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Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M 81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol) HI clouds with no obvious optical counterparts. We have searched 10 fields in the M81 group totalling 2.2 square degree, both targeting known dwarf spheroidal galaxies and blank fields around the central triplet. Our observations show that the new population of low-mass HI clouds appears to be confined to a region toward the South-East of the central triplet (at distances of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds are that they are related to the dSphs found to the South-East of M 81, that they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they are of primordial nature and provide fresh, unenriched material falling into the M 81 group, or that they are tidal debris from the 3-body interaction involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently favour the latter explanation.
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We present the initial results from an [O III] lambda 5007 survey for intra-group planetary nebulae in the M 81 group of galaxies. A total of 0.36 square degrees of the survey have been analyzed thus far, and a total of four intra-group candidates have been detected. These data allow us to probe the physics of galaxy interactions in small groups, and give us an upper limit for the density of intracluster starlight. We find that the M 81 group has less than 3% of its stars in an intra-group component; this is much less than the fraction found in richer galaxy clusters.
The Hubble Space Telescope observations of the nearby galaxy group M 81/M 82 and its vicinity indicate that the expansion outflow around the group is dominated by the antigravity of the dark energy background. The local density of dark energy in the area is estimated to be near the global dark energy density or perhaps exactly equal to it. This conclusion agrees with our previous results for the Local group vicinity and the vicinity of the Cen A/M 83 group.
We report the discovery of two kinematically anomalous atomic hydrogen (HI) clouds in M 100 (NGC 4321), which was observed as part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey in HI 21 cm at 3.3 km s$^{-1}$ spectroscopic and 44 arcsec$times$30 arcsec spatial resolution using the Karl G. Jansky Very Large Array. These clouds were identified as structures that show significant kinematic offsets from the rotating disk of M100. The velocity offsets of 40 km s$^{-1}$ observed in these clouds are comparable to the offsets seen in intermediate-velocity clouds (IVCs) in the circumgalactic medium (CGM) of the Milky Way and nearby galaxies. We find that one anomalous cloud in M 100 is associated with star-forming regions detected in H$alpha$ and far-ultraviolet imaging. Our investigation shows that anomalous clouds in M 100 may originate from multiple mechanisms, such as star formation feedback-driven outflows, ram-pressure stripping, and tidal interactions with satellite galaxies. Moreover, we do not detect any cool CGM at 38.8 kpc from the center of M 100, giving an upper limit of N(HI) $le$ $1.7times10^{13}$ cm$^{-2}$ (3$sigma$). Since M 100 is in the Virgo cluster, the non-existence of neutral/cool CGM is a likely pathway for turning it into a red galaxy.
A set of HI sources extracted from the north Galactic polar region by the ongoing ALFALFA survey has properties that are consistent with the interpretation that they are associated with isolated minihalos in the outskirts of the Local Group (LG). Unlike objects detected by previous surveys, such as the Compact High Velocity Clouds of Braun & Burton (1999), the HI clouds found by ALFALFA do not violate any structural requirements or halo scaling laws of the LambdaCDM structure paradigm, nor would they have been detected by extant HI surveys of nearby galaxy groups other than the LG. At a distance of d Mpc, their HI masses range between $5 x 10^4 d^2 and 10^6 d^2 solar and their HI radii between <0.4d and 1.6 d kpc. If they are parts of gravitationally bound halos, the total masses would be on order of 10^8--10^9 solar, their baryonic content would be signifcantly smaller than the cosmic fraction of 0.16 and present in a ionized gas phase of mass well exceeding that of the neutral phase. This study does not however prove that the minihalo interpretation is unique. Among possible alternatives would be that the clouds are shreds of the Leading Arm of the Magellanic Stream.
We present a new high-sensitivity HI observation toward nearby spiral galaxy M101 and its adjacent 2$^{circ}times$ 2$^{circ}$ region using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From the observation, we detect a more extended and asymmetric HI disk around M101. While the HI velocity field within the M101s optical disk region is regular, indicating that the relatively strong disturbance occurs in its outer disk. Moreover, we identify three new HI clouds located on the southern edge of the M101s HI disk. The masses of the three HI clouds are 1.3$times$10$^{7}$ $M_{odot}$, 2.4$times$10$^{7}$ $M_{odot}$, and 2.0$times$10$^{7}$ $M_{odot}$, respectively. The HI clouds similar to dwarf companion NGC 5477 rotate with the HI disk of M101. Unlike the NGC 5477, they have no optical counterparts. Furthermore, we detect a new HI tail in the extended HI disk of M101. The HI tail detected gives a reliable evidence for M101 interaction with the dwarf companion NGC 5474. We argue that the extra-planar gas (three HI clouds) and the HI tail detected in the M101s disk may origin from a minor interaction with NGC 5474.
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