517 research outputs found
Sub-mm clues to elliptical galaxy formation
There is growing evidence that, at the S(850) < 1 mJy level, the sub-mm
galaxy population (and hence a potentially significant fraction of the sub-mm
background) is associated with the star-forming Lyman-break population already
detected at optical wavelengths. However, the implied star-formation rates in
such objects (typically 3-30 solar masses per year) fall one or two orders of
magnitude short of the level of star-forming activity required to produce the
most massive elliptical galaxies on a timescale ~ 1 Gyr. If a significant
fraction of massive ellipticals did form the bulk of their stars in short-lived
massive starbursts at high redshift, then they should presumably be found among
the brighter, S(850) ~ 10 mJy sub-mm sources which are undoubtedly not part of
the Lyman-break population. A first powerful clue that this is indeed the case
comes from our major SCUBA survey of radio galaxies, which indicates that
massive dust-enshrouded star-formation in at least this subset of massive
ellipticals is largely confined to z > 2.5, with a mean redshift z = 3.5. While
radio selection raises concerns about bias, I argue that our current knowledge
of the brightest (S(850) ~ 10 mJy) sub-mm sources detected in unbiased SCUBA
imaging surveys indicates that they are also largely confined to this same
high-z regime. Consequently, while the most recent number counts imply such
extreme sources can contribute only 5-10% of the sub-mm background, their
comoving number density (in the redshift band 3 < z < 5) is 1-2 x 10^{-5} per
cubic megaparsec, sufficient to account for the formation of all ellipticals of
comparable mass to radio galaxies (~4L-star) in the present-day universe.Comment: 8 pages, 5 figures, UMass/INAOE conference proceedings on `Deep
millimeter surveys', eds. J. Lowenthal and D. Hughes, World Scientifi
The Sun, stellar-population models, and the age estimation of high-redshift galaxies
Given sufficiently deep optical spectroscopy, the age estimation of
high-redshif t () galaxies has been claimed to be a relatively robust
process (e.g. Dunlop et al. 1996) due to the fact that, for ages Gyr, the
near-ultraviolet light of a stellar population is expected to be dominated by
`well-understood' main-sequence (MS) stars. Recently, however, the reliability
of this process has been called into question by Yi et al (2000), who claim to
have developed models in which the spectrum produced by the main sequence
reddens much more rapidly than in the models of Jimenez et al (2000a), leading
to much younger age estimates for the reddest known high-redshift ellipticals.
In support of their revised age estimates, Yi et al cite the fact that their
models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the
solar spectrum is not reproduced by the Jimenez et al models until
Gyr. Here we confirm this discrepancy, but point out that this is in fact a
{\it strength} of the Jimenez et al models and indicative of some flaw in the
models of Yi et al (which, in effect, imply that the Sun will turn into a red
giant any minute now). We have also explored the models of Worthey (1994)
(which are known to differ greatly from those of Jimenez et al in the treatment
of post-MS evolution) and find that the main-sequence component of Worthey's
models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We
conclude that either the models of Yi et al are not as main-sequence dominated
at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these
models is in error by a factor possibly as high as two. (abridged)Comment: Submitted to MNRAS, final versio
Simulating the assembly of galaxies at redshifts z = 6 - 12
We use state-of-the-art simulations to explore the physical evolution of
galaxies in the first billion years of cosmic time. First, we demonstrate that
our model reproduces the basic statistical properties of the observed
Lyman-break galaxy (LBG) population at z = 6 - 8, including the evolving
ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and
the average specific star-formation rates (sSFR) of LBGs with M_{UV} < -18 (AB
mag). Encouraged by this success we present predictions for the behaviour of
fainter LBGs extending down to M_{UV} <= -15 (as will be probed with the James
Webb Space Telescope) and have interrogated our simulations to try to gain
insight into the physical drivers of the observed population evolution. We find
that mass growth due to star formation in the mass-dominant progenitor builds
up about 90% of the total z ~ 6 LBG stellar mass, dominating over the mass
contributed by merging throughout this era. Our simulation suggests that the
apparent "luminosity evolution" depends on the luminosity range probed: the
steady brightening of the bright end of the LF is driven primarily by genuine
physical luminosity evolution and arises due to a fairly steady increase in the
UV luminosity (and hence star-formation rates) in the most massive LBGs.
However, at fainter luminosities the situation is more complex, due in part to
the more stochastic star-formation histories of lower-mass objects; at this
end, the evolution of the UV LF involves a mix of positive and negative
luminosity evolution (as low-mass galaxies temporarily brighten then fade)
coupled with both positive and negative density evolution (as new low-mass
galaxies form, and other low-mass galaxies are consumed by merging). We also
predict the average sSFR of LBGs should rise from sSFR = 4.5 Gyr^-1 at z = 6 to
about 11 Gyr^-1 by z = 9.Comment: Accepted for publication in MNRA
Optical off-nuclear spectra of quasar hosts and radio galaxies
We present optical (~3200A to ~9000A) off-nuclear spectra of 26 powerful
active galaxies in the redshift range 0.1 < z < 0.3, obtained with the Mayall
and William Herschel 4-meter class telescopes. The sample consists of
radio-quiet quasars, radio-loud quasars (all with -23 > M_V > -26) and radio
galaxies of Fanaroff & Riley Type II (with extended radio luminosities and
spectral indices comparable to those of the radio-loud quasars). The spectra
were all taken approximately 5 arcseconds off-nucleus, with offsets carefully
selected so as to maximise the amount of galaxy light falling into the slit,
whilst simultaneously minimising the amount of scattered nuclear light. The
majority of the resulting spectra appear to be dominated by the integrated
stellar continuum of the underlying galaxies rather than by light from the
non-stellar processes occurring in the active nuclei, and in many cases a 4000A
break feature can be identified. The individual spectra are described in
detail, and the importance of the various spectral components is discussed.
Stellar population synthesis modelling of the spectra will follow in a
subsequent paper (Nolan et al. 2000).Comment: 23 pages, LaTeX, uses MNRAS style file, incorporates 71 postscript
figures, to be published in MNRAS. Contact author: [email protected]
Coherent control and feedback cooling in a remotely-coupled hybrid atom-optomechanical system
Cooling to the motional ground state is an important first step in the
preparation of nonclassical states of mesoscopic mechanical oscillators.
Light-mediated coupling to a remote atomic ensemble has been proposed as a
method to reach the ground state for low frequency oscillators. The ground
state can also be reached using optical measurement followed by feedback
control. Here we investigate the possibility of enhanced cooling by combining
these two approaches. The combination, in general, outperforms either
individual technique, though atomic ensemble-based cooling and feedback cooling
each individually dominate over large regions of parameter space.Comment: 28 pages, 5 figures, 2 tables. Updated to include exemplary
experimental parameters and expanded discussion of noise source
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