More Like this

1. The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated-light Spectroscopy

   https://doi.org/10.3847/1538-4357/acd9a9  

   Gibson, Benjamin J. ; Zasowski, Gail ; Seth, Anil ; Ashok, Aishwarya ; Goold, Kameron ; Wainer, Tobin ; Hasselquist, Sten ; Holtzman, Jon ; Imig, Julie ; Bizyaev, Dmitry ; et al ( July 2023 , The Astrophysical Journal)

   We present an analysis of nearly 1000 near-infrared, integrated-light spectra from APOGEE in the inner ∼7 kpc of M31. We utilize full-spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [α/M]. With this, we determine the mean kinematics, metallicities,αabundances, and ages of the stellar populations of M31's bar, bulge, and inner disk (∼4–7 kpc). We find a nonaxisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is less metal-rich (mean [M/H] =$-{0.149}_{-0.081}^{+0.067}$dex) than the disk, features minima in metallicity on either side of the bar ([M/H] ∼ −0.2), and is enhanced inαabundance (mean [α/M] =${0.281}_{-0.038}^{+0.035}$). The disk of M31 within ∼7 kpc is enhanced in both metallicity ([M/H] =$-{0.023}_{-0.052}^{+0.050}$) andαabundance ([α/M] =${0.274}_{-0.025}^{+0.020}$). Both of these structural components are uniformly old at ≃12 Gyr. We find the mean metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis (0.043 ± 0.021 dex kpc⁻¹). This gradient is the result of changing light contributions from the bulge and disk. The chemodynamics of stellar populations encodes information about a galaxy’s chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies.

    

   more » « less
2. DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of Our Nearest Neighbor

   https://doi.org/10.3847/1538-4357/aca5f8  

   Dey, Arjun ; Najita, Joan R. ; Koposov, Sergey E. ; Josephy-Zack, J. ; Maxemin, Gabriel ; Bell, Eric F. ; Poppett, C. ; Patel, E. ; Beraldo e Silva, L. ; Raichoor, A. ; et al ( February 2023 , The Astrophysical Journal)

   We present Dark Energy Spectroscopic Instrument (DESI) observations of the inner halo of M31, which reveal the kinematics of a recent merger—a galactic immigration event—in exquisite detail. Of the 11,416 sources studied in 3.75 hr of on-sky exposure time, 7438 are M31 sources with well-measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf, and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H] > − 0.5) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be${\log }_{10}{M}_{\mathrm{NFW}}(<125\mathrm{kpc})/M_{\odot }={11.80}_{-0.10}^{+0.12}$. The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.

    

   more » « less
3. Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana

   https://doi.org/10.3847/1538-4357/ad25ed  

   Fu, Sal Wanying ; Weisz, Daniel R. ; Starkenburg, Else ; Martin, Nicolas ; Mercado, Francisco J. ; Savino, Alessandro ; Boylan-Kolchin, Michael ; Côté, Patrick ; Dolphin, Andrew E. ; Longeard, Nicolas ; et al ( April 2024 , The Astrophysical Journal)

   We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with$〈\mathrm{[Fe/H]}〉=-{1.55}_{-0.04}^{+0.04}$and${\sigma }_{\mathrm{[Fe/H]}}={0.54}_{-0.03}^{+0.03}$. (ii) A metallicity gradient of −0.54 ± 0.07 dex$R_e^{-1}$(−2.1 ± 0.3 dex kpc⁻¹) over the extent of our imaging (∼2.5R_e), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores. (iii) Tucana’s bifurcated RGB has distinct metallicities: a blue RGB with$〈\mathrm{[Fe/H]}〉=-{1.78}_{-0.06}^{+0.06}$and${\sigma }_{\mathrm{[Fe/H]}}={0.44}_{-0.06}^{+0.07}$and a red RGB with$〈\mathrm{[Fe/H]}〉=-{1.08}_{-0.07}^{+0.07}$and${\sigma }_{\mathrm{[Fe/H]}}={0.42}_{-0.06}^{+0.06}$. (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼0.4 dex, but its blue RGB is chemically comparable to the satellites. Tucana’s MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well populated. (v) About 2% of Tucana’s stars have [Fe/H] < −3% and 20% have [Fe/H] > −1. We provide a catalog for community spectroscopic follow-up.

    

   more » « less
4. 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 center 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.

    

   more » « less
5. Hubble Space Telescope Imaging of Isolated Local Volume Dwarfs GALFA Dw3 and Dw4

   https://doi.org/10.3847/1538-4357/ac356c  

   Bennet, P. ; Sand, D. J. ; Crnojević, D. ; Weisz, D. R. ; Caldwell, N. ; Guhathakurta, P. ; Hargis, J. R. ; Karunakaran, A. ; Mutlu-Pakdil, B. ; Olszewski, E. ; et al ( January 2022 , The Astrophysical Journal)

   We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys on the Hubble Space Telescope. These galaxies were initially discovered as optical counterparts to compact Hiclouds in the GALFA survey. Both objects resolve into stellar populations which display old red giant branch (RGB), younger helium-burning, and massive main sequence stars. We use the tip of the RGB method to determine the distance to each galaxy, finding distances of${7.61}_{-0.29}^{+0.28}$Mpc and${3.10}_{-0.17}^{+0.16}$Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within ∼1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain Hiiregions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and Hαobservations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited Hiiregions. Compact Hisources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 and Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution.

    

   more » « less