J. H. van Gorkom

VLA observations of neutral hydrogen in Virgo Cluster galaxies. I. The Atlas

H I maps, velocity fields, position velocity plots, mosaics of the line channel, and H I proflies are presented for the 25 brightest spiral galaxies located in the center of the Virgo Cluster. A synthetic map presents the maps of all the individual galaxies. It confirms that the galaxies located within 3 deg of M87 have H I disks that are significantly smaller than the optical disks. Further away, a number of galaxies have a very asymmetric H I distribution and a sharp edge on the side which points toward M87. The anemic galaxies usually show a ring structure and a patchy aspect. Several gas-rich dwarf galaxies have been found close to H I deficient galaxies, and one has been found close to the cluster center. The galaxies located in the western part are larger than in the eastern part. Furthermore the most severely stripped spirals show signs of asymmetries in their gas distribution combined with enhanced star formation at the compressed side of the gas disk. This might suggest that the Virgo Cluster exhibits a mild form of ram-pressure enhanced star formation, thought to be an important process in clusters at higher redshifts. 65 refs.

HI, HII, and R-band observations of a galactic merger sequence

We present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp295, NGC4676, NGC520, NGC3921, and NGC7252. These data are supplemented by wide-field images taken through a narrow band Halpha filter, and by deep R-band surface photometry. In the early stages, large amounts of HI exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction induced starbursts. In the final stages there is little if any HI within the remnant bodies, and tidal material is seen moving inward. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However the observations of NGC520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors. Overall, large quantities of both gas and starlight are seen at large radii. Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.

Virgo galaxies with long one-sided H I tails

In a new H I imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo galaxies in Atomic gas), we find seven spiral galaxies with long H I tails. The morphology varies, but all the tails are extended well beyond the optical radii on one side. These galaxies are found in intermediate- to low-density regions (0.6-1 Mpc in projection from M87). The tails are all pointing roughly away from M87, suggesting that these tails may have been created by a global cluster mechanism. While the tidal effects of the cluster potential are too small, a rough estimate suggests that simple ram pressure stripping could have indeed formed the tails in all but two cases. At least three systems show H I truncation to within the stellar disk, providing evidence of a gas-gas interaction. Although most of these galaxies do not appear disturbed optically, some have close neighbors, suggesting that tidal interactions may have moved gas outward, making it more susceptible to the intracluster medium ram pressure or viscosity. Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that the galaxy is most likely affected by the combined effect of a gravitational interaction and ram pressure stripping. We conclude that these one-sided H I tail galaxies have recently arrived in the cluster, falling in on highly radial orbits. It appears that galaxies begin to lose their gas already at intermediate distances from the cluster center through ram pressure or turbulent viscous stripping and tidal interactions with their neighbors, or a combination of both.

DECLINING ROTATION CURVES - THE END OF A CONSPIRACY

New observations of H I rotation curves at the Very Large Array have uncovered two galaxies with rotation curves declining between 1 and 3 optical radii. The velocity decrease is large, more than 50 km s-1 (approximately 25% of the maximum rotation velocity), and is present on both sides of the galaxies; projection effects can be ruled out. We interpret the decrease in rotation velocity as an indication of a large ratio of luminous to dark mass in the luminous regions of these systems. An analysis of the current observations combined with rotation curves from the literature shows a clear correlation between peak circular velocity of a galaxy, its central surface brightness and the slope of the rotation curve in the outer parts. This correlation indicates a weakening of the well-known conspiracy between luminous and dark matter, and may provide evidence in favor of the idea that dark matter is baryonic. Although we do not sample the full morphological Hubble sequence, a strong correlation between slope of the rotation curve and morphological type is found. This result seems to support earlier suggestions that the ratio between the mass in dark and luminous matter might be the critical parameter that controls the Hubble sequence.

Environmentally Driven Evolution of Simulated Cluster Galaxies

Galaxies in clusters are gas deficient, and a number of possible explanations for this observation have been advanced, including galaxy-cluster tidal interactions, galaxy harassment, and ISM-ICM gas stripping. In this paper, we use a cosmological simulation of cluster formation and evolution in order to examine this issue from a theoretical standpoint. We follow a large number of galaxies over time and track each galaxys gas and stellar mass changes to discover what mechanism(s) dominate the evolution of the cluster galaxies. We find that while gas is lost due to a wide variety of mechanisms, the most common way is via a gas-only stripping event, and the amount of gas lost correlates with the ram pressure the galaxy is experiencing. Although this gas stripping occurs primarily in the central region ( -->r r > 2.4 Mpc) often accrete cool gas; the accretion stops between 1 and 2.4 Mpc, possibly indicating the onset of galaxy starvation.

