Arp 65 interaction debris: massive HI displacement and star formation

Context: Pre-merger interactions between galaxies can induce significant changes in the morphologies and kinematics of the stellar and ISM components. Large amounts of gas and stars are often found to be disturbed or displaced as tidal debris. This debris then evolves, sometimes forming stars and occasionally tidal dwarf galaxies. Here we present results from our HI study of Arp 65, an interacting pair hosting extended HI tidal debris. Aims: In an effort to understand the evolution of tidal debris produced by interacting pairs of galaxies, including in situ star and tidal dwarf galaxy formation, we are mapping HI in a sample of interacting galaxy pairs. The Arp 65 pair is one of them. Methods: Our resolved HI 21 cm line survey is being carried out using the Giant Metrewave Radio Telescope (GMRT). We used our HI survey data as well as available SDSS optical, Spitzer infra-red and GALEX UV data to study the evolution of the tidal debris and the correlation of HI with the star-forming regions within it. Results: In Arp 65 we see a high impact pre-merger interaction involving a pair of massive galaxies (NGC 90 and NGC 93) that have a stellar mass ratio of ~ 1:3. The interaction, which probably occurred ~ 1.0 -- 2.5 $\times$ 10$^8$ yr ago, appears to have displaced a large fraction of the HI in NGC 90 (including the highest column density HI) beyond its optical disk. We also find extended ongoing star formation in the outer disk of NGC 90. In the major star-forming regions, we find the HI column densities to be ~ 4.7 $\times$ 10$^{20}$ cm$^{-2}$ or lower. But no signature of star formation was found in the highest column density HI debris, SE of NGC 90. This indicates conditions within the highest column density HI debris remain hostile to star formation and it reaffirms that high HI column densities may be a necessary but not sufficient criterion for star formation.Comment: Accepted in A&

The curious case of J113924.74+164144.0: a possible new group of galaxies at z = 0.0693

J113924.74+164144.0 is an interesting galaxy at z = 0.0693, i.e. D_L ~ 305 Mpc, with tidal-tail-like extended optical features on both sides. There are two neighbouring galaxies, a spiral galaxy J113922.85+164136.3 which has a strikingly similar 'tidal' morphology, and a faint galaxy J113923.58+164129.9. We report HI 21 cm observations of this field to search for signatures of possible interaction. Narrow HI emission is detected from J113924.74+164144.0, but J113922.85+164136.3 shows no detectable emission. The total HI mass detected in J113924.74+164144.0 is 7.7 x 10^9 M_solar. The HI emission from the galaxy is found to be extended and significantly offset from the optical position of the galaxy. We interpret this as signature of possible interaction with the neighbouring spiral galaxy. There is also a possible detection of HI emission from another nearby galaxy J113952.31+164531.8 at z = 0.0680 at a projected distance of 600 kpc, and with a total HI mass of 5.3 x 10^9 M_solar, suggesting that all these galaxies form a loose group at z ~ 0.069.Comment: 5 pages, 1 table, 6 figures. Accepted for publication in MNRAS Letters. The definitive version will be available at http://www.blackwell-synergy.com

HI content in galaxies in loose groups

Gas deficiency in cluster spirals is well known and ram-pressure stripping is considered the main gas removal mechanism. In some compact groups too gas deficiency is reported. However, gas deficiency in loose groups is not yet well established. Lower dispersion of the member velocities and the lower density of the intra-group medium in small loose groups favour tidal stripping as the main gas removal process in them. Recent releases of data from HI Parkes all sky survey (HIPASS) and catalogues of nearby loose groups with associated diffuse X-ray emission have allowed us to test this notion. In this paper, we address the following questions: (a) do galaxies in groups with diffuse X-ray emission statistically have lower gas content compared to the ones in groups without diffuse X-ray emission? (b) does HI deficiency vary with the X-ray luminosity of the loose group in a systematic way? We find that (a) galaxies in groups with diffuse X-ray emission, on average, are HI deficient, and have lost more gas compared to those in groups without X-ray emission; the later are found not to have significant HI deficiency; (b) no systematic dependence of the HI deficiency with X-ray luminosity is found. Ram pressure assisted tidal stripping and evaporation by thermal conduction are the two possible mechanisms to account for this excess gas loss.Comment: 9 pages, 4 figures, accepted in MNRA

