The vertical extent and kinematics of the HI in NGC 2403

The neutral hydrogen line profiles along the major axis of the nearby spiral galaxy NGC 2403 show a wing towards the sytemic velocity. This asymmetry can be explained with the presence of an abnormally thick HI disk (FWHM ~ 5 kpc) or with a two-component structure: a thin disk and a slowly rotating, thicker (1-3 kpc) HI layer. The latter model gives a better representation of the observations. These results throw a new light on the disk-halo connection. In particular, the decrease of rotational velocity with height above the plane may be the result of a galactic fountain flow. A vertically extended, slowly rotating HI layer may be common among spiral galaxies with high levels of star formation.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and Astrophysics Letter

The Bright and the Dark Side of Malin 1

Malin 1 has long been considered a prototype giant, dark matter dominated Low Surface Brightness galaxy. Two recent studies, one based on a re-analysis of VLA HI observations and the other on an archival Hubble I-band image, throw a new light on this enigmatic galaxy and on its dark/luminous matter properties.Comment: 4 pages, 3 figures, to appear in the Proceedings of the 41st ESLAB Symposium "The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC, Noordwijk, N

The cold gaseous halo of NGC 891

We present HI observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30 % of the HI. This HI halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently counter-rotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing low angular momentum material needed to account for the observed rotation lag. The long HI filament and the counter-rotating clouds may be direct evidence of such accretion.Comment: Accepted for publication in The Astronomical Journal. High-resolution version available at http://www.astron.nl/~oosterlo/n891Pape

High-Latitude HI in the Low Surface Brightness Galaxy UGC7321

From the analysis of sensitive HI 21-cm line observations, we find evidence for vertically extended HI emission (|z|<~2.4 kpc) in the edge-on, low surface brightness spiral galaxy UGC7321. Three-dimensional modelling suggests that the HI disk of UGC7321 is both warped and flared, but that neither effect can fully reproduce the spatial distribution and kinematics of the highest z-height gas. We are able to model the high-latitude emission as an additional HI component in the form of a ``thick disk'' or ``halo'' with a FWHM~3.3 kpc. We find tentative evidence that the vertically extended gas declines in rotational velocity as a function of z, although we are unable to completely rule out models with constant V(z). In spite of the low star formation rate of UGC7321, energy from supernovae may be sufficient to sustain this high-latitude gas. However, alternative origins for this material, such as slow, sustained infall, cannot yet be excluded.Comment: to appear in the August 20 Astrophysical Journal; 17 pages; version with full resolution figures available at http://cfa-www.harvard.edu/~lmatthew

A Search for Extraplanar Dust in Nearby Edge-On Spirals

We present high resolution BV images of 12 edge-on spiral galaxies observed with the WIYN 3.5-m telescope. These images were obtained to search for extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those previously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include a sample of seven massive L_*-like spiral galaxies within D<25 Mpc that have inclinations i > 87 deg from the plane of the sky. We find that five of these seven systems show extraplanar dust, visible as highly-structured absorbing clouds against the background stellar light of the galaxies. The more prominent structures are estimated to have associated gas masses >10^5 M_sun; the implied potential energies are > 10^(52) ergs. All of the galaxies in our sample that show detectable halpha emission at large z also show extraplanar dust structures. None of those galaxies for which extraplanar halpha searches were negative show evidence for extensive high-z dust. The existence of extraplanar dust is a common property of massive spiral galaxies. We discuss several mechanisms for shaping the observed dust features, emphasizing the possibility that these dusty clouds represent the dense phase of a multiphase medium at high-z in spiral galaxies. The correlation between high-z dust and extraplanar Halpha emission may simply suggest that both trace the high-z interstellar medium in its various forms (or phases), the existence of which may ultimately be driven by vigorous star formation in the underlying disk. (Abstract abridged)Comment: 26 pages; 15 jpeg figures. To appear in The Astronomical Journal, May 1999. Gzipped tar files of high-resolution figures in postscript and jpeg formats are available at http://www.astro.wisc.edu/~howk/Papers/papers.html#surve

Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo

We present imaging Fabry-Perot observations of Halpha emission in the nearly edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial density profile along the major axis by examining position-velocity (PV) diagrams from the Fabry-Perot data cube as well as a CO 2-1 data cube from the literature. PV diagrams constructed parallel to the major axis are used to examine changes in azimuthal velocity as a function of height above the midplane. The results of this analysis reveal the presence of a vertical gradient in azimuthal velocity. The magnitude of this gradient is approximately 1 km/s/arcsec, or about 8 km/s/kpc, though a higher value of the gradient may be appropriate in localized regions of the halo. The evidence for an azimuthal velocity gradient is much stronger for the approaching half of the galaxy, although earlier slit spectra are consistent with a gradient on both sides. There is evidence for an outward radial redistribution of gas in the halo. The form of the rotation curve may also change with height, but this is not certain. We compare these results with those of an entirely ballistic model of a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity which is shallower than the observed gradient, indicating that an additional mechanism is required to further slow the rotation speeds in the halo.Comment: 18 pages, 18 figures. Uses emulateapj.cls. Accepted for publication in Ap

Accretion of gas onto nearby spiral galaxies

We present evidence for cosmological gas accretion onto spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extra-planar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extra-planar gas is produced by supernova feedback (galactic fountain) and only 10-20 % comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular-momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extra-planar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.Comment: 11 pages, 6 figures, accepted for publication in MNRA

The Role of a Hot Gas Environment on the Evolution of Galaxies

Most spiral galaxies are found in galaxy groups with low velocity dispersions; most E/S0 galaxies are found in galaxy groups with relatively high velocity dispersions. The mass of the hot gas we can observe in the E/S0 groups via their thermal X-ray emission is, on average, as much as the baryonic mass of the galaxies in these groups. By comparison, galaxy clusters have as much or more hot gas than stellar mass. Hot gas in S-rich groups, however, is of low enough temperature for its X-ray emission to suffer heavy absorption due to Galactic HI and related observational effects, and hence is hard to detect. We postulate that such lower temperature hot gas does exist in low velocity dispersion, S-rich groups, and explore the consequences of this assumption. For a wide range of metallicity and density, hot gas in S-rich groups can cool in far less than a Hubble time. If such gas exists and can cool, especially when interacting with HI in existing galaxies, then it can help link together a number of disparate observations, both Galactic and extragalactic, that are otherwise difficult to understand.Comment: 16 pages with one figure. ApJ Letters, in pres

Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383

We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha, broadband optical and near-infrared) observations of the classic barred spiral NGC 5383. We compare the observed central gas and dust morphology to the predictions of recent hydrodynamic simulations. In the nuclear region, our observations reveal three peaks lying along a S-shaped gas and dust distribution. In contrast, the model predicts a circumnuclear ring, not the observed S-shaped distribution; moreover, the predicted surface density contrast between the central gas accumulation and the bar dust lanes is an order of magnitude larger than observed. The discrepancies are not due to unexplored model parameter space or a nuclear bar but are probably due to the vigorous (7 solar masses per year) star formation activity in the center. As is common in similar bars, the star formation rate in the bar between the bar ends and the central region is low (~0.5 solar masses per yr), despite the high gas column density in the bar dust lanes; this is generally attributed to shear and shocks. We note a tendency for the HII regions to be associated with the spurs feeding the main bar dust lanes, but these are located on the leading side of the bar. We propose that stars form in the spurs, which provide a high column density but low shear environment. HII regions can therefore be found even on the leading side of the bar because the ionizing stars pass ballistically through the dust laneComment: Accepted for publication in The Astrophysical Journal, 33 pages (includes 10 figures