Isolated OB Associations in Stripped HI Gas Clouds

HST ACS/HRC images in UV (F250W), V (F555W), and I (F814W) resolve three isolated OB associations that lie up to 30 kpc from the stellar disk of the S0 galaxy NGC 1533. Previous narrow-band Halpha imaging and optical spectroscopy showed these objects as unresolved intergalactic HII regions having Halpha luminosities consistent with single early-type O stars. These young stars lie in stripped HI gas with column densities ranging from 1.5 - 2.5 * 10^20 cm^-2 and velocity dispersions near 30 km s^-1. Using the HST broadband colors and magnitudes along with previously-determined Halpha luminosities, we place limits on the masses and ages of each association, considering the importance of stochastic effects for faint (M_V >-8) stellar populations. The upper limits to their stellar masses range from 600 M_sun to 7000 M_sun, and ages range from 2 - 6 Myrs. This analysis includes an updated calculation of the conversion factor between the ionizing luminosity and the total number of main sequence O stars contained within an HII region. The photometric properties and sizes of the isolated associations and other objects in the HRC fields are consistent with those of Galactic stellar associations, open clusters and/or single O and B stars. We interpret the age-size sequence of associations and clustered field objects as an indication that these isolated associations are most likely rapidly dispersing. Furthermore, we consider the possibility that these isolated associations represent the first generation of stars in the HI ring surrounding NGC 1533. This work suggests star formation in the unique environment of a galaxy's outermost gaseous regions proceeds similarly to that within the Galactic disk and that star formation in tidal debris may be responsible for building up a younger halo component.Comment: 21 pages, 9 figures, 6 tables; accepted for publication in Ap

The Survey for Ionization in Neutral Gas Galaxies: I. Description and Initial Results

We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a census of star formation in HI-selected galaxies. The survey consists of H-alpha and R-band imaging of a sample of 468 galaxies selected from the HI Parkes All Sky Survey (HIPASS). The sample spans three decades in HI mass and is free of many of the biases that affect other star forming galaxy samples. We present the criteria for sample selection, list the entire sample, discuss our observational techniques, and describe the data reduction and calibration methods. This paper focuses on 93 SINGG targets whose observations have been fully reduced and analyzed to date. The majority of these show a single Emission Line Galaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs in a single field. All of the targets in this sample are detected in H-alpha indicating that dormant (non-star forming) galaxies with M(HI) > ~3e7 M_sun are very rare. A database of the measured global properties of the ELGs is presented. The ELG sample spans four orders of magnitude in luminosity (H-alpha and R-band), and H-alpha surface brightness, nearly three orders of magnitude in R surface brightness and nearly two orders of magnitude in H-alpha equivalent width (EW). The surface brightness distribution of our sample is broader than that of the Sloan Digital Sky Survey spectroscopic sample, the (EW) distribution is broader than prism-selected samples, and the morphologies found include all common types of star forming galaxies (e.g. irregular, spiral, blue compact dwarf, starbursts, merging and colliding systems, and even residual star formation in S0 and Sa spirals). (abridged)Comment: 28 pages, ApJS, in press. Full resolution version with all panels of Fig. 8 available at http://sungg.pha.jhu.edu/publications.html . On line data available at http://sungg.pha.jhu.edu/PubData/ . Author list corrected. Wrong value for f_ap used in eq. 7 now corrected; typos corrected, non-used references replaced, others update

The Survey for Ionization in Neutral Gas Galaxies- II. The Star Formation Rate Density of the Local Universe

