∼10 scale lengths, in the plane of the disk). The line-of-sight velocities of RGB stars show the presence of two kinematical components. One component is consistent with rotation in the plane of M33's Hidisk and has a velocity dispersion (∼19 km s⁻¹), consistent with that observed in a comparison sample of younger stars, while the second component has a significantly higher velocity dispersion. A two-component fit to the RGB velocity distribution finds that the high-dispersion component has a velocity dispersion of${59.3}_{-2.5}^{+2.6}$km s⁻¹and rotates very slowly in the plane of the disk (consistent with no rotation at the <1.5σlevel), which favors interpreting it as a stellar halo rather than a thick disk population. A spatial analysis indicates that the fraction of RGB stars in the high-velocity-dispersion component decreases with increasing radius over the range covered by the spectroscopic sample. Our spectroscopic sample establishes that a significant high-velocity-dispersion component is present in M33's RGB population from near M33's centermore »to at least the radius where M33's Hidisk begins to warp at 30′ (∼7.5 kpc) in the plane of the disk. This is the first detection and spatial characterization of a kinematically hot stellar component throughout M33's inner regions.

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2. MAGAZ3NE: High Stellar Velocity Dispersions for Ultramassive Quiescent Galaxies at z ≳ 3*

   https://doi.org/10.3847/1538-4357/ac8747  

   Forrest, Ben ; Wilson, Gillian ; Muzzin, Adam ; Marchesini, Danilo ; Cooper, M. C. ; Marsan, Z. Cemile ; Annunziatella, Marianna ; McConachie, Ian ; Zaidi, Kumail ; Gomez, Percy ; et al ( October 2022 , The Astrophysical Journal)

   In this work, we publish stellar velocity dispersions, sizes, and dynamical masses for eight ultramassive galaxies (UMGs;$\log (M_{*}/M_{\odot })$> 11),z≳ 3) from the Massive Ancient Galaxies Atz> 3 NEar-infrared (MAGAZ3NE) Survey, more than doubling the number of such galaxies with velocity dispersion measurements at this epoch. Using the deep Keck/MOSFIRE and Keck/NIRES spectroscopy of these objects in theHandKbandpasses, we obtain large velocity dispersions of ∼400 km s⁻¹for most of the objects, which are some of the highest stellar velocity dispersions measured and ∼40% larger than those measured for galaxies of similar mass atz∼ 1.7. The sizes of these objects are also smaller by a factor of 1.5–3 compared to this samez∼ 1.7 sample. We combine these large velocity dispersions and small sizes to obtain dynamical masses. The dynamical masses are similar to the stellar masses of these galaxies, consistent with a Chabrier initial mass function (IMF). Considered alongside previous studies of massive quiescent galaxies across 0.2 <z< 4.0, there is evidence for an evolution in the relation between the dynamical mass–stellar mass ratio and velocity dispersion as a function of redshift. This implies an IMF with fewer low-mass stars (e.g., Chabrier IMF) for massive quiescentmore »galaxies at higher redshifts in conflict with the bottom-heavy IMF (e.g., Salpeter IMF) found in their likelyz∼ 0 descendants, though a number of alternative explanations such as a different dynamical structure or significant rotation are not ruled out. Similar to data at lower redshifts, we see evidence for an increase of IMF normalization with velocity dispersion, though thez≳ 3 trend is steeper than that forz∼ 0.2 early-type galaxies and offset to lower dynamical-to-stellar mass ratios.

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3. Galactic Kinematics and Observed Flare Rates of a Volume-complete Sample of Mid-to-late M Dwarfs: Constraints on the History of the Stellar Radiation Environment of Planets Orbiting Low-mass Stars

   https://doi.org/10.3847/1538-4357/ac77f9  

   Medina, Amber A. ; Winters, Jennifer G. ; Irwin, Jonathan M. ; Charbonneau, David ( August 2022 , The Astrophysical Journal)

