156 research outputs found
Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters
We investigate the relationship between brightest cluster galaxies (BCGs) and
their host clusters using a sample of nearby galaxy clusters from the
Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged
with the Southern Observatory for Astrophysical Research (SOAR) in R band to
investigate the mass of the old stellar population. Using a metric radius of
12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster
mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found
that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray
emission, including all of the cool core (CC) clusters. We also found an
unexpected correlation between the BCG metric luminosity and the core gas
density for non-cool core (non-CC) clusters, following a power law of n_e
\propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation
is not easily explained by star formation (which is weak in non-CC clusters) or
overall cluster mass (which is not correlated with core gas density). The trend
persists even when the BCG is not located near the peak of the X-ray emission,
so proximity is not necessary. We suggest that, for non-CC clusters, this
correlation implies that the same process that sets the central entropy of the
cluster gas also determines the central stellar density of the BCG, and that
this underlying physical process is likely to be mergers.Comment: 16 pages, 8 figures, accepted Astrophysical Journa
AGC 226067: A possible interacting low-mass system
We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source
AGC 226067. Originally identified as an ultra-compact high velocity cloud and
candidate Local Group galaxy, AGC 226067 is spatially and kinematically
coincident with the Virgo cluster, and the identification by multiple groups of
an optical counterpart with no resolved stars supports the interpretation that
this systems lies at the Virgo distance (D=17 Mpc). The combined observations
reveal that the system consists of multiple components: a central HI source
associated with the optical counterpart (AGC 226067), a smaller HI-only
component (AGC 229490), a second optical component (AGC 229491), and extended
low surface brightness HI. Only ~1/4 of the single-dish HI emission is
associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which
is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x
10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has
M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g =
3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC
226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC
229491 are separated by only 0.5'. The current data do not resolve the HI in
AGC 229490 and its origin is unclear. We discuss possible scenarios for this
system of objects: an interacting system of dwarf galaxies, accretion of
material onto AGC 226067, or stripping of material from AGC 226067.Comment: Accepted for publication in A&A. 6 pages, 4 figure
The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
We present VLA HI spectral line imaging of 5 sources discovered by ALFALFA.
These targets are drawn from a larger sample of systems that were not uniquely
identified with optical counterparts during ALFALFA processing, and as such
have unusually high HI mass to light ratios. These candidate "Almost Dark"
objects fall into 4 categories: 1) objects with nearby HI neighbors that are
likely of tidal origin; 2) objects that appear to be part of a system of
multiple HI sources, but which may not be tidal in origin; 3) objects isolated
from nearby ALFALFA HI detections, but located near a gas-poor early-type
galaxy; 4) apparently isolated sources, with no object of coincident redshift
within ~400 kpc. Roughly 75% of the 200 objects without identified counterparts
in the .40 database (Haynes et al. 2011) fall into category 1. This
pilot sample contains the first five sources observed as part of a larger
effort to characterize HI sources with no readily identifiable optical
counterpart at single dish resolution. These objects span a range of HI mass
[7.41 < log(M) < 9.51] and HI mass to B-band luminosity ratios (3 <
M/L < 9). We compare the HI total intensity and velocity
fields to SDSS optical imaging and to archival GALEX UV imaging. Four of the
sources with uncertain or no optical counterpart in the ALFALFA data are
identified with low surface brightness optical counterparts in SDSS imaging
when compared with VLA HI intensity maps, and appear to be galaxies with clear
signs of ordered rotation. One source (AGC 208602) is likely tidal in nature.