We derive observed Halpha and R band luminosity densities of an HI-selected sample of nearby galaxies using the SINGG sample to be l_Halpha' = (9.4 +/- 1.8)e38 h_70 erg s^-1 Mpc^-3 for Halpha and l_R' = (4.4 +/- 0.7)e37 h_70 erg s^-1 A^-1 Mpc^-3 in the R band. This R band luminosity density is approximately 70% of that found by the Sloan Digital Sky Survey. This leads to a local star formation rate density of log(SFRD) = -1.80 +0.13/-0.07(random) +/- 0.03(systematic) + log(h_70) after applying a mean internal extinction correction of 0.82 magnitudes. The gas cycling time of this sample is found to be t_gas = 7.5 +1.3/-2.1 Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW(Halpha) = 28.8 +7.2/-4.7 A (21.2 +4.2/-3.5 A without internal dust correction). As with similar surveys, these results imply that SFRD(z) decreases drastically from z ~ 1.5 to the present. A comparison of the dynamical masses of the SINGG galaxies evaluated at their optical limits with their stellar and HI masses shows significant evidence of downsizing: the most massive galaxies have a larger fraction of their mass locked up in stars compared with HI, while the opposite is true for less massive galaxies. We show that the application of the Kennicutt star formation law to a galaxy having the median orbital time at the optical limit of this sample results in a star formation rate decay with cosmic time similar to that given by the SFRD(z) evolution. This implies that the SFRD(z) evolution is primarily due to the secular evolution of galaxies, rather than interactions or mergers. This is consistent with the morphologies predominantly seen in the SINGG sample.Comment: 15 pages, 5 figures, ApJ in press. Data available at http://sungg.pha.jhu.edu/PubData/ Corrected: Minor typos and formatting issues fixe

Gas Accretion and Star Formation Rates

Cosmological numerical simulations of galaxy evolution show that accretion of metal-poor gas from the cosmic web drives the star formation in galaxy disks. Unfortunately, the observational support for this theoretical prediction is still indirect, and modeling and analysis are required to identify hints as actual signs of star-formation feeding from metal-poor gas accretion. Thus, a meticulous interpretation of the observations is crucial, and this observational review begins with a simple theoretical description of the physical process and the key ingredients it involves, including the properties of the accreted gas and of the star-formation that it induces. A number of observations pointing out the connection between metal-poor gas accretion and star-formation are analyzed, specifically, the short gas consumption time-scale compared to the age of the stellar populations, the fundamental metallicity relationship, the relationship between disk morphology and gas metallicity, the existence of metallicity drops in starbursts of star-forming galaxies, the so-called G dwarf problem, the existence of a minimum metallicity for the star-forming gas in the local universe, the origin of the alpha-enhanced gas forming stars in the local universe, the metallicity of the quiescent BCDs, and the direct measurements of gas accretion onto galaxies. A final section discusses intrinsic difficulties to obtain direct observational evidence, and points out alternative observational pathways to further consolidate the current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by Springe

The Survey for Ionization in Neutral-Gas Galaxies: III. Diffuse, Warm Ionized Medium and Escape of Ionizing Radiation

We use the first data release from the SINGG H-alpha survey of HI-selected galaxies to study the quantitative behavior of the diffuse, warm ionized medium (WIM) across the range of properties represented by these 109 galaxies. The mean fraction f_WIM of diffuse ionized gas in this sample is 0.59+/- 0.19, slightly higher than found in previous samples. Since lower surface-brightness galaxies tend to have higher f_WIM, we believe that most of this difference is due to selection effects favoring large, optically-bright, nearby galaxies with high star-formation rates. As found in previous studies, there is no appreciable correlation with Hubble type or total star-formation rate. However, we find that starburst galaxies, defined here by an H-alpha surface brightness > 2.5x 10^39 erg s^-1 kpc^-2 within the H-alpha half-light radius, do show much lower fractions of diffuse H-alpha emission. The cause apparently is not dominated by a lower fraction of field OB stars. However, it is qualitatively consistent with an expected escape of ionizing radiation above a threshold star-formation rate, predicted from our model in which the ISM is shredded by pressure-driven supernova feedback. The HI gas fractions in the starburst galaxies are also lower, suggesting that the starbursts are consuming and ionizing all the gas, and thus promoting regions of density-bounded ionization. If true, these effects imply that some amount of Lyman continuum radiation is escaping from most starburst galaxies, and that WIM properties and outflows from mechanical feedback are likely to be pressure-driven. However, in view of previous studies showing that the escape fraction of ionizing radiation is generally low, it is likely that other factors also drive the low fractions of diffuse ionized gas in starbursts.Comment: 16 pages plus separate 2-page PostScript table. Accepted to the Astrophysical Journa

Cationic polyelectrolytes: A new look at their possible roles as opsonins, as stimulators of respiratory burst in leukocytes, in bacteriolysis, and as modulators of immune-complex diseases (A review hypothesis)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44497/1/10753_2004_Article_BF00915991.pd