Oct 09

"Star Formation in Galaxies"
Christopher McKee, University of California at Berkeley

Star formation is at the nexus of astrophysics: stars are believed to
be responsible for the re-ionization of the universe, they created all the heavy elements, they control the formation and evolution of galaxies, and the formation of stars naturally leads to the formation of planets. It is therefore critical to understand the rate at which stars form. Observations of the star formation rate are encapsulated in the Kennicutt-Schmidt relations between the star formation rate and the amount of gas in a galaxy. Observations have also shown
that star formation is inefficient. This inefficiency, and the Kennicutt-Schmidt relations, are naturally explained in the theory of turbulence-regulated star formation. Progress is being made in developing a fully self-consistent theory, including a prediction for the molecular content of galaxies. Star formation in turn is a major driver of turbulence in giant molecular clouds, the sites of star formation. The observed star formation rate is generally inferred from observations of the ionized gas produced by massive stars. However, massive stars can form only in regions of high surface density (~ 1 g/cm^2 or greater). This threshold for massive star formation leads to a truncation of the initial mass function at high masses in the outer parts of galaxies, and can account for the difference in the star formation rates inferred from observations of UV continuum emission by GALEX and of Halpha emission from ionized gas.

Oct 16

"Halo Density Reduction by Baryonic Settling?" http://arxiv.org/abs/0808.3449
John Jardell, University of Texas at Austin

We test the proposal by El-Zant et al that the dark matter density of halos could be reduced through dynamical friction acting on heavy baryonic clumps in the early stages of galaxy formation. Using N-body simulations, we confirm that the inner halo density cusp is flattened to 0.2 of the halo break radius by the settling of a single clump of mass > 0.5% of the halo mass. We also find that an ensemble of 50 clumps each having masses > 0.2% can flatten the cusp to almost the halo break radius on a time scale of ~ 9 Gyr, for an NFW halo of concentration 15. We summarize some of the difficulties that need to be overcome if this mechanism is to resolve the apparent conflict between the observed inner densities of galaxy halos and the predictions of LCDM.

Oct 23

"Quasars from SDSS: Beyond Eigenvector 1"
Randi Worhatch, University of Texas at Austin

In 1992, Boroson & Green (BG92) discovered the so-called eigenvector 1; AGN exhibit an anticorrelation between [O III] 5007 Angstroms and optical Fe II emission, coupled with the width of broad-line H_beta, such that stronger [O III] corresponds to broader H_beta and weaker Fe II. They showed (see also Boroson 2000) that these relationships are related to black hole mass or Eddington accretion ratio, and thus to the central engine. Surprisingly, the physics at sub-parsec scales is correlated with narrow line emission arising up to hundreds of parsecs away.

We revisit this fundamental relationship using the large numbers of quasar spectra provided by the Sloan Digital Sky Survey. Our sample includes 9508 spectroscopically-identified QSOs from Data Release 5 with a redshift range of 0.1 < z < 0.53, on which we perform spectral principal component analysis (SPCA). We find a subset of objects with extremely large [O III]/H_beta flux ratios that behave independent of H_beta linewidth. These objects drive nonlinear correlations among the data, and mask the correlation of FWHM of H_beta for the rest of the data set. However, these objects comprise less than a few percent of the entire data set, and thus are rare enough that they did not drive the relationships seen in previous, smaller samples such as BG92. These strong [O III]/H_beta objects do not seem to differ from the data set significantly in other spectral properties, such as luminosity, redshift, line shapes, and continuum slope. We conclude that they are not a special class of objects, just an extreme end of the normal quasar distribution.

Our conclusions demonstrate that for a more representative sample, the relationship between the broad- and narrow-line regions is more complex than suggested by early eigenvector analysis. The implications for accretion physics and AGN feedback are discussed.

Nov 20

"HI Observations of Superthin Galaxies"
Juan Uson, National Radio Astronomy Observatory (NRAO)

Superthin galaxies are bulgeless, late-type spiral galaxies seen edge-on. HI synthesis observations probe the kinematic structure of their interstellar medium. I shall present the results of high-sensitivity VLA observations of UGC7321, IC2233, UGD3967, and UGC10043. Two of these galaxies are rather isolated while the other two are the dominant members of small groups.

We have detected a pattern of "corrugations" in IC 2233, the first such detection in neutral Hydrogen in an external galaxy. UGC3967, the "Integral Sign" galaxy shows a spectacular S-shaped bending that has been triggered by the passage of an intruder through its disk. UGC10043, not quite a superthin galaxy, has presented us with an unexpected, rich interaction with a previously unknown companion which is reminiscent of the M51 system (the Whirlpool Galaxy) "seen from the side."