1,553 research outputs found
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
Starbursts and Star Clusters in the Ultraviolet
Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies
reveal them to be highly irregular, even after excluding compact sources
(clusters and resolved stars). Most (7/9) are found to have a similar intrinsic
effective surface brightnesses, suggesting that a negative feedback mechanism
is setting an upper limit to the star formation rate per unit area. All
starbursts in our sample contain UV bright star clusters indicating that
cluster formation is an important mode of star formation in starbursts. On
average about 20% of the UV luminosity comes from these clusters. The brightest
clusters, or super star clusters (SSC), are preferentially found at the very
heart of starbursts. The size of the nearest SSCs are consistent with those of
Galactic globular clusters. The luminosity function of SSCs is well represented
by a power law with a slope alpha ~ -2. There is a strong correlation between
the far infrared excess and the UV spectral slope. The correlation is well
modeled by a geometry where much of their dust is in a foreground screen near
to the starburst, but not by a geometry of well mixed stars and dust.Comment: 47 pages, text only, LaTeX with aaspp.sty (version 3.0), compressed
postscript figures available at
ftp://eta.pha.jhu.edu/RecentPublications/meurer
Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies
HI-Selected galaxies obey a linear relationship between their maximum
detected radius Rmax and rotational velocity. This result covers measurements
in the optical, ultraviolet, and HI emission in galaxies spanning a factor of
30 in size and velocity, from small dwarf irregulars to the largest spirals.
Hence, galaxies behave as clocks, rotating once a Gyr at the very outskirts of
their discs. Observations of a large optically-selected sample are consistent,
implying this relationship is generic to disc galaxies in the low redshift
Universe. A linear RV relationship is expected from simple models of galaxy
formation and evolution. The total mass within Rmax has collapsed by a factor
of 37 compared to the present mean density of the Universe. Adopting standard
assumptions we find a mean halo spin parameter lambda in the range 0.020 to
0.035. The dispersion in lambda, 0.16 dex, is smaller than expected from
simulations. This may be due to the biases in our selection of disc galaxies
rather than all halos. The estimated mass densities of stars and atomic gas at
Rmax are similar (~0.5 Msun/pc^2) indicating outer discs are highly evolved.
The gas consumption and stellar population build time-scales are hundreds of
Gyr, hence star formation is not driving the current evolution of outer discs.
The estimated ratio between Rmax and disc scale length is consistent with
long-standing predictions from monolithic collapse models. Hence, it remains
unclear whether disc extent results from continual accretion, a rapid initial
collapse, secular evolution or a combination thereof.Comment: 14 pages, 7 figures, 3 in colour. Published in MNRAS. This v2
corrects wrong journal in the references section (all instances of
"Astrophysics and Space Sciences" should have been ApJ). The Posti+2017 has
also been updated. An erratum has been submitted to MNRA
Dust emission from the lensed Lyman break galaxy cB58
We detect 1.2mm continuum emission from dust in the gravitationally lensed
Lyman break galaxy MS 1512+36-cB58. Our detected flux is surprisingly low:
relative to local starburst galaxies, cB58 appears to produce somewhat less
far-IR emission than its UV reddening predicts. After comparing several
different estimates of the source's dust content, we conclude that the apparent
discrepancy is most likely related to uncertainty in its UV spectral slope.
Alternate scenarios to account for a far-IR "deficit" which rely on a high dust
temperature or differential magnification are less satisfactory. Our result
underscores one of the risks inherent in characterizing the cosmic star
formation history from rest-UV data alone.Comment: 5 pages, 1 figure, accepted by A&A Letter
FUSE Observations of Outflowing OVI in the Dwarf Starburst Galaxy NGC1705
We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxy
NGC 1705. These data allow us for the first time to probe the coronal-phase gas
(T = 10E5 to 10E6 K) that may dominate the radiative cooling of the
supernova-heated ISM and thereby determine the dynamical evolution of
starburst-driven outflows. We detect a broad (100 km/s) and blueshifted (by 80
km/s) OVI absorption-line arising in the previously-known galactic outflow. The
properties of the OVI absorption are inconsistent with the standard superbubble
model in which this gas arises in a conductive interface inside the outer
shell. We show that the superbubble in NGC 1705 is blowing out of the galaxy
ISM. During blow-out, coronal-phase gas can be created by hydrodynamical mixing
as hot gas rushes out through fissures in the fragmenting shell of cool gas. As
the coronal gas cools radiatively, it can naturally produce the observed OVI
column density and outflow speed. The OVI data show that the cooling rate in
the coronal-phase gas is less than about 10% of the supernova heating rate.
Since the X-ray luminosity from hotter gas is even smaller, we conclude that
radiative losses are insignificant. The outflow should be able to vent its
metals and kinetic energy out of the galaxy. This process has potentially
important implications for the evolution of dwarf galaxies and the IGM.Comment: ApJ (in press
Spectral Energy Distributions of starburst galaxies in the 900-1200 A range
We present the 970-1175 A spectral energy distributions (SEDs) of 12
starburst galaxies observed with the Far Ultraviolet Spectroscopic Explorer
FUSE. We take benefit of the high spectral resolution of FUSE to estimate a
continuum as much as possible unaffected by the interstellar lines. The
continuum is rather flat with, in few cases, a decrease at lambda <~1050 A, the
amplitude of which being correlated with various indicators of the dust
extinction. The far-UV SEDs are compared with synthetic population models. The
galaxies with almost no extinction have a SED consistent with an on-going star
formation over some Myrs. We derive a mean dust attenuation law in the
wavelength range 965-1140 A by comparing the SED of obscured galaxies to an
empirical dust-free SED. The extinction is nearly constant longward of 1040 A
but rises at shorter wavelengths. We compare our results with other studies of
the extinction for galaxies and stars in this wavelength range.Comment: 11 pages, 6 postscript figures, accepted for publication in Astronomy
& Astrophysic
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