The Void Galaxy Survey: Optical Properties and H I Morphology and Kinematics

We have carefully selected a sample of 60 galaxies that reside in the deepest underdensities of geometrically identified voids within the Sloan Digital Sky Survey. HI imaging of 55 galaxies with the Westerbork Synthesis Radio Telescope reveals morphological and kinematic signatures of ongoing interactions and gas accretion. We probe a total volume of 485 Mpc(3) within the voids, with an angular resolution of 8 kpc at an average distance of 85 Mpc. We reach column density sensitivities of 5 x 10(19) cm(-2), corresponding to an HI mass limit of 3 x 10(8) M-circle dot. We detect HI in 41 galaxies, with total masses ranging from 1.7 x 10(8) to 5.5 x 10(9) M-circle dot. The upper limits on the 14 non-detections are not inconsistent with their luminosities, given their expected HI mass-to-light ratios. We find that the void galaxies are generally gas-rich, low-luminosity, blue disk galaxies, with optical and HI properties that are not unusual for their luminosity and morphology. The sample spans a range of absolute magnitudes (-16.1 > M-r > -20.4) and colors (0.06 <g - r <0.87), and includes disk and irregular galaxies. We also identify three as early-type galaxies, all of which are not detected in HI. All galaxies have stellar masses less than 3 x 10(10) M-circle dot, and many have kinematic and morphological signs of ongoing gas accretion, suggesting that the void galaxy population is still in the process of assembling. The small-scale clustering in the void, within 600 kpc and 200 km s(-1), is similar to that in higher density regions, and we identify 18 HI-rich neighboring galaxies in the voids. Most are within 100 kpc and 100 km s(-1) of the targeted galaxy, and we find no significant population of HI-rich low-luminosity galaxies filling the voids, contrary to what is predicted by simulations.

VLA neutral hydrogen imaging of compact groups of galaxies. II - HCG 31, 44, and 79

Neutral hydrogen images of three compact groups of galaxies are presented: HCG 31, 44, and 79. The images were obtained with the very large array (VLA), an on-line Hanning smoothing was applied to the data, and the H I spectral channel was isolated. The images were made on the Pipeline, and were produced by means of a method described by Gorkon and Ekers (1988). The images of HCG 44 are compared with earlier Arecibo observations. The H I emission in HCG 44 is discovered within the galaxies, whereas the emission in 31 and 79 can be found throughout the group in clouds that are larger than the galaxies. Evidence of a relationship between the compact groups is found in the H I data, and the groups are considered to be merging into a single object. Some of the groups are theorized to be young amorphous galaxies where the H I is still bound to individual galaxies, and which have just begun to condense from the intergalactic medium. The kinematics of the gas are shown to vary, and a common gaseous envelope contains the dwarf galaxies. 42 refs.

RADIO CONTINUUM OBSERVATIONS OF THE VIRGO CLUSTER SPIRAL NGC 4522: THE SIGNATURE OF RAM PRESSURE

Radio continuum observations at 20 and 6 cm of the highly inclined Virgo spiral galaxy NGC 4522 are presented. Both 20 and 6 cm total emission distributions are asymmetric with an extended component to the west, where extraplanar atomic gas and Hα emission are found. The 6 cm polarized emission is located at the eastern edge of the galactic disk. Its peak is located about 1 kpc to the east of the total emission peak. We argue that this phenomena is a characteristic feature for cluster galaxies that are experiencing significant pressure from the intracluster medium. Polarized radio continuum emission is thus a powerful tool to detect interactions of spiral galaxies with the cluster ICM. The degree of polarization decreases from the east to the west. The flattest spectral index between 20 and 6 cm coincides with the peak of the 6 cm polarized emission. These findings are consistent with a picture of a large-scale shock due to ram pressure located at the east of the galaxy where cosmic rays are accelerated. We conclude that it is likely that the galaxy experiences active ram pressure.

VLA H I observations of the radio galaxy Centaurus A

This paper presents 21-cm line observations made with the VLA of the nearby radio galaxy Centaurus A (NGC 5128). The H I velocities in the inner parts agree well with optical emission-line velocities and indicate that the dust lane is a warped, rotating system. The H I velocities in the outer parts do not agree with this interpretation which may indicate that in the outer regions the gas disk is still unstable and has not yet settled into regular orbits. This then would favor a scenario in which the gas and dust is a recently captured system. The total mass of the galaxy as derived from the H I rotation curve is significantly less than the mass estimate based on X-ray observations. 40 refs.

Only the Lonely: H I Imaging of Void Galaxies

Void galaxies, residing within the deepest underdensities of the Cosmic Web, present an ideal population for the study of galaxy formation and evolution in an environment undisturbed by the complex processes modifying galaxies in clusters and groups, as well as provide an observational test for theories of cosmological structure formation. We have completed a pilot survey for the H I imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in H I in local (d <100 Mpc) voids. H I masses range from 3.5 x 10(8) to 3.8 x 10(9) M-circle dot, with one nondetection with an upper limit of 2.1 x 10(8) M-circle dot. Our galaxies were selected using a structural and geometric technique to produce a sample that is purely environmentally selected and uniformly represents the void galaxy population. In addition, we use a powerful new backend of the Westerbork Synthesis Radio Telescope that allows us to probe a large volume around each targeted galaxy, simultaneously providing an environmentally constrained sample of fore-and background control samples of galaxies while still resolving individual galaxy kinematics and detecting faint companions in H I. This small sample makes up a surprisingly interesting collection of perturbed and interacting galaxies, all with small stellar disks. Four galaxies have significantly perturbed H I disks, five have previously unidentified companions at distances ranging from 50 to 200 kpc, two are in interacting systems, and one was found to have a polar H I disk. Our initial findings suggest void galaxies are a gas-rich, dynamic population which present evidence of ongoing gas accretion, major and minor interactions, and filamentary alignment despite the surrounding underdense environment.