6,410 research outputs found
Local Starbursts in a Cosmological Context
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
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
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
Far-ultraviolet spectra obtained with show that the strong
1036 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 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 (). Alternatively, the areal
covering factor of opaque material is typically 94%. Thus, the fraction
of ionizing stellar photons that escape the ISM of each galaxy is small: our
conservative estimates typically yield . Inclusion of
extinction due to dust will further decrease . An analogous analysis
of the rest-UV spectrum of the star-forming galaxy at =2.7
leads to similar constraints on . 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 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
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