421 research outputs found
A systematic investigation of edge-on starburst galaxies: Evidence for supernova-driven superwinds
We are completing a project designed to realistically assess the global/cosmological significance of superwinds by attempting to systematize our understanding of them (determine their incidence rate and the dependence of their properties on the star-formation that drives them). Specifically, we are analyzing data from an optical spectroscopic and narrow-band imaging survey of an infrared flux-limited sample of about 50 starburst galaxies whose stellar disks are viewed nearly edge-on. This edge-on orientation is crucial because the relevant properties of the superwind can be far more easily measured when the flow is seen in isolation against the sky rather than projected onto the much brighter gas associated with the starburst galaxy itself
Are Cosmological Gas Accretion Streams Multiphase and Turbulent?
Simulations of cosmological filamentary accretion reveal flows ("streams") of
warm gas, ~ K, which are efficient in bringing gas into galaxies. We
present a phenomenological scenario where gas in such flows -- if it is shocked
as it enters the halo as we assume -- become biphasic and, as a result,
turbulent. We consider a collimated stream of warm gas that flows into a halo
from an over dense filament of the cosmic web. The post-shock streaming gas
expands because it has a higher pressure than the ambient halo gas, and
fragments as it cools. The fragmented stream forms a two phase medium: a warm
cloudy phase embedded in hot post-shock gas. We argue that the hot phase
sustains the accretion shock. A fraction of the initial kinetic energy of the
infalling gas is converted into turbulence among and within the warm clouds.
The thermodynamic evolution of the post-shock gas is largely determined by the
relative timescales of several processes -- the cooling, the expansion of the
post-shock gas, the amount of turbulence in the clouds, and the halo dynamics.
We expect the gas to become multiphase when the cooling and dynamical times are
of the same order-of-magnitude. In this framework, we show that this occurs in
the important mass range of ~ to M , where the bulk
of stars have formed in galaxies. Gas accreting along cosmic web filaments may
eventually lose coherence and mix with the ambient halo gas. Through both the
phase separation and "disruption" of the stream, the accretion efficiency onto
a galaxy in a halo dynamical time is lowered. De-collimating flows make the
direct interaction between galaxy feedback and accretion streams more likely,
thereby further reducing the overall accretion efficiency. Moderating the gas
accretion efficiency through these mechanisms may help to alleviate a number of
significant challenges in theoretical galaxy formation.Comment: 13 pages, 8 figures, Submitted to A&A, New version includes new
figure
O VI Emission Imaging of a Galaxy with the Hubble Space Telescope: a Warm Gas Halo Surrounding the Intense Starburst SDSS J115630.63+500822.1
We report results from a new HST study of the OVI 1032,1038\AA\ doublet in
emission around intensely star-forming galaxies. The programme aims to
characterize the energy balance in starburst galaxies and gas cooling in the
difficult-to-map coronal temperature regime of 2-5 x K. We present the
first resolved image of gas emission in the OVI line. Our target, SDSS
J1156+5008, is very compact in the continuum but displays OVI emission to radii
of 23 kpc. The surface brightness profile is well fit by an exponential with a
scale of 7.5kpc. This is ten times the size of the photoionized gas, and we
estimate that 1/6 the total OVI luminosity comes from resonantly scattered
continuum radiation. Spectroscopy - which closely resembles a stacked sample of
archival spectra - confirms the OVI emission, and determines the column density
and outflow velocity from blueshifted absorption. The combination of
measurements enables several new calculations with few assumptions. The OVI
regions fill only ~ of the volume. By comparing the cooling time with
the cloud sound-crossing time, the cooling distance with the size, and the
pressure in the OVI and nebular gas, we conclude that the OVI-bearing gas
cannot have been lifted to the scale height at this temperature, and must be
cooling in situ through this coronal temperature regime. The coronal phase
contains ~1% of the ionized mass, and its kinetic energy is currently ~1% of
the budget set by supernova feedback. However a much larger amount of the gas
must have cooled through this phase during the star formation episode. The
outflow exceeds the escape velocity and the gas may become unbound, but it will
recombine before it escapes and become visible to Lyman (and OI) spectroscopy.
