6,377 research outputs found

    Local Starbursts in a Cosmological Context

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    In this contribution I introduce some of the major issues that motivate the conference, with an emphasis on how starbursts fit into the ``big picture''. I begin by defining starbursts in several different ways, and discuss the merits and limitations of these definitions. I will argue that the most physically useful definition of a starburst is its ``intensity'' (star formation rate per unit area). This is the most natural parameter to compare local starbursts with physically similar galaxies at high redshift, and indeed I will argue that local starbursts are unique laboratories to study the processes at work in the early universe. I will describe how NASA's GALEX mission has uncovered a rare population of close analogs to Lyman Break Galaxies in the local universe. I will then compare local starbursts to the Lyman-Break and sub-mm galaxies high redshift populations, and speculate that the multidimensional ``manifold'' of starbursts near and far can be understood largely in terms of the Schmidt/Kennicutt law and galaxy mass-metallicity relation. I will briefly summarize he properties of starburst-driven galactic superwinds and their possible implications for the evolution of galaxies and the IGM. These complex multiphase flows are best studied in nearby starbursts, where we can study the the hot X-ray gas that contains the bulk of the energy and newly produced metals.Comment: Proceedings of the Conference "Starbursts: Fropm 30 Doradus to Lyman Break Galaxies

    The Excess Far-Infrared Emission of AGN in the Local Universe

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    We have cross-correlated the Sloan Digital Sky Survey (SDSS) second data release spectroscopic galaxy sample with the IRAS faint-source catalogue (FSC). Optical emission line ratios are used to classify the galaxies with reliable IRAS 60 and 100 microns detections into AGN and normal star-forming galaxies. We then create subsamples of normal galaxies and AGN that are very closely matched in terms of key physical properties such as stellar mass, redshift, size, concentration and mean stellar age (as measured by absorption line indicators in the SDSS spectra). We then quantify whether there are systematic differences between the IR luminosities of the galaxies and the AGN in the matched subsamples. We find that the AGN exhibit a significant excess in far-IR emission relative to the star-forming galaxies in our sample. The excesses at 60 and 100 microns are 0.21 +/- 0.03 dex and 0.12 +/- 0.035 dex in log[L(60)/M*] and log[L(100)/M*], respectively. We then discuss whether the far-IR excess is produced by radiation from the active nucleus that is absorbed by dust or alternatively, by an extra population of young stars that is not detectable at optical wavelengths.Comment: 12 pages, 14 figures, accepted by MNRA

    The Discovery of an Active Galactic Nucleus in the Late-type Galaxy NGC 3621: Spitzer Spectroscopic Observations

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    We report the discovery of an Active Galactic Nucleus (AGN) in the nearby SAd galaxy NGC 3621 using Spitzer high spectral resolution observations. These observations reveal the presence of [NeV] 14 um and 24 um emission which is centrally concentrated and peaks at the position of the near-infrared nucleus. Using the [NeV] line luminosity, we estimate that the nuclear bolometric luminosity of the AGN is ~ 5 X 10^41 ergs s^-1, which corresponds based on the Eddington limit to a lower mass limit of the black hole of ~ 4 X 10^3 Msun. Using an order of magnitude estimate for the bulge mass based on the Hubble type of the galaxy, we find that this lower mass limit does not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Mutli-wavelength follow-up observations of NGC 3621 are required to obtain more precise estimates of the bulge mass, black hole mass, accretion rate, and nuclear bolometric luminosity. The discovery reported here adds to the growing evidence that a black hole can form and grow in a galaxy with no or minimal bulge.Comment: 5 pages, 7 figures, Accepted for publication in ApJ Letter

    On the Escape of Ionizing Radiation from Starbursts

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    Far-ultraviolet spectra obtained with FUSEFUSE show that the strong CIIλCII\lambda1036 interstellar absorption-line is essentially black in five of the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM below the Lyman-edge will be significantly larger than in the CIICII line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a a uniform absorbing slab, the implied optical depth at the Lyman edge is huge (τ0102\tau_0 \geq 10^2). Alternatively, the areal covering factor of opaque material is typically \geq 94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield fesc6f_{esc} \leq 6%. Inclusion of extinction due to dust will further decrease fescf_{esc}. An analogous analysis of the rest-UV spectrum of the star-forming galaxy MS1512CB58MS 1512-CB58 at zz =2.7 leads to similar constraints on fescf_{esc}. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at zz \geq 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary - but not sufficient - part of the process for creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.Comment: 17 pages; ApJ, in pres

    Removing the Veil of Ignorance in Assessing the Distributional Impacts of Social Policies

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    This paper summarizes our recent research on evaluating the distributional consequences of social programs. This research advances the economic policy evaluation literature beyond estimating assorted mean impacts to estimate distributions of outcomes generated by different policies and determine how those policies shift persons across the distributions of potential outcomes produced by them. Our approach enables analysts to evaluate the distributional effects of social programs without invoking the 'Veil of Ignorance' assumption often used in the literature in applied welfare economics. Our methods determine which persons are affected by a given policy, where they come from in the ex-ante outcome distribution and what their gains are. We apply our methods to analyze two proposed policy reforms in American education. These reforms benefit the middle class and not the poor.
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