HST Imaging of Decoupled Dust Clouds in the Ram Pressure Stripped Virgo Spirals NGC 4402 and NGC 4522

We present the highest-resolution study to date of the ISM in galaxies undergoing ram pressure stripping, using HST BVI imaging of NGC 4522 and NGC 4402, Virgo Cluster spirals that are well-known to be experiencing ICM ram pressure. We find that throughout most of both galaxies, the main dust lane has a fairly well-defined edge, with a population of GMC-sized (tens- to hundreds-of-pc scale), isolated, highly extincting dust clouds located up to ~1.5 kpc radially beyond it. Outside of these dense clouds, the area has little or no diffuse dust extinction, indicating that the clouds have decoupled from the lower-density ISM material that has already been stripped. Several of the dust clouds have elongated morphologies that indicate active ram pressure, including two large (kpc-scale) filaments in NGC 4402 that are elongated in the projected ICM wind direction. We calculate a lower limit on the HI + H_2 masses of these clouds based on their dust extinctions and find that a correction factor of ~10 gives cloud masses consistent with those measured in CO for clouds of similar diameters, probably due to the complicating factors of foreground light, cloud substructure, and resolution limitations. Assuming that the clouds' actual masses are consistent with those of GMCs of similar diameters (~10^4-10^5 M_sun), we estimate that only a small fraction (~1-10%) of the original HI + H_2 remains in the parts of the disks with decoupled clouds. Based on H-alpha images, a similar fraction of star formation persists in these regions, 2-3% of the estimated pre-stripping star formation rate. We find that the decoupled cloud lifetimes may be up to 150-200 Myr.Comment: 20 pages, 22 figure

Research in far ultraviolet filtering for space optical systems

Design and fabrication of multilayer interference filters for ultraviole

Massive Star Formation Rates and Radial Distributions from Halpha Imaging of 84 Virgo Cluster and Isolated Spiral Galaxies

The massive star formation properties of 55 Virgo Cluster and 29 isolated S0-Scd bright (M(B) < -18) spiral galaxies are compared via analyses of R and Halpha surface photometry and integrated fluxes as functions of Hubble type and central R light concentration (bulge-to-disk ratio). In the median, the total normalized massive star formation rates (NMSFRs) in Virgo Cluster spirals are reduced by factors up to 2.5 compared to isolated spiral galaxies of the same type or concentration, with a range from enhanced (up to 2.5 times) to strongly reduced (up to 10 times). Within the inner 30% of the optical disk, Virgo Cluster and isolated spirals have similar ranges in NMSFRs, with similar to enhanced median NMSFRs for Virgo galaxies. NMSFRs in the outer 70% of the optical disk are reduced in the median by factors up to 9 for Virgo Cluster spirals, with more severely reduced star formation at progressively larger disk radii. Thus the reduction in total star formation of Virgo Cluster spirals is caused primarily by spatial truncation of the star-forming disks. The correlation between HI deficiency and R light central concentration is much weaker than the correlation between HI deficiency and Hubble type. ICM-ISM stripping of the gas from spiral galaxies is likely responsible for the truncated star-forming disks of Virgo Cluster spirals. This effect may be responsible for a significant part of the morphology-density relationship.Comment: Revised version accepted by Astrophysical Journal. 16 pages, 12 figures. Revised version is significantly shorter. Two figures were added, several figures were cut. Other revised material now appears in a second paper: Halpha Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies, also accepted by Astrophysical Journa

The Trouble with Hubble Types in the Virgo Cluster

Quantitative measures of central light concentration and star formation activity are derived from R and Halpha surface photometry of 84 bright S0-Scd Virgo Cluster and isolated spiral galaxies. For isolated spirals, there is a good correlation between these two parameters and assigned Hubble types. In the Virgo Cluster, the correlation between central light concentration and star formation activity is significantly weaker. Virgo Cluster spirals have systematically reduced global star formation with respect to isolated spirals, with severe reduction in the outer disk, but normal or enhanced activity in the inner disk. Assigned Hubble types are thus inadequate to describe the range in morphologies of bright Virgo Cluster spirals. In particular, spirals with reduced global star formation activity are often assigned misleading early-type classifications, irrespective of their central light concentrations. 45+-25% of the galaxies classified as Sa in the Virgo Cluster sample have central light concentrations more characteristic of isolated Sb-Sc galaxies. The misleading classification of low concentration galaxies with low star formation rates as early-type spirals may account for part of the excess of `early-type' spiral galaxies in clusters. Thus the morphology-density relationship is not all due to a systematic increase in the bulge-to-disk ratio with environmental density.Comment: Accepted for publication in ApJ Letters. Latex aaspp4.sty, 9 pages, 2 Postscript (embedded) figures. Also available at http://www.astro.yale.edu/koopmann/preprint.htm