The mapping of this gas represents a crucial step in further constraining
galaxy formation scenarios and guiding the development of future satellites.Comment: Accepted by the Astrophysical Journal. 25 pages, 11 figures. Section
7 presents calculated properties of warm halo gas. Version 2 fixes PDF
compatibility issue for some PDF viewer
The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies
We compare the star-formation history and dynamics of the Milky Way (MW) with
the properties of distant disk galaxies. During the first ~4 Gyr of its
evolution, the MW formed stars with a high star-formation intensity (SFI),
Sigma_SFR~0.6 Msun/yr/kpc2 and as a result, generated outflows and high
turbulence in its interstellar medium. This intense phase of star formation
corresponds to the formation of the thick disk. The formation of the thick disk
is a crucial phase which enables the MW to have formed approximately half of
its total stellar mass by z~1 which is similar to "MW progenitor galaxies"
selected by abundance matching. This agreement suggests that the formation of
the thick disk may be a generic evolutionary phase in disk galaxies. Using a
simple energy injection-kinetic energy relationship between the 1-D velocity
dispersion and SFI, we can reproduce the average perpendicular dispersion in
stellar velocities of the MW with age. This relationship, its inferred
evolution, and required efficiency are consistent with observations of galaxies
from z~0-3. The high turbulence generated by intense star formation naturally
resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism
that links the evolution of MW to the observed characteristics of distant disk
galaxies.Comment: 5 pages, 4 figures; accepted to ApJ Letter
The slowing down of galaxy disks in dissipationless minor mergers
We have investigated the impact of dissipationless minor galaxy mergers on
the angular momentum of the remnant. Our simulations cover a range of initial
orbital characteristics and the system consists of a massive galaxy with a
bulge and disk merging with a much less massive (one-tenth or one-twentieth)
gasless companion which has a variety of morphologies (disk- or
elliptical-like) and central baryonic mass concentrations. During the process
of merging, the orbital angular momentum is redistributed into the internal
angular momentum of the final system; the internal angular momentum of the
primary galaxy can increase or decrease depending on the relative orientation
of the orbital spin vectors (direct or retrograde), while the initially
non-rotating dark matter halo always gains angular momentum. The specific
angular momentum of the stellar component always decreases independent of the
orbital parameters or morphology of the satellite, the decrease in the rotation
velocity of the primary galaxy is accompanied by a change in the anisotropy of
the orbits, and the ratio of rotation speed to velocity dispersion of the
merger remnant is lower than the initial value, not only due to an increase in
the dispersion but also to the slowing -down of the disk rotation. We briefly
discuss several astrophysical implications of these results, suggesting that
minor mergers do not cause a "random walk" process of the angular momentum of
the stellar disk component of galaxies, but rather a steady decrease. Minor
mergers may play a role in producing the large scatter observed in the
Tully-Fisher relation for S0 galaxies, as well as in the increase of the
velocity dispersion and the decrease in at large radii as observed
in S0 galaxies.Comment: 10 pages, 10 figures, accepted for publication in A&
The Panchromatic Starburst Intensity Limit At Low And High Redshift
The integrated bolometric effective surface brightness S_e distributions of
starbursts are investigated for samples observed in 1. the rest frame
ultraviolet (UV), 2. the far-infrared and H-alpha, and 3. 21cm radio continuum
emission. For the UV sample we exploit a tight empirical relationship between
UV reddening and extinction to recover the bolometric flux. Parameterizing the
S_e upper limit by the 90th percentile of the distribution, we find a mean
S_{e,90} = 2.0e11 L_{sun}/kpc^2 for the three samples, with a factor of three
difference between the samples. This is consistent with what is expected from
the calibration uncertainties alone. We find little variation in S_{e,90} with
effective radii for R_e ~ 0.1 - 10 kpc, and little evolution out to redshifts z
~ 3. The lack of a strong dependence of S_{e,90} on wavelength, and its
consistency with the pressure measured in strong galactic winds, argue that it
corresponds to a global star formation intensity limit (\dot\Sigma_{e,90} ~ 45
M_{sun}/kpc^2/yr) rather than being an opacity effect. There are several
important implications of these results: 1. There is a robust physical
mechanism limiting starburst intensity. We note that starbursts have S_e
consistent with the expectations of gravitational instability models applied to
the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral
bulges can plausibly be built with maximum intensity bursts, while normal
spiral disks can not. 3. The UV extinction of high-z galaxies is significant,
implying that star formation in the early universe is moderately obscured.
After correcting for extinction, the observed metal production rate at z ~ 3
agrees well with independent estimates made for the epoch of elliptical galaxy
formation.Comment: 31 pages Latex (aas2pp4.sty,psfig.sty), 9 figures, accepted for
publication in the Astronomical Journa
On the Structure and Morphology of the `Diffuse Ionized Medium' in Star-Forming Galaxies
Deep H images of a sample of nearby late-type spiral galaxies have
been analyzed to characterize the morphology and energetic significance of the
``Diffuse Ionized Medium'' (DIM). We find that the DIM properties can be
reasonably unified as a function of relative surface brightness, by using a new
method to quantify the DIM importance in galaxies. This new approach is more
consistent with the fundamentally morphological definition of the DIM as being
`Diffuse', compared to the traditional way adopted in previous studies that
could only isolate the DIM based on an absolute surface brightness criterion.
Our results suggest that the variation of the DIM's significance among the
galaxies is small enough so that the fractional contribution of the DIM to the
global H luminosity in the galaxies is fairly constant, as has been
observed. We found a smooth structural transition from HII regions to the DIM,
suggesting that the ionizing energy for the DIM mainly comes from HII regions.Comment: 30 pages, 12 figures, AASTeX styl
Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
We determine the ensemble properties of z~5 Lyman break galaxies (LBGs)
selected as V-band dropouts to i(AB)<26.3 in the Chandra Deep Field South using
their rest-frame UV-to-visible SEDs. By matching the selection and performing
the same analysis that has been used for z~3 samples, we show clear differences
in the properties of two samples of LBGs which are separated by ~1Gyr in
lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and
have lower stellar masses (10^9Msol) than their z~3 counterparts. The
difference in mass is significant even when considering the presence of an
older, underlying population in both samples. Such young and moderately massive
systems dominate the luminous z~5 LBG population (>70%), whereas they comprise
<30% of LBG samples at z~3. This result is robust under all reasonable
modelling assumptions. These intense starbursts appear to be experiencing their
first (few) generations of large-scale star formation and are accumulating
their first significant stellar mass. Their dominance in luminous LBG samples
suggests that z~5 witnesses a period of wide-spread, recent galaxy formation.
As such, z~5 LBGs are the likely progenitors of the spheroidal components of
present-day massive galaxies. This is supported by their high stellar mass
surface densities, their core phase-space densities, as well as the ages of
stars in the bulge of our Galaxy and other massive systems. Their high star
formation rates per unit area suggest that these systems host outflows or winds
that enrich the intra- and inter-galactic media with metals. Their estimated
young ages are consistent with inefficient metal-mixing on galaxy-wide scales.
Therefore these galaxies may contain a significant fraction of metal-free stars
as has been proposed for z~3 LBGs (Jimenez & Haiman 2006). [Abridged]Comment: Accepted for publication in MNRAS. 21 pages, 9 postscript figures.
For a PDF file with high resolution figures, see
http://www-astro.physics.ox.ac.uk/~averma
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