   We present a study of the relationship between Galactic kinematics, flare rates, chromospheric activity, and rotation periods for a volume-complete, nearly all-sky sample of 219 single stars within 15 pc and with masses between 0.1 and 0.3M_⊙observed during the primary mission of TESS. We find all stars consistent with a common value ofα= 1.984 ± 0.019 for the exponent of the flare frequency distribution. Using our measured stellar radial velocities and Gaia astrometry, we determine GalacticUVWspace motions. We find 78% of stars are members of the Galactic thin disk, 7% belong to the thick disk, and for the remaining 15% we cannot confidently assign membership to either component. If we assume star formation has been constant in the thin disk for the past 8 Gyr, then based on the fraction that we observe to be active, we estimate the average age at which these stars transition from the saturated to the unsaturated flaring regime to be 2.4 ± 0.3 Gyr. This is consistent with the ages that we assign from Galactic kinematics: we find that stars with rotation periodP_rot< 10 days have an age of 2.0 ± 1.2 Gyr, stars with 10 days <P_rot≤ 90 days have an agemore »of 5.6 ± 2.7 Gyr, and stars withP_rot> 90 days have an age of 12.9 ± 3.5 Gyr. We find that the average age of stars withP_rot< 10 days increases with decreasing stellar mass from 0.6 ± 0.3 Gyr (0.2–0.3M_⊙) to 2.3 ± 1.3 Gyr (0.1–0.2M_⊙).

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4. A Glimpse of the Stellar Populations and Elemental Abundances of Gravitationally Lensed, Quiescent Galaxies at z ≳ 1 with Keck Deep Spectroscopy

   https://doi.org/10.3847/1538-4357/acc79b  

   Zhuang, Zhuyun ; Leethochawalit, Nicha ; Kirby, Evan N. ; Nightingale, J. W. ; Steidel, Charles C. ; Glazebrook, Karl ; Barone, Tania M. ; Skobe, Hannah ; Sweet, Sarah M. ; Nanayakkara, Themiya ; et al ( May 2023 , The Astrophysical Journal)

   Gravitational lenses can magnify distant galaxies, allowing us to discover and characterize the stellar populations of intrinsically faint, quiescent galaxies that are otherwise extremely difficult to directly observe at high redshift from ground-based telescopes. Here, we present the spectral analysis of two lensed, quiescent galaxies atz≳ 1 discovered by theASTRO 3D Galaxy Evolution with Lensessurvey:AGEL1323 (M_*∼ 10^11.1M_⊙,z= 1.016,μ∼ 14.6) andAGEL0014 (M_*∼ 10^11.5M_⊙,z= 1.374,μ∼ 4.3). We measured the age, [Fe/H], and [Mg/Fe] of the two lensed galaxies using deep, rest-frame-optical spectra (S/N ≳40 Å⁻¹) obtained on the Keck I telescope. The ages ofAGEL1323 andAGEL0014 are${5.6}_{-0.8}^{+0.8}$Gyr and${3.1}_{-0.3}^{+0.8}$Gyr, respectively, indicating that most of the stars in the galaxies were formed less than 2 Gyr after the Big Bang. Compared to nearby quiescent galaxies of similar masses, the lensed galaxies have lower [Fe/H] and [Mg/H]. Surprisingly, the two galaxies have comparable [Mg/Fe] to similar-mass galaxies at lower redshifts, despite their old ages. Using a simple analytic chemical evolution model connecting the instantaneously recycled element Mg with the mass-loading factors of outflows averaged over the entire star formation history, we found that the lensed galaxies may have experienced enhanced outflows during their star formation compared to lower-redshift galaxies,more »which may explain why they quenched early.

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5. Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

   https://doi.org/10.3847/1538-4357/ac7c6c  

   Jones, Michael G. ; Sand, David J. ; Bellazzini, Michele ; Spekkens, Kristine ; Karunakaran, Ananthan ; Adams, Elizabeth A. K. ; Battaglia, Giuseppina ; Beccari, Giacomo ; Bennet, Paul ; Cannon, John M. ; et al ( August 2022 , The Astrophysical Journal)

   We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with Hiobservations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M_*∼ 10⁵M_⊙), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich,$12+\log (\mathrm{O}/\mathrm{H})>8.2$, Hiiregions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in Hi, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, inmore »particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.

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