We find no "dark galaxies" in this limited sample. The present observations
reveal complex sources with suppressed star formation, highlighting both the
observational difficulties and the necessity of synthesis follow-up
observations to understand these extreme objects. (abridged)Comment: Astronomical Journal, in pres
The dwarf galaxy population as revealed by ALFALFA
The combination of sensitivity and large sky coverage of the ALFALFA HI survey has enabled the detection of difficult to observe low mass galaxies in large numbers, including dwarf galaxies overlooked in optical surveys. Three different, but connected, studies of dwarf galaxies from the ALFALFA survey are of particular interest: SHIELD (Survey of HI in Extremely Low-mass Dwarfs), candidate gas-rich ultra- faint dwarf galaxies, and the (Almost) Dark population. SHIELD is a systematic multiwavelength study of all dwarf galaxies from ALFALFA with MHI < 107.2 M☉ and clear optical counterparts. Candidate gas-rich ultra-faint dwarf galaxies extend the dwarf galaxy population to even lower masses. These galaxies are identified as isolated HI clouds with no discernible optical counterpart but subsequent observations reveal that some are extremely faint, gas- dominated galaxies. Leo P, discovered first as an HI detection, and then found to be an actively star-forming galaxy, bridges the gap between these candidate galaxies and the SHIELD sample. The (Almost) Dark sample consists of galaxies whose optical counterparts are overlooked in current optical surveys but which are clear detections in ALFALFA. This sample includes field gas-rich ultra-diffuse galaxies. Coma P, with a peak surface brightness of only ~26.4 mag arcsec-2 in g', demonstrates the sort of extreme low surface brightness galaxy that can be discovered in an HI survey
The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging
We present Hubble Space Telescope (HST) observations of the low surface
brightness (SB) galaxy Coma P. This system was first discovered in the Arecibo
Legacy Fast ALFA HI survey and was cataloged as an (almost) dark galaxy because
it did not exhibit any obvious optical counterpart in the available survey data
(e.g., Sloan Digital Sky Survey). Subsequent WIYN pODI imaging revealed an
ultra-low SB stellar component located at the center of the HI detection. We
use the HST images to produce a deep color-magnitude diagram (CMD) of the
resolved stellar population present in Coma P. We clearly detect a red stellar
sequence that we interpret to be a red giant branch, and use it to infer a tip
of the red giant branch (TRGB) distance of 5.50 Mpc. The new
distance is substantially lower than earlier estimates and shows that Coma P is
an extreme dwarf galaxy. Our derived stellar mass is only 4.3 10
, meaning that Coma P has an extreme HI-to-stellar mass ratio of 81.
We present a detailed analysis of the galaxy environment within which Coma P
resides. We hypothesize that Coma P formed within a local void and has spent
most of its lifetime in a low-density environment. Over time, the gravitational
attraction of the galaxies located in the void wall has moved it to the edge,
where it had a recent "fly-by" interaction with M64. We investigate the
possibility that Coma P is at a farther distance and conclude that the
available data are best fit by a distance of 5.5 Mpc.Comment: 22 pages, 11 figures. Accepted for publication in the Astronomical
Journa
VLA Imaging of H i-bearing Ultra-Diffuse Galaxies from the ALFALFA Survey
Ultra-diffuse galaxies have generated significant interest due to their large optical extents and low optical surface brightnesses, which challenge galaxy formation models. Here we present resolved synthesis observations of 12 H i-bearing ultra-diffuse galaxies (HUDs) from the Karl G. Jansky Very Large Array (VLA), as well as deep optical imaging from the WIYN 3.5-meter telescope at Kitt Peak National Observatory. We present the data processing and images, including total intensity H i maps and H i velocity fields. The HUDs show ordered gas distributions and evidence of rotation, important prerequisites for the detailed kinematic models in Mancera PiËœna et al. (2019b). We compare the H i and stellar alignment and extent, and find the H i extends beyond the already extended stellar component and that the H i disk is often misaligned with respect to the stellar one, emphasizing the importance of caution when approaching inclination measurements for these extreme sources. We explore the H i mass-diameter scaling relation, and find that although the HUDs have diffuse stellar populations, they fall along the relation, with typical global H i surface densities. This resolved sample forms an important basis for more detailed study of the H i distribution in this extreme extragalactic population
VLA Imaging of H I-bearing Ultra-diffuse Galaxies from the ALFALFA Survey
Ultra-diffuse galaxies have generated significant interest due to their large optical extents and low optical surface brightnesses, which challenge galaxy formation models. Here we present resolved synthesis observations of 12 H i-bearing ultra-diffuse galaxies (HUDs) from the Karl G. Jansky Very Large Array (VLA), as well as deep optical imaging from the WIYN 3.5-meter telescope at Kitt Peak National Observatory. We present the data processing and images, including total intensity H i maps and H i velocity fields. The HUDs show ordered gas distributions and evidence of rotation, important prerequisites for the detailed kinematic models in Mancera PiËœna et al. (2019b). We compare the H i and stellar alignment and extent, and find the H i extends beyond the already extended stellar component and that the H i disk is often misaligned with respect to the stellar one, emphasizing the importance of caution when approaching inclination measurements for these extreme sources. We explore the H i mass-diameter scaling relation, and find that although the HUDs have diffuse stellar populations, they fall along the relation, with typical global H i surface densities. This resolved sample forms an important basis for more detailed study of the H i distribution in this extreme extragalactic population
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