8 research outputs found
Secular Evolution and the Growth of Pseudobulges in Disk Galaxies
Galaxy evolution is in transition from an early universe dominated by
hierarchical clustering to a future dominated by secular processes. These
result from interactions involving collective phenomena such as bars, oval
disks, spiral structure, and triaxial dark halos. This paper summarizes a
review by Kormendy & Kennicutt (2004) using, in part, illustrations of
different galaxies. In simulations, bars rearrange disk gas into outer rings,
inner rings, and galactic centers, where high gas densities feed starbursts.
Consistent with this picture, many barred and oval galaxies have dense central
concentrations of gas and star formation rates that can build bulge-like
stellar densities on timescales of a few billion years. We conclude that
secular evolution builds dense central components in disk galaxies that look
like classical, merger-built bulges but that were made slowly out of disk gas.
We call these pseudobulges. Many pseudobulges can be recognized because they
have characteristics of disks: (1) flatter shapes than those of classical
bulges, (2) correspondingly large ratios of ordered to random velocities, (3)
small velocity dispersions, (4) spiral structure or nuclear bars, (5) nearly
exponential brightness profiles, and (6) starbursts. These structures occur
preferentially in barred and oval galaxies in which secular evolution should be
most rapid. Thus a variety of observational and theoretical results contribute
to a new paradigm of secular evolution that complements hierarchical
clustering.Comment: 19 pages, 9 Postscript figures; requires kapproc.cls and procps.sty;
to appear in "Penetrating Bars Through Masks of Cosmic Dust: The Hubble
Tuning Fork Strikes a New Note", ed. Block, Freeman, Puerari, Groess, and
Block, Dordrecht: Kluwer, in press; for a version with full resolution
figures, see http://chandra.as.utexas.edu/~kormendy/ar3ss.htm
Nearly 100% of the sky is covered by Lyman-α emission around high redshift galaxies
International audienceGalaxies are surrounded by large reservoirs of gas, mostly hydrogen, fed by inflows from the intergalactic medium and by outflows due to galactic winds. Absorption-line measurements along the sightlines to bright and rare background quasars indicate that this circumgalac-tic medium pervades far beyond the extent of starlight in galaxies, but very little is known about the spatial distribution of this gas. A new window into circumgalactic environments was recently opened with the discovery of ubiquitous extended Lyman-α emission from hydrogen around high-redshift galaxies[1, 2], facilitated by the extraordinary sensitivity of the MUSE instrument at the ESO Very Large Telescope[3]. Due to the faintness of this emission, such measurements were previously limited to especially favourable systems[4-6] or to massive statistical averaging[7, 8]. Here we demonstrate that low surface brightness Lyman-α emission surrounding faint galaxies at redshifts between 3 and 6 adds up to a projected sky coverage of nearly 100%. The corresponding rate of incidence (the mean number of Lyman-α emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high column density absorbers frequently detected in the spectra of distant quasars[9-12]. This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected also in emission
AGN Feedback in Elliptical Galaxies: Numerical Simulations
The importance of feedback (radiative and mechanical) from massive black
holes at the centers of elliptical galaxies is not in doubt, given the well
established relation among black hole mass and galaxy optical luminosity. Here,
with the aid of high-resolution hydrodynamical simulations, we discuss how this
feedback affects the hot ISM of isolated elliptical galaxies of different mass.
The cooling and heating functions include photoionization plus Compton heating,
the radiative transport equations are solved, and the mechanical feedback due
to the nuclear wind is also described on a physical basis; star formation is
considered. In the medium-high mass galaxies the resulting evolution is highly
unsteady. At early times major accretion episodes caused by cooling flows in
the recycled gas produced by stellar evolution trigger AGN flaring: relaxation
instabilities occur so that duty cycles are small enough to account for the
very small fraction of massive ellipticals observed to be in the QSO-phase,
when the accretion luminosity approaches the Eddington luminosity. At low
redshift all models are characterized by smooth, very sub-Eddington mass
accretion rates. The mass accumulated by the central black hole is limited to
range observed today, even though the mass lost by the evolving stellar
population is roughly two order of magnitude larger than the black hole masses
observed in elliptical galaxies.Comment: 20 pages, 4 (low-resolution) figures. Abbreviated version of the
article to appear in book "Hot Interstellar Matter in Elliptical Galaxies",
D.-W. Kim and S. Pellegrini eds., Astrophysics and Space Science Library
(ASSL), Springe