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VLA neutral hydrogen observations of the shell elliptical NGC 3656 reveal an edge-on, warped minor axis gaseous disk (M_HI ~ 2.10^9 Msun) extending 7 kpc. HI is also found outside the optical image, on two complexes to the North-East and North-West that seem to trace an outer broken HI disk or ring, or possibly one or two tidal tails. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell has traces of HI with velocities bracketing the stellar velocities, providing evidence for a dynamical association of HI and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase wrapping origin for the shell. We detect five dwarf galaxies with HI masses ranging from 2.10^8 Msun to 2.10^9 Msun all within 180 kpc from NGC 3656 and all within the velocity range (450 kms) of the HI of NGC 3656. For the NGC 3656 group to be bound requires a total mass of 3-7.4x10^{12} Msun, yielding a mass to light ratio from 125 to 300. The overall HI picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin, or possibly an ongoing process of multiple merger with nearby dwarfs.
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SHARC II, 350-micron continuum and archival HST J-H band maps are presented of NGC 3656, the brightest of our sample of six elliptical galaxies for which resolved CO gas disks have recently been detected with 7-spatial-resolution, interferometry mapping. These gas disks confirm the conclusions of earlier results showing optical dust lanes and unresolved CO that implied the common existence of molecular gas in ellipticals and the disk-like structure of this gas. The presented SHARC II mapping results provide the best to date resolved FIR-submm extent of NGC 3656 and of any elliptical galaxy > 40 Mpc, showing that dust of 29 K exists out to at least 1.8 kpc in this galaxy. These new data are used in conjunction with the archival HST maps and other published data to determine dust properties and associations with galactic structures, including dominant heating sources such as nuclear-activity, star-formation or diffuse-stellar radiation.
We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.
We present a wide-field, high spatial and velocity resolution map of the entire extended HI distribution of the Local Group dwarf galaxy NGC 6822. The observations were obtained with the Australia Telescope Compact Array in mosaicing mode. The interstellar medium of NGC 6822 is shaped by the presence of numerous H{sc i} holes and shells, including a supergiant shell, and the effects of tidal interaction, in the form of a tidal arm and an infalling or interacting HI complex. The HI shell is situated outside the optical galaxy and occupies roughly a quarter of the area of the main HI disk. It measures 2.0 times 1.4 kpc, making it one of the largest supergiant HI shells ever found. The giant hole shows no signs of expansion and no obvious creation mechanism is evident from our data. If star formation was the cause, an energy equivalent of ~100 supernovae (10^53 erg) is needed to create the hole. We derive an upper limit for the age of order 100 Myr. The presence of a possible tidal arm indicates that NGC 6822 may recently have undergone some interaction. An HI complex located in the north-west of the galaxy may be the interaction partner. We argue that it is likely that these features were created about 100 Myr ago in an event that also enhanced the star formation rate
We present X-ray spectroscopic and morphological analyses using Chandra ACIS and ROSAT observations of the giant elliptical galaxy NGC 4472 in the Virgo cluster. We discuss previously unobserved X-ray structures within the extended galactic corona. In the inner 2 of the galaxy, we find X-ray holes or cavities with radii of ~2 kpc, corresponding to the position of radio lobes. These holes were produced during a period of nuclear activity that began 1.2 x 10^7 years ago and may be ongoing. We also find an asymmetrical edge in the galaxy X-ray emission 3 (14 kpc) northeast of the core and an ~8 tail (36 kpc) extending southwest of the galaxy. These two features probably result from the interaction of NGC 4472 gas with the Virgo gas, which produces compression in the direction of NGC 4472s infall and an extended tail from ram pressure stripping. Assuming the tail is in pressure equilibrium with the surrounding gas, we compute its angle to our line of sight and estimate that its true extent exceeds 100 kpc. Finally, in addition to emission from the nucleus (first detected by Soldatenkov, Vikhlinin & Pavlinsky), we detect two small extended sources within 10 of the nucleus of the galaxy, both of which have luminosities of ~7 x 10^38 erg/s.
We have mapped the emission from atomic hydrogen at 21 cm from the galaxy NGC 3783 with the Australia Telescope Compact Array. Our main results are: a) the HI morphology is irregular and perturbed, gathered in three blobs apparently unrelated to the optical morphology; b) the observed HI velocity distribution indicates a normal disk in differential rotation with a constant velocity out to a radius of 160 (30 kpc), c) the inclination of the disk is about 25 deg with the kinematic major axis at a position angle slightly different from that of the stellar bar, d) the HI mass inside a radius of 18 is only 2.1 10^7 Msun, the total HI mass within 180 is 1.1 10^9 Msun and the dynamical mass is 2 10^{11} Msun. The bulk of the gas in NGC 3783 is outside the diameter of the stellar bar; e) Numerical simulations of the gas flow in the barred potential derived from the red image indicate that the pattern speed is Omega_p = 38 km/s/kpc: the ring of Halpha emitting regions encircling the bar would then correspond to UHR, and the Halpha accumulation in the center to a nuclear ring. Various possibilities are discussed to account for the active nucleus fuelling.
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