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 L. M. Young Physics , 1999, DOI: 10.1086/301187 Abstract: Previous searches for atomic gas in our Galaxy's dwarf spheroidal companions have not been complete enough to settle the question of whether or not these galaxies have HI, especially in their outer parts. We present new observations of the dwarf spheroidals Sextans, Leo I, Ursa Minor, and Draco, using the NRAO 140-foot telescope to search much farther in radius than has been done before. The new data go out to at least 2.5 times the core radius in all cases, and well beyond even the tidal radius in two cases. These observations give HI column density limits of 2-6 x 10^17 atoms cm^-2. Unless HI is quite far from the galaxies' centers, we conclude that these galaxies don't contain significant amounts of atomic gas at the present time. We discuss whether the observations could have missed some atomic gas.
 Physics , 1999, Abstract: We have measured the 21-cm line of Galactic HI over more than 50 square degrees in the direction of the Sagittarius dwarf spheroidal galaxy. The data show no evidence of HI associated with the dwarf spheroidal which might be consider analogous to the Magellanic Stream as it is associated in both position and velocity with the Large Magellanic Cloud. Nor do the HI data show evidence for any disturbance in the Milky Way disk gas that can be unambiguously assigned to interaction with the dwarf galaxy. The data shown here limit the HI mass at the velocity of the Sagittarius dwarf to <7000 solar masses over some 18 square degrees between Galactic latitudes -13 degrees and -18 degrees.
 Physics , 1998, DOI: 10.1086/300540 Abstract: Neutral hydrogen (HI) has been detected in the Local Group dwarf spheroidal galaxy Sculptor with the 64 m Parkes single dish and mapped with the Australia Telescope synthesis array. Most of the detected HI is in two clouds \sim 15'-20' away from the optical center. The gas is observed at the same systemic velocity than the stars but at \ge 125 \kms away from the Magellanic Stream components in that region. A lower limit to the HI mass of 3.0 \times 10^4 \Msol is derived from the synthesis observation for an M_HI/L_B \simeq 0.02. This amount of HI is compatible with mass loss expected from normal giants even if only 10% of the gas is retained by the galaxy in its neutral form.
 Physics , 2003, DOI: 10.1086/377312 Abstract: New observations of the neutral hydrogen (HI) in and around the line of sight of the Sculptor dwarf spheroidal (dSph) are presented. The data obtained with the single-dish Parkes telescope cover a large area of 7\degr x 7\degr in the direction of the dwarf, and have resolutions of 15\farcm x 1.12 km/s. The Australia Telescope Compact Array (ATCA) was used to map a smaller area of 2\degr x 2\degr centered on the direction of the dwarf with higher resolutions (350\arcsec x 140\arcsec x 1.65 km/s). Many HI structures having velocities outside the range of the normal Galactic disk velocities were detected, including the two Sculptor clouds (northeast and southwest) of Carignan et al. (1998, C98). The present study shows the total extent of the C98 clouds. We derived heliocentric radial velocities for the NE and SW clouds of 100.2\pm 0.9 km/s and 105.1\pm 0.3 km/s, respectively. The intensity-weighted mean HI velocity for both clouds is 104.1\pm 0.4 km/s. The mass of each cloud is (4.1\pm 0.2) x 10^4 M_\odot (NE cloud) and (1.93\pm 0.02) x 10^5 M_\odot (SW cloud) at the Sculptor dSph distance (79 kpc).
 Physics , 2011, Abstract: In the cosmological paradigm, cold Dark Matter (DM) dominates the mass content of the Universe and is present at every scale. Candidates for DM include many extensions of the standard model, with a weakly interacting massive particle (WIMP) in the mass range from 50 GeV to greater than 10 TeV. The self-annihilation of WIMPs in astrophysical regions of high DM density can produce secondary particles, including very high energy (VHE) gamma rays, with energies up to the dark matter particle mass. The VERITAS array of Cherenkov telescopes, designed for the detection of VHE gamma rays in the 100 GeV-10 TeV energy range, is an appropriate instrument for the detection of DM and is complementary to Fermi-LAT. Dwarf spheroidal galaxies (dSphs) of the Local Group are potentially the best targets to search for the annihilation signature of DM due to their proximity and large DM content. We report on the latest VERITAS observations of dSphs and discuss the results in the framework of WIMP models.
