Sep 16

"Connecting Star Formation, Galaxy Evolution and the X-Ray Background"
David Ballantyne, Georgia Institute of Technology

As we now know, the growth of galaxies and their central black holes is connected through some unknown mechanism. The cosmic X-ray background encodes within it the entire history of accretion onto supermassive massive black holes and so provides an unique view of the role of AGN in the assembly of galaxies. This talk presents results of recent work that is attempting to exploit the information contained in the X-ray background (and ancillary multiwavelength studies) to construct a testable scenario for the evolution of AGN and their host galaxies.

Sep 30

"The Star Formation Rate and Gas Surface Density Relation in the Milky Way: Implications for Extragalactic Studies"
Amanda Heiderman et al., University of Texas at Austin

We investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star forming regions using a sample of young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of objects located in 20 large molecular clouds from the Spitzer cores to disks (c2d) and Gould's Belt (GB) surveys. These data allow us to probe the regime of low-mass star formation essentially invisible to tracers of high-mass star formation used to establish extragalactic SFR-gas relations. We estimate the gas surface density (Sigma_gas) from extinction (Av) maps and YSO SFR surface densities (Sigma_SFR) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into evenly spaced contour levels of Av , counting only Class I and Flat SED YSOs, which have not yet migrated from their birthplace. For a sample of massive star forming clumps, we derive SFRs from the total infrared luminosity and use HCN gas maps to estimate gas surface densities. We find that c2d and GB clouds lie above the extragalactic SFR-gas relations (e.g., Kennicutt-Schmidt Law) by factors up to 17. Cloud regions with high Sigma_gas lie above extragalactic relations up to a factor of 54 and overlap with high-mass star forming regions. We use 12CO and 13CO gas maps of the Perseus and Ophiuchus clouds from the COMPLETE survey to estimate gas surface densities and compare to measurements from Av maps. We find that 13CO, with the standard conversions to total gas, underestimates the Av-based mass by factors of 4-5. 12CO may underestimate the total gas mass at Sigma_gas > 200 Msun pc-2 by >30%; however, this small difference in mass estimates does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Sigma_th) at 129+-14 Msun pc-2. At Sigma_gas > Sigma_th, the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Sigma_gas is below Sigma_th in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation (Sigma_SFR~Sigma_gas1.4) and a linear relation between dense gas and star formation is assumed, the fraction of dense star forming gas (f_dense) increases as ~Sigma_gas0.4. When Sigma_gas reaches ~300Sigma_th, f_dense is ~1, creating a maximal starburst.

paper by: Amanda Heiderman (UT), Neal Evans (UT), Lori Allen (NOAO), Tracy Huard (UMD), and Mark Heyer (UMass)

Reference: http://arxiv.org/abs/1009.1621

Oct 14

"The 21 cm High Velocity Clouds, Dark Matter Minihaloes, and the Lowest-Mass Dwarf Galaxies: A Continuum, or Distinct Phenomena?"
John Barentine, University of Texas at Austin

Clouds of neutral hydrogen at anomalous velocities with respect Milky Way rotation models were first observed almost a half century ago, but a full understanding of their nature and origin remains elusive. A key unknown in comparing their properties with model predictions is their distances, with proposals ranging from near the Galactic plane to the edge of the Local Group. Our recent work has placed the first meaningful constraints distances to some cloud complexes and individual clouds, demonstrating that some low metallicity clouds are falling into the Milky Way for the first time in their histories while others represent either tidal stripping of hydrogen from gravitationally bound dwarf galaxies or cycling of gas between the disk and halo. New results from Lambda-CDM cosmological simulations and observations of anomalous-velocity H I gas in the "M81 Filament" suggest an inconsistency in the number of clouds observed relative to expectations. Alongside discoveries of "ultra-faint" dwarfs in the SDSS that question the lower limit of halo mass for dwarfs, the relation to the HVCs that apparently never form stars becomes suggestive. I will review these results and comment on our ongoing work to better understand and attack the problem.

Nov 4

"Simulations of the Magellanic Stream in a First Infall Scenario"
Gurtina Besla, Harvard University

Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Magellanic Clouds are either on their first passage or on an eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2 Gyr). Without a short period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds ~150 degrees across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. Both Clouds are modeled as N-body systems using cosmologically motivated density profiles and infall masses. The orbit of the SMC about the LMC is chosen such that resonances maximize the efficiency of LMC tides. Contrary to previous models, the orbit of the LMC about the MW is not assumed to be a free parameter and is instead determined by the HST proper motions. The N-body simulations demonstrate that it is possible to explain the origin of the Stream under the assumption that the Clouds have not been long term satellites of the MW. More generally, they show that gas stripping is expected to occur between any gas-rich dwarf galaxy pair.

Nov 18

TCC Presentation: "Exploring the Dark Universe with Gravitational Lensing"
Sherry Suyu, University of California, Santa Barbara

Strong gravitational lens systems provide a tool for measuring cosmological parameters and for probing galaxy mass distributions. In the first part of my talk, I will show how strong lens systems with measured time delays between the multiple images can be used to measure the "time-delay distance" to the lens, and thus the Hubble constant. In particular, I will present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (i) new, deep HST observations, (ii) a new velocity dispersion measurement of the primary lens galaxy, (iii) an updated study of the lens environment, and (iv) time-delay measurements. By modeling the primary lens galaxy stellar dynamics and the density environment of the lens, we break the mass-sheet degeneracy present in the lensing analysis and measure cosmological parameters through the time-delay distance. I will present the resulting constraints on cosmological parameters, particularly the Hubble constant and the dark energy equation of state, from the combination of B1608+656 and the Wilkinson Microwave Anisotropy Probe data. In the second part of my talk, I will focus on using gravitational lensing to probe the properties of the dark matter halos of galaxies, independent of their light profiles. In particular, I will present a measurement of the dark matter halo size of the satellite galaxy in the massive elliptical lens SL2S J08544-0121.

Dec 2

"Electromagnetic Flares from the Tidal Disruption of Stars by Massive Black Holes"
Linda Strubbe, University of California, Berkeley

A star that wanders too close to a massive black hole (BH) gets shredded by the BH's tidal gravity. Stellar gas soon falls back to the BH at a rate initially exceeding the Eddington rate, releasing a flare of energy as gas accretes. How often this process occurs is uncertain at present, as is the physics of super-Eddington accretion (which is relevant for BH growth and feedback at high redshift as well). Excitingly, transient surveys like the Palomar Transient Factory (PTF), Pan-STARRS and LSST should shed light on these questions soon -- in anticipation, I predict photometric and spectroscopic properties of tidal flares. Early on, much of the falling-back gas should blow away in a wind, producing luminous optical emission imprinted with blueshifted UV absorption lines. If the shock at pericenter is unable to thermalize, photons should Compton upscatter to produce hard X-ray emission, erasing the UV lines but providing X-ray absorption lines. The photometric and spectroscopic signatures are significantly different for ~10^5 - 10^6 Msun BHs relative to 10^7 - 10^8 Msun BHs, and offer a new avenue for learning about these small BHs and their host galaxies. I will describe predicted detection rates for PTF, Pan-STARRS and LSST, and discuss the substantial challenge of disentangling these events from supernovae. These surveys should significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of IMBHs, and the role of tidal disruption in the growth of massive BHs.