University of Washington

ASTRONOMY DEPARTMENT

Observing the star formation rate and metallicity since the earliest times in the universe is crucial to understanding galaxy formation and evolution. I will present recent results from our investigation into the chemical history of galaxies both nearby and over cosmic time. In nearby galaxies, we analyse chemical abundances in galaxy mergers. I show that disrupted abundance gradients are a smoking gun for galactic-scale gas flows in merging galaxies. I present the results of our large observational investigation into the global chemical history of star-forming galaxies between 0

Massive star clusters are found at the centers of a majority of spiral and elliptical galaxies with masses similar to or smaller than Milky Way. Recent studies have shown that these nuclear cluster masses scale with galaxy or bulge mass in the same way as supermassive black holes, and thus seem connected to the overall evolution of their host galaxy. The link between black holes and nuclear star clusters is largely unknown, but we have found that they appear to frequently co-habitate. We have assembled a wide range of observations of the nearest nuclear star clusters to understand their formation and the process of mass accretion on parsec scales at the centers of galaxies. Using adaptive optics assisted integral-field spectroscopy, we resolve the kinematics and structure of the nuclear star clusters, measure their masses and constrain the mass of any coincident black holes.

The variety of observations of Active Galactic Nuclei (AGN) show that the nuclear activity is powered by a central massive black hole that drives radio emitting jets and ionizes surrounding line-emitting clouds. This central engine is surrounded by an obscuring torus, comprised of optically thick dusty clouds in a rotating configuration. The torus dynamical origin, and especially its vertical support, present a serious challenge.
We have recently developed the formalism for radiative transfer in clumpy media, and in this talk I show that past problems with modeling the AGN infrared emission find a natural explanation in clumpy torus models. Furthermore, the clumpy model may also provide the answer for the torus dynamical origin and solidify the case for a paradigm shift: the torus is apparently just the dusty region of wind outflow from the AGN accretion disk in which the clouds are optically thick.

A few star clusters in the Magellanic Clouds present composite structures in red clump region of their colour magnitude diagrams. The most striking case is NGC419 in the SMC, where the HST/ACS/HRC images reveal a red clump composed of a main blob together with a marked faint feature, which cannot be accounted for by field contamination, binaries, photometric errors, and the like.  We show that the dual structure of the red clump corresponds to the simultaneous presence of stars which passed through electron degeneracy after central H-exhaustion, and stars which did not. This rare occurrence in a single cluster allows us to set stringent constraints to its age and to the efficiency of convective core overshooting during the main sequence. In this talk, we present a more detailed analysis of NGC419 together with a first analysis of other rich LMC clusters which are apparently in the same phase, namely NGC1751, 1783, 1806, 1846, 1852, and 1917. Moreover, we compare these Magellanic Cloud cases with their likely Galactic counterparts, NGC752 and 7789. Age determinations for these clusters are largely independent of reddening, distance, binary fraction, and the efficiency of convective core overshooting. Therefore, they have an extraordinary potential as absolute calibration marks in the age scale of intermediate-age populations.

Star formation is complex, involving processes ranging from atomic reactions to the gravitational stability of entire galaxies. This makes it very hard to model even with the fastest computers. Instead, computer simulations of galaxy evolution usually resort to empirical based prescriptions to set the amount of ISM that is converted into stars (the Star Formation Law - SFL) and the mass distribution of stars formed (the Initial Mass Function - IMF). These "laws" have proved remarkably resilient, having been proposed over 50 years ago. I will show results and ongoing work from the Survey of Ionization in Neutral Gas Galaxies (SINGG) and the Survey of Ultraviolet emission in Neutral Gas Galaxies (SUNGG) which survey the star formation properties of galaxies as traced by H-alpha and Ultraviolet emission. Our simple neutral hydrogen (HI) only selection criteria results in a sample that captures all types of star forming galaxies, while the use of two star formation tracers and integrated HI fluxes provide new constraints on both the SFL and IMF. We find strong correlations between the Halpha/FUV flux ratio and the optical surface brightness of galaxies. The only plausible explanation of this result is that the IMF is not constant. This result has enormous implications for galaxy evolution. Meanwhile strong correlations between the HI, star-formation, and old stellar population properties of galaxies are tighter than the Kennicutt-Schmidt Star Formation Law. The existence of an HI - star formation connection has been somewhat of a mystery since stars form in the molecular not the neutral ISM, while the highest mass stars and the HI have very different distributions in galaxies. I will argue that the IMF and the other star formation scaling relations result from pressure-regulated star-formation in a disk maintained at near critical dynamical stability.

