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Register a new userHIPASS Detection of an Intergalactic Gas Cloud in the NGC 2442 Group
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We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of a gas cloud associated with the asymmetric spiral galaxy NGC 2442. This object, designated HIPASS J0731-69, contains ~10^9 M_sun of HI, or nearly one-third as much atomic gas as NGC 2442 itself. No optical counterpart to any part of HIPASS J0731-69 has yet been identified, consistent with the gas being diffuse, and with its stream-like kinematics. If the gas in HIPASS J0731-69 was once part of NGC 2442, then it was most likely a fairly recent tidal encounter with a moderately massive companion which tore it loose, although the possibility of ram-pressure stripping cannot be ruled out. This discovery highlights the potential of the HIPASS data for yielding new clues to the nature of some of the best-known galaxies in the local universe.
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A mosaic of six HI line observations with the Australia Telescope Compact Array is used to search for intergalactic gas in the NGC 1808 group of galaxies. Within the field of view of about 1.4 deg x 1.2 deg, no emission from intergalactic HI gas is detected, either in the form of tidal plumes or tails, intergalactic HI clouds, or as gas associated with tidal dwarf galaxies, with a 5-sigma limiting sensitivity of about 3x10^{18} cm^{-2} (or 1.4x10^7 M_sun at a distance of 10.9 Mpc, for the given beam size of 127.9 x 77.3). The HI data of NGC 1792 and NGC 1808, with a velocity resolution of 6.6 km/s, confirm the results of earlier VLA observations. Simultaneous wide-band 1.34 GHz continuum observations also corroborate the results of earlier studies. However, the continuum flux of NGC 1808 measured by us is almost 20% higher than reported previously. No radio continuum emission was detected from the type Ia supernova SN1993af in the north-eastern spiral arm of NGC 1808. A comparison of NGC 1792 and NGC 1808 shows that it is not primarily the total energy input that makes the big difference between the starburst-related outflow in NGC 1808 and the absence of such extraplanar features in NGC 1792, but the area over which the energy released by stellar winds and supernovae is injected into the ISM.
We use smoothed particle hydrodynamics (SPH) models to study the large-scale morphology and dynamical evolution of the intergalactic gas in Stephans Quintet, and compare to multiwavelength observations. Specifically, we model the formation of the hot X-ray gas, the large-scale shock, and emission line gas as the result of NGC 7318b colliding with the group. We also reproduce the N-body model of Renaud and Appleton for the tidal structures in the group.
We have carried out a new photometric study of the remote Galactic globular cluster NGC 2419, using proprietary and archive B,V,I time-series CCD photometry of the cluster, that allowed us to discover a large number of new variable stars and to obtain a new colour magnitude diagram that reaches V ~26 mag over a field of 50X43 square arcmin centered on NGC 2419. The new variables include 39 RR Lyrae and 11 SX Phoenicis stars. The pulsation properties of the new RR Lyrae stars confirm and strengthen the classification of NGC 2419 as an Oosterhoff type II cluster.
We present the HI eXtreme (HIX) galaxy survey targeting some of the most HI rich galaxies in the southern hemisphere. The 13 HIX galaxies have been selected to host the most massive HI discs at a given stellar luminosity. We compare these galaxies to a control sample of average galaxies detected in the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001). As the control sample is matched in stellar luminosity, we find that the stellar properties of HIX galaxies are similar to the control sample. Furthermore, the specific star formation rate and optical morphology do not differ between HIX and control galaxies. We find, however, the HIX galaxies to be less efficient in forming stars. For the most HI massive galaxy in our sample (ESO075-G006, $rm log M_{HI} [M_{odot}] = 10.8$) the kinematic properties are the reason for inefficient star formation and HI excess. Examining the Australian Telescope Compact Array (ATCA) HI imaging and Wide Field Spectrograph (WiFeS) optical spectra of ESO075-G006 reveals an undisturbed galaxy without evidence for recent major, violent accretion events. A tilted-ring fit to the HI disc together with the gas-phase oxygen abundance distribution supports the scenario that gas has been constantly accreted onto ESO07-G006 but the high specific angular momentum makes ESO075-G006 very inefficient in forming stars. Thus a massive HI disc has been built up.
A deep Chandra observation of the X-ray bright group, NGC 5044, shows that the central region of this group has been strongly perturbed by repeated AGN outbursts. These recent AGN outbursts have produced many small X-ray cavities, cool filaments and cold fronts. We find a correlation between the coolest X-ray emitting gas and the morphology of the Ha filaments. The Ha filaments are oriented in the direction of the X-ray cavities, suggesting that the warm gas responsible for the Halpha emission originated near the center of NGC 5044 and was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed spectroscopic analysis shows that the central region of NGC 5044 contains spatially varying amounts of multiphase gas. The regions with the most inhomogeneous gas temperature distribution tend to correlate with the extended 235 MHz and 610 MHz radio emission detected by the GMRT. This may result from gas entrainment within the radio emitting plasma or mixing of different temperature gas in the regions surrounding the radio emitting plasma by AGN induced turbulence. Accounting for the effects of multiphase gas, we find that the abundance of heavy elements is fairly uniform within the central 100 kpc, with abundances of 60-80% solar for all elements except oxygen, which has a significantly sub-solar abundance. In the absence of continued AGN outbursts, the gas in the center of NGC 5044 should attain a more homogeneous distribution of gas temperature through the dissipation of turbulent kinetic energy and heat conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044 indicates that the time between recent AGN outbursts has been less than approximately 10e8 yr.
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