1,177 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
Estimating commitment in a digital market place environment
The future generation of mobile communication shall be a convergence of mobile telephony and information systems which promises to change people's lives by enabling them to access information when, where and how they want. It presents opportunities to offer multimedia applications and services that meet end-toend service requirements. The Digital Marketplace framework will enable users to have separate contracts for different services on a per call basis. In order for such a framework to function appropriately, there has to be some means for the network operator to know in advance if its network will be able to support the user requirements. This paper discusses the methods by which the network operator will be able to determine if the system will be able to support another user of a certain service class and therefore negotiate parameters like commitment, QoS and the associated cost for providing the service, thus making the Digital Marketplace wor
Synthetic stellar populations: single stellar populations, stellar interior models and primordial proto-galaxies
We present a new set of stellar interior and synthesis models for predicting
the integrated emission from stellar populations in star clusters and galaxies
of arbitrary age and metallicity. This work differs from existing spectral
synthesis codes in a number of important ways, namely (1) the incorporation of
new stellar evolutionary tracks, with sufficient resolution in mass to sample
rapid stages of stellar evolution; (2) a physically consistent treatment of
evolution in the HR diagram, including the approach to the main sequence and
the effects of mass loss on the giant and horizontal-branch phases. Unlike
several existing models, ours yield consistent ages when used to date a coeval
stellar population from a wide range of spectral features and colour indexes.
We rigorously discuss degeneracies in the age-metallicity plane and show that
inclusion of spectral features blueward of 4500 AA, suffices to break any
remaining degeneracy and that with moderate S/N spectra (10 per 20AA,
resolution element) age and metallicity are not degenerate. We also study
sources of systematic errors in deriving the age of a single stellar population
and conclude that they are not larger than 10-15%. We illustrate the use of
single stellar populations by predicting the colors of primordial
proto-galaxies and show that one can first find them and then deduce the form
of the IMF for the early generation of stars in the universe. Finally, we
provide accurate analytic fitting formulas for ultra fast computation of colors
of single stellar populations. The models can be found at
http://www.physics.upenn.edu/~rauljComment: MNRAS in pres
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
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