 Physics , 2003, DOI: 10.1086/368363 Abstract: Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we explore in 40 early- and late-type dwarfs in the Local Group and nearby field. Although dSphs formed stars over extended periods, today all but one are free of detectable interstellar matter (ISM), even in the Fornax dSph, where stars still formed 100 Myr ago. Combining metallicities for red giants with HI data from the literature, we show that the well-known offset in luminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr) galaxies exists also when comparing only their old stellar populations: dSphs have higher mean stellar metallicities for a fixed luminosity. Evidently younger dSphs experienced more efficient enrichment than young dIrrs. Dwarf galaxies, whose locus in the L-Z diagram is consistent with that of dSphs even for baryonic luminosities, are the transition-type dwarfs'' Phoenix, DDO210, LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellar masses, low angular momentum, and HI contents of less than a few 10^6 solar masses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphs if their gas were removed; they are likely dSph progenitors. As gas removal is key, we consider the empirical evidence for various gas removal processes. We suggest that internal gas removal mechanisms are inadequate and favor ram pressure stripping to make dSphs. A combination of initial conditions and environment seems to support the formation of dSphs, which appear to form from small galaxies with active early star formation, whose evolution halts due to externally induced gas loss. Transition-type dwarfs then are dSphs that kept their ISM, and therefore should replace dSphs in isolated locations where stripping is ineffective. (Abridged)
 Physics , 1998, DOI: 10.1046/j.1365-8711.1998.01905.x Abstract: We present 21cm HI line observations of the blue compact dwarf galaxy NGC1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the HII morphology and kinematics. In contrast, most of the HI lies in a rotating disk. An extraplanar HI spur accounts for ~ 8% of the total HI mass, and is possibly associated with the HII outflow. The inferred mass loss rate out of the galaxy's core is significant ~ 0.2 - 2 M_sun/yr, but does not dominate the HI dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density \rho_0 \approx 0.1 M_sun/pc^3: ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM halos may regulate the morphology of dwarf galaxies by setting the critical surface density for disk star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC1705.
 Physics , 2013, DOI: 10.1093/mnras/stu285 Abstract: Dwarf spheroidal satellite galaxies have emerged a powerful probe of small-scale dark matter clustering and of cosmic reionization. They exhibit structural and chemical continuity with dwarf irregular galaxies in the field and with spheroidal galaxies in high-density environments. By combining empirical constraints derived for star formation at low gas column densities and metallicities in the local universe with a model for dark matter and baryonic mass assembly, we provide an analytical description of how the dwarf spheroidals acquired their stellar content. Their progenitors formed stars until the gas content, initially reduced from the cosmic average by the thermal pressure of the reionized intergalactic medium, was finally ram pressure stripped during the progenitors' accretion on to the host galaxy. Dwarf spheroidal satellites of differing luminosities seem to share very similar most massive progenitor histories that reach thresholds for gas cooling by atomic line emission at epochs at which the Lagrangian volume of the Local Group should have been reionized. We hypothesize that dwarf spheroidals formed the bulk of their stars in partially rotationally supported HI disks in a reionized universe. This model provides an explanation for the "common mass scale" relation and reproduces the empirical luminosity-size and luminosity-metallicity relations. Explosive feedback phenomena, such as outflows driven by the concerted action of supernovae, need not have been significant in the dwarf spheroidals' formation. We further speculate that the true pre-reionization fossils should exhibit a structure distinct from that of the dwarf spheroidals, e.g., in the form of dense isolated or nuclear star clusters.
 Physics , 2013, Abstract: Dwarf spheroidal galaxies are characterized by a large measured mass-to-light ratio and are not expected to be the site of high-luminosity non-thermal high-energy gamma-ray emissions. Therefore they are among the most promising candidates for indirect searches of dark matter particle annihilation signals in gamma rays. The Sagittarius dwarf spheroidal galaxy has been regularly observed by the High Energy Stereoscopic System (H.E.S.S.) of Cherenkov telescopes for more than 90 hours, searching for TeV gamma-ray emission from annihilation of dark matter particles. In absence of a significant signal, new constraints on the annihilation crosssection of the dark matter particles applicable for Majorana Weakly Interacting Massive Particles (WIMPs) are derived.
 Physics , 1998, Abstract: Our current knowledge of M31's dwarf spheroidal companions is reviewed. Two topics of recent interest constitute the bulk of this review. First, color-magnitude diagrams reaching below the horizontal branch have been constructed for two M31 dwarf spheroidals based on images from HST/WFPC2. The horizontal branches are predominantly red in both galaxies, redder than expected for their metallicity based on Galactic globular clusters. Thus, the second parameter effect is seen in the M31 halo. Second, recent surveys have revealed three new dwarf spheroidal companions to M31. Thus, dwarf spheroidal galaxies are not as rare around M31 as previously thought and as a result, some properties of the M31 companion system have changed.
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