Using the group crossing time t c as an age indicator for galaxy groups, we have investigated the correlation between t c and the group spiral fraction, as well as between t c and the neutral hydrogen gas fraction of galaxy groups. Our galaxy group sample is selected from the SDSS DR7 catalog, and the group spiral fraction is derived from the Galaxy Zoo morphological data set. We found that the group spiral galaxy fraction is correlated with the group crossing time. We further cross-matched the latest released ALFALFA 70% H i source catalog with the SDSS group catalog and have identified 172 groups from the SDSS survey whose total H i mass can be derived by summing up the H i mass of all the H i sources within the group radius. For the galaxies not detected in ALFALFA, we estimate their H i masses based on the galaxies’ optical colors and magnitudes. Our sample groups contain more than eight member galaxies, they cover a wide range of halo masses and are distributed in different cosmic environments. We derived the group H i mass fraction, which is the ratio of group H i mass to the group virial mass. We found a correlation between the H i mass fraction and the group crossing time. Our results suggest that long timescale mechanisms such as starvation seem to play a more important role than short timescale processes like stripping in depleting H i gas in the SDSS galaxy groups.

UCHII G25.4NW is a bright IR source in the inner Galaxy region. New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the UCHII region up to 95 km s −1 , and there is not any other absorptions up to the tangential velocity. It reveals that G25.4NW is at a near-side distance of 5.7 kpc, and it is located in the inner Galactic molecular ring region. Using the new distance, the bolometric luminosity of G25.4NW is estimated as 10 5.6 L ⊙ , which corresponds to an O6 star. It contains 460 M ⊙ of ionized gas. High-resolution 13 CO image from the Galactic Ring Survey (GRS) reveals that G25.4NW is part of a more extended star-formation complex with about 10 4 M ⊙ molecular gas.

We report the discovery of a possible accretion stream toward a Milky Way–type galaxy M106 based on very deep H i imaging data with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The accretion stream extends for about 130 kpc in projection length and it is similar to the Magellanic stream in many respects. We provide unambiguous evidence based on the stream morphology, kinematics and local star formation activity to show that the H i gas is being accreted onto the disk of M106. Such a long continuous flow of gas provides a unique opportunity to probe the circumgalactic medium (CGM) and reveals how the gas stream traverses the hot halo and CGM, and eventually reaches the galaxy disk. The source of the stream appears to be from M106's satellite galaxy NGC 4288. We argue that the stream of gas could be due to the tidal interaction with NGC 4288, or with a high speed encounter near this system. Close to the position of UGC 7356 the stream bifurcates into two streams. The second stream may be gas tidally stripped from UGC 7356 or due to an interaction with UGC 7356. Our results show that high-sensitivity H i imaging is crucial in revealing low column density accretion features in nearby galaxies.