Nuclear Star Clusters

The centers of most galaxies in the local universe are occupied by compact, barely resolved sources. Based on their structural properties, position in the fundamental plane, and integrated spectra, these sources clearly have a stellar origin. They are therefore called "nuclear star clusters" (NCs) or "stellar nuclei". NCs are found in galaxies of all Hubble types, suggesting that their formation is intricately linked to galaxy evolution. Here, I review some recent studies of NCs, describe ideas for their formation and subsequent growth, and touch on their possible evolutionary connection with both supermassive black holes and globular clusters.Comment: invited talk at IAU Symp. 266 "Star Clusters: Galactic Building Blocks through Space and Time

A Radio View of the Sky: the Cosmic History of Star-Forming and AGN Galaxies

We explore the cosmic evolution of radio detected star forming (SF) galaxies and active galactic nuclei (AGN), drawn from the VLA-COSMOS survey, out to z = 1.3. We present the 20 cm radio luminosity function for these populations, and find that SF galaxies evolve much more rapidly than low-power radio AGN. Our results imply that weak radio AGN (L_(1.4GHz) ≤ 5×10^(25) WHz^(−1)) occur in the most massive galaxies already at z ~ 1. They may significantly contribute to the heating of their surrounding medium and thus inhibit gas accretion onto their host galaxies, as recently suggested for the ‘radio mode’ AGN feedback in cosmological models

Molecular Gas and Star Formation in the Host Galaxy of I Zw 1

A recent analysis of high angular resolution NIR imaging and spectroscopic data in conjunction with Plateau de Bure interferometric mm-line observations indicate the presence of a circum-nuclear starburst ring of about 1.5 arcsec (1.5 kpc) diameter in I Zw 1. High angular resolution NIR imaging, using the MPE SHARP camera at the ESO NTT, HST V-band images, as well as NIR spectroscopy with MPE 3D provide an improved analysis of the star formation activity in the disk and nucleus of I Zw 1. We present first results from subarcsecond interferometric imaging in the 12CO(2-1) line using the Plateau de Bure Interferometer.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Cyanoacetylene in IC 342: An Evolving Dense Gas Component with Starburst Age

We present the first images of the J=5-4 and J=16-15 lines of the dense gas tracer, cyanoacetylene, HC_3N, in an external galaxy. The central 200 pc of the nearby star-forming spiral galaxy, IC 342, was mapped using the VLA and the Plateau de Bure Interferometer. HC_3N(5-4) line emission is found across the nuclear mini-spiral, but is very weak towards the starburst site, the location of the strongest mid-IR and radio emission. The J=16-15 and 10-9 lines are also faint near the large HII region complex, but are brighter relative to the 5-4 line, consistent with higher excitation. The brightest HC_3N emission is located in the northern arm of the nuclear minispiral, 100 pc away from the radio/IR source to the southwest of the nucleus. This location appears less affected by ultraviolet radiation, and may represent a more embedded, earlier stage of star formation. HC_3N excitation temperatures are consistent with those determined from C^{18}O; the gas is dense, 10^{4-5}/cc, and cool, T_K ~< 40 K. So as to not violate limits on the total H_2 mass determined from C^{18}O, at least two dense components are required to model IC 342's giant molecular clouds. These observations suggest that HC_3N(5-4) is an excellent probe of the dense, quiescent gas in galaxies. The high excitation combined with faint emission towards the dense molecular gas at the starburst indicates that it currently lacks large masses of very dense gas. We propose a scenario where the starburst is being caught in the act of dispersing or destroying its dense gas in the presence of the large HII region. This explains the high star formation efficiency seen in the dense component. The little remaining dense gas appears to be in pressure equilibrium with the starburst HII region.Comment: Accepted, AJ. 12 pages, 5 figure

CONSTITUTIONAL LAW-TAX EXEMPTION CONTRACT

A charter granted in 1863 by the State of Georgia to the Atlantic Coast Line Railroad Company provided as follows: The stock of said company shall be subject to a tax not exceeding ½ per cent per annum on the net proceeds of its investments. In 1931, the Georgia legislature levied a tax of 5½ per cent on corporate net income. The railroad brought an action seeking to have an assignment under this tax declared invalid, on the theory that the tax as applied to the plaintiff railroad violated the contract clause of the federal Constitution. The Georgia Supreme Court concluded that the tax exemption of the 1863 charter did not apply. On appeal, held, affirmed. The Supreme Court agreed with the state court\u27s opinion that in view of the fact that the income tax had not been a part of the state system of taxation at the time of the charter, it could not have been within the legislative intent. Atlantic Coast Line Railroad Co. v. Phillips, (U.S. 1947) 67 S.Ct. 1584

Understanding the central kinematics of globular clusters with simulated integrated-light IFU observations

The detection of intermediate mass black holes in the centres of globular clusters is highly controversial, as complementary observational methods often deliver significantly different results. In order to understand these discrepancies, we develop a procedure to simulate integral field unit (IFU) observations of globular clusters: Simulating IFU Star Cluster Observations (SISCO). The input of our software are realistic dynamical models of globular clusters that are then converted in a spectral data cube. We apply SISCO to Monte Carlo cluster simulations from Downing et al. (2010), with a realistic number of stars and concentrations. Using independent realisations of a given simulation we are able to quantify the stochasticity intrinsic to the problem of observing a partially resolved stellar population with integrated-light spectroscopy. We show that the luminosity-weighted IFU observations can be strongly biased by the presence of a few bright stars that introduce a scatter in the velocity dispersion measurements up to $\simeq$40% around the expected value, preventing any sound assessment of the central kinematic and a sensible interpretation of the presence/absence of an intermediate mass black hole. Moreover, we illustrate that, in our mock IFU observations, the average kinematic tracer has a mass of $\simeq$0.75 solar masses, only slightly lower than the mass of the typical stars examined in studies of resolved line-of-sight velocities of giant stars. Finally, in order to recover unbiased kinematic measurements we test different masking techniques that allow us to remove the spaxels dominated by bright stars, bringing the scatter down to a level of only a few percent. The application of SISCO will allow to investigate state-of-the-art simulations as realistic observations.Comment: 13 pages, 9 figures, 1 table. Accepted for publication in MNRA