HI in Arp72 and similarities with M51-type systems

We present neutral hydrogen (H{\sc i}) observations with the Giant Metrewave Radio Telescope ({\it GMRT}) of the interacting galaxies NGC5996 and NGC5994, which make up the Arp72 system. Arp72 is an M51-type system and shows a complex distribution of H{\sc i} tails and a bridge due to tidal interactions. H{\sc i} column densities ranging from 0.8$-1.8\times10^{20}$ atoms cm$^{-2}$ in the eastern tidal tail to 1.7$-2\times10^{21}$ atoms cm$^{-2}$ in the bridge connecting the two galaxies, are seen to be associated with star-forming regions. We discuss the morphological and kinematic similarities of Arp72 with M51, the archetypal example of the M51-type systems, and Arp86, another M51-type system studied with the {\it GMRT}, and suggest that a multiple passage model of Salo & Laurikainen may be preferred over the classical single passage model of Toomre & Toomre, to reproduce the H{\sc i} features in Arp72 as well as in other M-51 systems depicting similar optical and H{\sc i} features.Comment: 8 pages, 6 figures, accepted for publication in MNRA

Rejuvenating Star Formation Activity in an Early-type Dwarf Galaxy, LEDA 1915372, with Accreted HI Gas

We report a rare astrophysical phenomenon, in which an early-type dwarf galaxy (dE), LEDA 1915372, is accreting gas from a nearby star-forming dwarf galaxy, MRK 0689, and is rejuvenating star-formation activity at the center. Both LEDA 1915372 and MRK 0689 have similar brightness of $M_{r}$ = $-$16.99 and $-$16.78 mag, respectively. They are located in a small group environment, separated by a sky-projected distance of 20.27 kpc (up to 70 kpc in three dimension), and have a relative line-of-sight radial velocity of 6 km/s. The observation of 21 cm emission with the Giant Metrewave Radio Telescope provides strong evidence of interaction between the pair dwarf galaxies in terms of neutral hydrogen (HI) morphology and kinematics. In particular, the HI map reveals that the two galaxies are clearly connected by a gas bridge, and the gas components of both LEDA 1915372 and MRK 0689 share a common direction of rotation. We also find that the HI emission peak deviates from LEDA 1915372 toward its optical blue plume, suggesting a tidal origin of ongoing central star formation. Our findings provide a new path to the formation of blue-cored dEs.Comment: Accepted for publication in ApJ

Searching for axion dark matter with MeerKAT Radio Telescope

Axions provide a natural and well-motivated dark matter candidate, with the capability to convert directly to photons in the presence of an electromagnetic field. A particularly compelling observational target is the conversion of dark matter axions into photons in the magnetospheres of highly magnetised neutron stars, which is expected to produce a narrow spectral peak centred at the frequency of the axion mass. We point the MeerKAT radio telescope towards the isolated neutron star J0806.4$-$4123 for $10$-hours of observation and obtain the radio spectra in the frequency range $769$-$1051$ MHz. By modelling the conversion process of infalling axion dark matter (DM), we then compare these spectra to theoretical expectations for a given choice of axion parameters. Whilst finding no signal above $5\sigma$ in the data, we provide a unique constraint on the Primakoff coupling of axion DM, $g_{{\rm a}\gamma\gamma}\lesssim 9.3 \times 10^{-12}\,{\rm GeV}^{-1}$ at the $95\%$ confidence level, in the mass range $3.18$-$4.35\,\mu$eV. This result serves the strongest constraint in the axion mass range $4.20$-$4.35\,\mu$eV.Comment: 6 pages, 3 figures, accepted by Physical Review