Jeffrey D. P. Kenney

The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment

CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies.

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.

Hα Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies

We describe the various Hα morphologies of Virgo Cluster and isolated spiral galaxies and associate the Hα morphologies with the types of environmental interactions that have altered the cluster galaxies. The spatial distributions of Hα and R-band emission are used to divide the star formation morphologies of the 52 Virgo Cluster spiral galaxies into several categories: normal (37%), anemic (6%), enhanced (6%), and (spatially) truncated (52%). Truncated galaxies are further subdivided on the basis of their inner star formation rates into truncated/normal (37%), truncated/compact (6%), truncated/anemic (8%), and truncated/enhanced (2%). The fraction of anemic galaxies is relatively small (6%-13%) in both environments, suggesting that starvation is not a major factor in the reduced star formation rates of Virgo spiral galaxies. The majority of Virgo spiral galaxies have their Hα disks truncated (52%), whereas truncated Hα disks are rarer in isolated galaxies (12%). Most of the Hα-truncated galaxies have relatively undisturbed stellar disks and normal to slightly enhanced inner disk star formation rates, suggesting that intracluster medium-interstellar medium (ICM-ISM) stripping is the main mechanism causing the reduced star formation rates of Virgo spiral galaxies. Several of the truncated galaxies are peculiar, with enhanced central star formation rates, disturbed stellar disks, and barlike distributions of luminous H II complexes inside the central 1 kpc but no star formation beyond, suggesting that recent tidal interactions or minor mergers have also influenced their morphology. Two highly inclined Hα-truncated spiral galaxies have numerous extraplanar H II regions and are likely in an active phase of ICM-ISM stripping. Several spiral galaxies have one-sided Hα enhancements at the outer edge of their truncated Hα disks, suggesting modest local enhancements in their star formation rates due to ICM-ISM interactions. Low-velocity tidal interactions and perhaps outer cluster H I accretion seem to be the triggers for enhanced global star formation in four Virgo galaxies. These results indicate that most Virgo spiral galaxies experience ICM-ISM stripping, many experience significant tidal effects, and many experience both.

The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution

Stellar bars drive gas into the circumnuclear (CN) region of galaxies. To investigate the fate of the CN gas and star formation (SF), we study a sample of barred nonstarbursts and starbursts with high-resolution CO, optical, Hα, radio continuum, Brγ, and HST data, and find the following. (1) The inner kiloparsec of bars differs markedly from the outer disk. It hosts molecular gas surface densities Σgas-m of 500-3500 M☉ pc-2, gas mass fractions of 10%-30%, and epicyclic frequencies of several 100-1000 km s-1 kpc-1. Consequently, in the CN region gravitational instabilities can only grow at high gas densities and on short timescales, explaining in part why powerful starbursts reside there. (2) Across the sample, we find bar pattern speeds with upper limits of 43-115 km s-1 pc-1 and outer inner Lindblad resonance radii of >500 pc. (3) Barred starbursts and nonstarbursts have CN SF rates of 3-11 and 0.1-2 M☉ yr-1, despite similar CN gas masses. The Σgas-m value in the starbursts is larger (1000-3500 M☉ pc-2) and close to the Toomre critical density over a large region. (4) Molecular gas makes up 10%-30% of the CN dynamical mass and fuels large CN SF rates in the starbursts, building young, massive, high-V/σ components. Implications for secular evolution along the Hubble sequence are discussed.

Global properties of infrared bright galaxies

Infrared flux densities of 182 galaxies, including 50 galaxies in the Virgo cluster, were analyzed using IRAS data for 12, 25, 60, and 100 microns, and the results were compared with data listed in the Point Source Catalog (PSC, 1985). In addition, IR luminosities, L(IRs), colors, and warm dust masses were derived for these galaxies and were compared with the interstellar gas masses and optical luminosities of the galaxies. It was found that, for galaxies whose optical diameter measures between 5 and 8 arcmin, the PSC flux densities are underestimated by a factor of 2 at 60 microns, and by a factor of 1.5 at 100 microns. It was also found that, for 49 galaxies, the mass of warm dust correlated well with the H2 mass, and that L(IR) correlated with L(H-alpha), demonstrating that the L(IR) measures the rate of star formation in these galaxies. 74 refs.

