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&

Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy

Using the Westerbork Synthesis Radio Telescope, we have detected HI 21 cm-line absorption in the Warm Neutral Medium of the Galaxy toward the extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s with a full width at half maximum of 53+/-6 km/s, is associated with the Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3). The estimated spin temperature of the gas is 3600+/-360 K. The volume density is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273 and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the 3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600 K. In addition, we have also detected HI 21 cm-line absorption from high velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of -117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature components in the high velocity clouds with spin temperatures of greater than 1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table

Spatial and Kinematical Lopsidedness of Atomic Hydrogen in the Ursa Major Group of Galaxies

We have carried out the harmonic analysis of the atomic hydrogen (HI) surface density maps and the velocity fields for 11 galaxies belonging to the Ursa Major group, over a radial range of 4-6 disc scalelengths in each galaxy. This analysis gives the radial variation of spatial lopsidedness, quantified by the Fourier amplitude A$_1$ of the m=1 component normalised to the average value. The kinematical analysis gives a value for the elongation of the potential to be $\sim 10$. The mean amplitude of spatial lopsidedness is found to be $\sim 0.14$ in the inner disc, similar to the field galaxies, and is smaller by a factor of $\sim 2$ compared to the Eridanus group galaxies. It is also shown that the the average value of A$_1$ does not increase with the Hubble type, contrary to what is seen in field galaxies. We argue that the physical origin of lopsidedness in the Ursa Major group of galaxies is tidal interactions, albeit weaker and less frequent than in Eridanus. Thus systematic studies of lopsidedness in groups of galaxies can provide dynamical clues regarding the interactions and evolution of galaxies in a group environment.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication by MNRA

A search for HI 21cm absorption toward the highest redshift radio loud objects

We have searched for HI 21cm absorption toward the two brightest radio AGN at high redshift, J0924--2201 at $z = 5.20$, and J0913+5919 at $z=5.11$, using the Giant Meter Wave Radio Telescope (GMRT). These data set a 3$\sigma$ upper limit to absorption of $< 30$ at 40 km s$^{-1}$ resolution for the 30 mJy source J0913+5919, and $< 3$ for the 0.55 Jy source J0924--2201 at 20 km s$^{-1}$ resolution. For J0924--2201, limits to broader lines at the few percent level are set by residual spectral baseline structure. For J0924--2201 the column density limit per 20 km s $^{-1}$ channel is: N(HI) $< 2.2\times 10^{18} \rm T_s$ cm$^{-2}$ over a velocity range of -700 km s$^{-1}$ to $+1180$ km s$^{-1}$ centered on the galaxy redshift determined through CO emission, assuming a covering factor of one. For J0913+5919 the column density limit per 40 km s$^{-1}$ channel is: N(HI) $< 2.2\times 10^{19} \rm T_s$ cm$^{-2}$ within $\pm 2400$ km s$^{-1}$ of the optical redshift. These data rule out any cool, high column density HI clouds within roughly $\pm 1000$ km s$^{-1}$ of the galaxies, as are often seen in Compact Steep Spectrum radio AGN, or clouds that might correspond to residual gas left over from cosmic reionization.Comment: accepted to the Astronomical Journal; 7 figures; 9 pages aastex forma

Low-frequency radio emission in the massive galaxy cluster MACS J0717.5 + 3745

International audienceTo investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5 + 3745, located at z = 0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5 + 3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process. Methods: Continuum observations were carried out using GMRT at 0.235 and 0.61 GHz on MACS J0717.5 + 3745 and archival data from the VLA (0.074 and 1.42 GHz) and WSRT (0.325 GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used. Results: A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index α0.2350.61˜-1.17±0.37. We have detected a giant radio halo within the cluster system with a linear size of 1.58 Mpc and a "Chair-shaped" filament structure between the merging subclusters of linear size 853 kpc at 0.235 GHz. This is the most powerful halo ever observed with P1.4 = 9.88 × 1025 WHz-1 and an equipartition magnetic field estimate of ~6.49 μG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events

On the Origin of the Wide HI Absorption Line Toward Sgr A*

We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119 +/- 42 km/s. Such a wide line is absent in the spectra at positions beyond about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius approx. 1.3') surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.Comment: LaTeX, 12 pages and 9 figures, accepted for publication in J. Astrophys. Ast

Radio continuum and far-infrared emission from the galaxies in the Eridanus group

The Eridanus galaxies follow the well-known radio-FIR correlation. Majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies having a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron. March, 200

GMRT detection of HI 21 cm associated absorption towards the z=1.2 red quasar 3C 190

We report the GMRT detection of associated HI 21 cm-line absorption in the z=1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect to the systemic redshift. The absorption, at $\sim$ 647.7 MHz, is broad and complex, spanning a velocity width of $\sim$ 600 \kms. Since the core is self-absorbed at this frequency, the absorption is most likely towards the hotspots. Comparison of the radio and deep optical images reveal linear filaments in the optical which overlap with the brighter radio jet towards the south-west. We therefore suggest that most of the HI 21 cm-line absorption could be occurring in the atomic gas shocked by the south-west jet.Comment: 8 pages, 1 fugure. To appear in Journal of Astrophysics and Astronom

Multiwavelength observation of a new black hole candidate: EXS 1737.9-2952

We report a multiwavelength analysis of an unusual high-energy transient: EXS 1737.9-2952. Due to the features this source exhibited in the hard X-ray domain similar to another source in the Galactic center region (1E1740.9 - 2952), and in order to study the molecular gas toward this X-ray source, we performed an observation of the EXS region in 1993 August, using the Swedish-ESO Submillimeter Telescope (SEST) located in La Silla (Chilean Andes). We observed a cloud, at the "forbidden" velocity of 135 km s-1, using 12CO (1-0) transitions, giving a maximum column density of 8 &#215; 1021 cm-2. In 1994 we conducted other observations to search for higher density regions inside the cloud, using HCO+ and CS lines, but they were unsuccessful: we concluded that the cloud could be associated with the X-ray source and its mean density is of the order of &#8776;103 cm-3. In 1994 April, we performed a set of observations of the field containing EXS, at 20 cm and 6 cm, using the VLA in its A configuration, and found four possible radio candidates for an association with the EXS X-ray source, one of them (source 3) being extended, exhibiting a head-tail morphology, and a having a thermal spectrum with a spectral index &#8776;-0.7. We reproduced our observation in 1994 November and December, using the C configuration at 6 cm, in order to investigate on possible variability and extension of these sources and found a marginal indication in the 20 cm image that source 3 may have a weak second component displaced about 15". Nevertheless, this indication is too faint to associate this source definitely with EXS since no significant variation was detected. In addition, during the 1994 November-December observation, two more extended sources were detected but their association with EXS is unlikely. We also analyzed the Einstein/IPC image of the 5 &#963; EXS error box which does not exhibit, at the time of the observation, any significant low-energy X-ray counterpart to EXS. A nearby pulsar PSR 1737-30 in the ROSAT catalog is outside this error box. Finally, IRAS maps of the EXS region do not show any IR contribution at the location of the radio sources. We conclude that (1) EXS 1737.9-2952 is a high-energy transient, (2) a persistent counterpart at other wavelengths is not demonstrated, and (3) EXS, when flaring, as well as other GC gamma-ray sources, could possibly contribute to the 511 keV bulge emission