I will present results on the relation between gas and star formation (star formation law) in nearby galaxies using new, high-resolution radio data to trace the atomic and molecular gas (`The HI Nearby Galaxy Survey' and the `HERA CO Line Extragalactic Survey'). These results include a detailed pixel-by-pixel study of the star formation (Schmidt/Kennicutt) law on sub-kpc resolution, using THINGS HI, HERACLES CO, Spitzer IR and GALEX UV data. We make these measurements at high resolution in a systematic way across the entire star forming disks of a sample of ~20 nearby galaxies, including spirals and dwarfs. I will show that a Schmidt law with power law index N~1 relates star formation surface density and molecular gas surface density in the star forming disks and that the star formation efficiency, i.e. the star formation rate per unit total gas mass, and the ratio of H2-to-HI are both strong functions of radius and thus environment in a galaxy. I will compare these measurements to proposed drivers of star and cloud formation and will focus in particular on the impact of metallicity, the interstellar radiation field, spiral density waves, ISM pressure and large-scale instabilities, all quantities that have been predicted to affect the conversion of HI into H2 or the ability of gas to form stars. Eventually, I will briefly present first results of ongoing work where we extend these studies into the extreme environments of outer galaxy disks.

We have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby Seyfert 1 galaxies with expected masses in the range 10^6-10^7 Msun, and also the well-studied nearby AGN NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the subsequent response in the broad optical emission lines. I will summarize the observing program and the analysis of the light curves for the objects in our sample, discuss the subsequent black hole mass measurements, and describe ongoing work to understand the detailed structure and kinematics of the broad line region gas in these objects. I will also describe ongoing work to update black hole scaling relationships (i.e., the AGN black hole mass--stellar velocity dispersion relationship and the AGN broad line region radius -- luminosity relationship) by including these new results and thus extending the range of these scaling relationships by a factor of ~10 on the low end.

Finally, computer simulations can merge two black holes in full general relativity -- and the latest results reveal a big surprise: when two black holes merge, the new black hole gets a gravitational wave kick with a velocity as high as 4000 km/s. A kick this fast can send even a supermassive black hole careening out of its home galaxy. How, then, do galaxies - especially low mass ones in the early universe - retain supermassive black holes after they merge? We will explore this and other consequences of gravitational wave recoil in this talk.

The chemical evolution of the Galaxy and the early Universe is a key topic in modern astrophysics. Since the most metal-poor Galactic stars are the local equivalent of the high-redshift Universe, they can be employed to reconstruct the onset of the chemical and dynamical formation processes of the Galaxy, the origin and evolution of the elements, and associated nucleosynthesis processes. They also provide constraints on the nature of the first stars and SNe, the initial mass function, and early star formation processes. The discovery of two astrophysically very important metal-poor objects recently lead to a significant advance regarding these topics. One object is the most iron-poor star yet found (with [Fe/H]=-5.4). The other star displays the strongest known over abundances of heavy neutron-capture elements, such as uranium, and nucleo-chronometry yields a stellar age of ~13 Gyr. Metal-poor stars, once also identified in dwarf galaxies, are vital probes also for near-field cosmology. Their chemical signatures now suggest that systems like these were building blocks of the Milky Way's low-metallicity halo. This opens a new window to study galaxy formation through stellar chemistry.