The effects of environment on the molecular and atomic gas properties of large Virgo cluster spirals

The effect of the Virgo environment on the ISM of Virgo disks is presently ascertained by comparing the molecular and atomic gas properties of 40 Virgo cluster spiral galaxies with their optical properties. Virgo Sc galaxies fainter than B(T)exp 0 = 12 are found to have significantly weaker CO emission/unit area and unit mass than brighter Sc galaxies, which have a fairly good correlation between CO luminosity and optical luminosity. An anticorrelation is noted between CO and H I emission in Virgo Sc galaxies with B(T)exp 0 = 11-12. Comparisons of the CO/H I flux ratios with the H I defficiency parameter indicate that there is no CO deficiency in H I-deficient Virgo spirals. 93 refs.

Cospatial counterrotating stellar disks in the Virgo E7/S0 galaxy NGC 4550

Two cospatial stellar disks have been discovered, one orbiting prograde, one orbiting retrograde, in NGC 4550, an E7/SO galaxy in the core of the Virgo Cluster. One of the stellar disks is coincident with a gas disk. Absorption and emission-line velocities for the two counterrotating components have been measured over about one-third of the optical diameter (30″). It is proposed that after the initial stellar disk of NGC 4550 was formed, a substantial amount of counterrotating gas was acquired ≥10 9 yr ago. It then settled to the plane via dissipation; the bulk of the counterrotating stars most likely formed after the gas settled to the plane

Twin peaks of CO emission in the central regions of barred galaxies

New high-resolution (∼2″) CO maps of the central regions of three barred galaxies show that the strongest CO emission arises from twin peaks, which are oriented perpendicular to the large-scale stellar bars and located where dust lanes intersect nuclear rings of H II regions. These twin gas concentrations can be explained by the crowding of gas streamlines near inner Lindblad resonances. In a fourth barred system, the nuclear starburst galaxy NGC 3504, a large concentration of molecular gas is centered on the nucleus, apparently inside an inner Lindblad resonance

Kinematic Disturbances in Optical Rotation Curves among 89 Virgo Disk Galaxies

For 89 galaxies, mostly spirals, in the Virgo Cluster region, we have obtained optical long-slit major-axis spectra of the ionized gas. We find the following: (1) One-half of the Virgo galaxies we observed have regular rotation patterns, while the other half exhibit kinematic disturbances ranging from mild to major. Velocity complexities are generally consistent with those resulting from tidal encounters or accretion events. Since kinematic disturbances are expected to fade within ~109 yr, many Virgo galaxies have experienced several significant kinematic disturbances during their lifetimes. (2) There is no strong correlation of rotation curve complexity with Hubble type, with galaxy luminosity, with local galaxy density, or with H I deficiency. (3) A few Virgo galaxies have ionized gas of limited extent, with velocities exceptionally low for their luminosities. In these galaxies the gas must be not rotationally supported. (4) There is a remarkable difference in the distribution of galaxy systemic velocity for galaxies with regular rotation curves and galaxies with disturbed rotation curves. Galaxies with regular rotation patterns show a flat distribution with velocities ranging from V0 = -300 km s-1 to V0 = +2500 km s-1; galaxies with disturbed kinematics have a Gaussian distribution that peaks at V0 = +1172 ± 100 km s-1, close to the cluster mean velocity. This latter distribution is virtually identical to the distribution of systemic velocity for elliptical galaxies in Virgo. However, disturbed galaxies are less concentrated to the cluster core than are the ellipticals; those near the periphery have velocities closer to the mean cluster velocity. Thus, spirals with disturbed kinematics are preferentially on radial orbits, which bring them to the denser core, where tidal interactions are strong and/or more common. Because they spend much time near apocenter, we observe them near the cluster periphery. Some may be falling into the core for the first time. These observations suggest that for a nonvirialized cluster like Virgo, galaxies may encounter either local (nearby galaxies) or global (cluster-related) interactions. These interactions may alter the morphology of the galaxy and may also play a role in driving the Virgo Cluster toward dynamical equilibrium.

An Atlas of Hα and R Images and Radial Profiles of 63 Bright Virgo Cluster Spiral Galaxies

Narrowband Hα and broadband R images and radial profiles are presented for 63 bright spiral galaxies in the Virgo Cluster. The sample is complete for Sb-Scd galaxies with B ≤ 12 and inclination ≤75°. Isophotal radii, disk scale lengths, concentration parameters, and integrated fluxes are derived for the sample galaxies.