J. M. van der Hulst

The effects of interactions on spiral galaxies. II - Disk star-formation rates

H-alpha emission-line and IRAS far-IR observations of interacting spiral and irregular galaxies are here used to assess the influence of interactions on their global star-formation rates. Two samples of interacting galaxies were observed: a complete sample of close pairs, and an Arp atlas sample of peculiar systems. When compared to a control sample of single galaxies, both samples of interacting systems exhibit systematically higher levels of H-alpha and infrared emission on average, and a larger dispersion in emission properties. Emission levels in the very active system are much more strongly correlated with the properties of the interaction than with the internal properties of the galaxies themselves. Strong disk emission is almost always accompanied by unusually strong nuclear activity. Simple star-formation burst models can reproduce the observed H-alpha equivalent widths and broadband colors of most of the galaxies. The bursts are relatively short (few times 10 million yr) and rarely involve more than 1-2 percent of a galaxys total mass. 56 references.

HI Observations of Low Surface Brightness Galaxies: Probing Low Density Galaxies

We present Very Large Array (VLA) and Westerbork Synthesis Radio Telescope (WSRT) 21-cm HI observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low HI surface densities, about a factor of similar to 3 lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts, The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120 km s(-1), and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities, We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both M(HI)/L(B) and M(dyn)/L(B) depend strongly on central surface brightness, consistent with the surface brightness-mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.

The Westerbork HI survey of spiral and irregular galaxies - I. HI imaging of late-type dwarf galaxies

Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 late-type dwarf galaxies. These observations are part of the WHISP project (Westerbork Hi Survey of Spiral and Irregular Galaxies). Here we present Hi maps, velocity fields, global profiles and radial surface density profiles of Hi, as well as Hi masses, Hi radii and line widths. For the late-type galaxies in our sample, we find that the ratio of Hi extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8 +/- 0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in Hi with a typical M-HI/L-B of 1.5. The relative H I content M-HI/L-R increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average H I column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided H I distribution, and about half shows signs of kinematic lopsidedness.

THE TULLY-FISHER RELATION FOR LOW SURFACE BRIGHTNESS GALAXIES - IMPLICATIONS FOR GALAXY EVOLUTION

We present the B-band Tully-Fisher relation for low surface brightness (LSB) galaxies. These LSB galaxies follow the same Tully-Fisher relation as normal spiral galaxies. This implies that the mass-to-light ratio (M/L) of LSB galaxies is typically a factor of 2 larger than that of normal galaxies of the same total luminosity and morphological type. Since the dynamical mass of a galaxy is related to the rotational velocity and scalelength via M proportional to V(2)h, at fixed linewidth LSB galaxies must be twice as large as normal galaxies. This is confirmed by examining the relation between scalelength and linewidth for LSB and normal galaxies. The universal nature of the Tully-Fisher relation can be understood if LSB galaxies are galaxies with low mass surface density, sigma (

HI holes and high-velocity clouds in the spiral galaxy NGC 6946

) over bar. The mass surface density apparently controls the luminosity evolution of a galaxy in such a way as to keep the product sigma (

The PAH emission spectra of Large Magellanic Cloud H II regions

) over bar M/L constant.

VLA H I observations of the radio galaxy Centaurus A

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 h at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity HI (up to about 100 km s(-1)) and find 121 HI holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with HI holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the HI holes. There are also indications, especially in the outer regions, -an extended HI plume, velocity anomalies, sharp edges, and large-scale asymmmetries-pointing to tidal encounters and recent gas accretion.

Discovery of a Group of Star-forming Dwarf Galaxies in A1367

A set of ISOPHOT spectra from a sample of H II regions in the Large Magellanic Cloud (LMC) is presented. In all the spectra, emission bands arising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearly present. These features are observed to vary considerably in relative strength to each other from source to source and even within 30 Doradus. The LMC spectra have been compared with ISO-SWS spectra from Galactic H II regions and with the ISOCAM observation towards a quiescent molecular cloud in the SMC (Reach et al. 2000). A correlation is found between the I-7.7/I-11.2 versus I-6.2/I-11.2 and the I-8.6/I-11.2 versus I-6.2/I-11.2 ratios. A segregation between the sources in the different types of environment (Milky Way - LMC - SMC) is present. Furthermore, within the LMC observations, a clear distinction between 30 Doradus and non-30 Doradus pointings is found. We discuss the variations in the relative strength of the PAH features in view of the different physical environments and highlight the relation with the PAH/dust ratio and the extinction curve. We conclude that 1) the same conditions responsible for the observed trends in the relative PAH-feature strengths also affect the carrier of the 2175 Angstrom bump leading to the differences in strength of the latter, and 2) the molecular structure is the major cause of the observed variations in the relative strength of the PAH features. In the SMC and 30 Doradus compact PAH species dominate, while PAHs with an open, uneven structure are the dominant ones in Galactic H II regions and the non-30 Dor LMC sources.

A radio continuum and infrared study of Galactic HII regions

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.

An Interacting Galaxy System along a Filament in a Void

We describe the properties of a remarkable group of actively star-forming dwarf galaxies and H II galaxies in the A1367 cluster, which were discovered in a large-scale Halpha imaging survey of the cluster. Approximately 30 Halpha-emitting knots were identified in a region approximately 150 kpc across, in the vicinity of the spiral galaxies NGC 3860, CGCG 97-125, and CGCG 97-114. Follow-up imaging and spectroscopy reveal that some of the knots are associated with previously uncataloged dwarf galaxies (M-B = -15.8 to -16.5), while others appear to be isolated H II galaxies or intergalactic H II regions. Radial velocities obtained for several of the knots show that they are physically associated with a small group or subcluster, including CGCG 97-114 and CGCG 97-125. No comparable concentration of emission-line objects has been found elsewhere in any of the eight northern Abell clusters surveyed to date. The strong H emission in the objects and their high spatial density argue against this being a group of normal, unperturbed dwarf galaxies. Emission-line spectra of several of the knots also show some to be anomalously metal-rich relative to their luminosities. The results suggest that many of these objects were formed or triggered by tidal interactions or mergers involving CGCG 97-125 and other members of the group. A Westerbork Synthesis Radio Telescope H I map of the region shows direct evidence for tidal interactions in the group. These objects may be related to the tidal dwarf galaxies found in some interacting galaxy pairs, merger remnants, and compact groups. They could also represent evolutionary precursors to the class of isolated ultracompact dwarf galaxies that have been identified in the Fornax Cluster.