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Ultrafaint Dwarf Galaxy Candidates in the M81 Group: Signatures of Group Accretion
The faint and ultrafaint dwarf galaxies in the Local Group form the observational bedrock upon which our understanding of small-scale cosmology rests. In order to understand whether this insight generalizes, it is imperative to use resolved-star techniques to discover similarly faint satellites in nearby galaxy groups. We describe our search for ultrafaint galaxies in the M81 group using deep ground-based resolved-star data sets from Subaruâs Hyper Suprime-Cam. We present one new ultrafaint dwarf galaxy in the M81 group and identify five additional extremely low surface brightness candidate ultrafaint dwarfs that reach deep into the ultrafaint regime to M V ⌠â 6 (similar to current limits for Andromeda satellites). These candidatesâ luminosities and sizes are similar to known Local Group dwarf galaxies Tucana B, Canes Venatici I, Hercules, and Boötes I. Most of these candidates are likely to be real, based on tests of our techniques on blank fields. Intriguingly, all of these candidates are spatially clustered around NGC 3077, which is itself an M81 group satellite in an advanced state of tidal disruption. This is somewhat surprising, as M81 itself and its largest satellite M82 are both substantially more massive than NGC 3077 and, by virtue of their greater masses, would have been expected to host as many or more ultrafaint candidates. These results lend considerable support to the idea that satellites of satellites are an important contribution to the growth of satellite populations around Milky Way-mass galaxies
Ultrafaint Dwarf Galaxy Candidates in the M81 Group: Signatures of Group Accretion
The faint and ultrafaint dwarf galaxies in the Local Group form the
observational bedrock upon which our understanding of small-scale cosmology
rests. In order to understand whether this insight generalizes, it is
imperative to use resolved-star techniques to discover similarly faint
satellites in nearby galaxy groups. We describe our search for ultrafaint
galaxies in the M81 group using deep ground-based resolved-star data sets from
Subaru's Hyper Suprime-Cam. We present one new ultrafaint dwarf galaxy in the
M81 group and identify five additional extremely low surface brightness
candidate ultrafaint dwarfs that reach deep into the ultrafaint regime to (similar to current limits for Andromeda satellites). These
candidates' luminosities and sizes are similar to known Local Group dwarf
galaxies Tucana B, Canes Venatici I, Hercules, and Bo\"otes I. Most of these
candidates are likely to be real, based on tests of our techniques on blank
fields. Intriguingly, all of these candidates are spatially clustered around
NGC 3077, which is itself an M81 group satellite in an advanced state of tidal
disruption. This is somewhat surprising, as M81 itself and its largest
satellite M82 are both substantially more massive than NGC 3077 and by virtue
of their greater masses, would have been expected to host as many or more
ultrafaint candidates. These results lend considerable support to the idea that
satellites of satellites are an important contribution to the growth of
satellite populations around Milky Way-mass galaxies.Comment: The Astrophysical Journal Letters; in press. 11 pages, 4 figures, 1
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COOL-LAMPS III: Discovery of a 25".9 Separation Quasar Lensed by a Merging Galaxy Cluster
In the third paper from the COOL-LAMPS Collaboration, we report the discovery
of COOL J0542-2125, a gravitationally lensed quasar at , observed as
three images due to an intervening massive galaxy cluster at . The
lensed quasar images were identified in a search for lens systems in recent
public optical imaging data and have separations on the sky up to 25".9, wider
than any previously known lensed quasar. The galaxy cluster acting as a strong
lens appears to be in the process of merging, with two sub-clusters separated
by Mpc in the plane of the sky, and their central galaxies showing a
radial velocity difference of km/s. Both cluster cores show
strongly lensed images of an assortment of background sources, as does the
region between them. A preliminary strong lens model implies masses of $M(<250\
\rm{kpc}) = 1.79^{+0.16} _{-0.01} \times 10^{14} M_{\odot}M(<250\
\rm{kpc}) = 1.48^{+0.04}_{-0.10} \times 10^{14} M_{\odot}$ for the East and
West sub-clusters, respectively. This line of sight is also coincident with a
ROSAT ALL-sky Survey source, centered between the two confirmed cluster halos
reminiscent of other major cluster-scale mergers.Comment: 13 pages, 6 figures. Submitted to Ap
COOL-LAMPS. VI. Lens Model and New Constraints on the Properties of COOL J1241+2219, a Bright z = 5 Lyman Break Galaxy and its z = 1 Cluster Lens
We present a strong lensing analysis of COOL J1241+2219, the brightest known gravitationally lensed galaxy at z â„ 5, based on new multiband Hubble Space Telescope (HST) imaging data. The lensed galaxy has a redshift of z = 5.043, placing it shortly after the end of the âEpoch of Reionization,â and an AB magnitude z AB = 20.47 mag (Khullar et al.). As such, it serves as a touchstone for future research of that epoch. The high spatial resolution of HST reveals internal structure in the giant arc, from which we identify 15 constraints and construct a robust lens model. We use the lens model to extract the cluster mass and lensing magnification. We find that the mass enclosed within the Einstein radius of the z = 1.001 cluster lens is M(<5.âł77)=1.079â0.007+0.023Ă1013Mâ , significantly lower than other known strong lensing clusters at its redshift. The average magnification of the giant arc is ăÎŒ arcă = 76â20+40 , a factor of 2.4â0.7+1.4 greater than previously estimated from ground-based data; the flux-weighted average magnification is ăÎŒ arcă = 92â31+37 . We update the current measurements of the stellar mass and star formation rate (SFR) of the source for the revised magnification to log(Mâ/Mâ)= 9.7 ± 0.3 and SFR = 10.3â4.4+7.0 M â yrâ1, respectively. The powerful lensing magnification acting upon COOL J1241+2219 resolves the source and enables future studies of the properties of its star formation on a clump-by-clump basis. The lensing analysis presented here will support upcoming multiwavelength characterization with HST and JWST data of the stellar mass assembly and physical properties of this high-redshift lensed galaxy
COOL-LAMPS. IV. A Sample of Bright Strongly Lensed Galaxies at 3 < z < 4
We report the discovery of five bright, strong gravitationally lensed galaxies at 3 < z < 4: COOL J0101+2055 (z = 3.459), COOL J0104â0757 (z = 3.480), COOL J0145+1018 (z = 3.310), COOL J0516â2208 (z = 3.549), and COOL J1356+0339 (z = 3.753). These galaxies have magnitudes of rAB, zAB < 21.81 mag and are lensed by galaxy clusters at 0.26 < z < 1. This sample nearly doubles the number of known bright lensed galaxies with extended arcs at 3 < z < 4. We characterize the lensed galaxies using ground-based grz/giy imaging and optical spectroscopy. We report model-based magnitudes and derive stellar masses, dust content, and star formation rates via stellar population synthesis modeling. Building lens models based on ground-based imaging, we estimate source magnifications ranging from âŒ29 to âŒ180. Combining these analyses, we derive demagnified stellar masses in the range and star formation rates in the youngest age bin in the range , placing the sample galaxies on the massive end of the star-forming main sequence in this redshift interval. In addition, three of the five galaxies have strong Lyα emissions, offering unique opportunities to study Lyα emitters at high redshift in future work
Saying Hallo to M94's Stellar Halo: Investigating the Accretion History of the Largest Pseudobulge Host in the Local Universe
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 âŒ2.8 Ă 10 ^8 M _â , with a median metallicity of [M/H] = â1.4. 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