51 research outputs found
Cluster Galaxies Die Hard
We investigate how the specific star formation rates of galaxies of different
masses depend on cluster-centric radius and on the central/satellite dichotomy
in both field and cluster environments. Recent data from a variety of sources,
including the cluster catalogue of von der Linden et al. are compared to the
semi-analytic models of De Lucia & Blaizot. We find that these models predict
too many passive satellite galaxies in clusters, too few passive central
galaxies with low stellar masses, and too many passive central galaxies with
high masses. We then outline a series of modifications to the model necessary
to solve these problems: a) Instead of instantaneous stripping of the external
gas reservoir after a galaxy becomes a satellite, the gas supply is assumed to
decrease at the same rate that the surrounding halo loses mass due to tidal
stripping, b) The AGN feedback efficiency is lowered to bring the fraction of
massive passive centrals in better agreement with the data. We also allow for
radio mode AGN feedback in satellite galaxies. c) We assume that satellite
galaxies residing in host haloes with masses below 10^12 M_sun do not undergo
any stripping. We highlight the fact that in low mass galaxies, the external
reservoir is composed primarily of gas that has been expelled from the galactic
disk by supernovae driven winds. This gas must remain available as a future
reservoir for star formation, even in satellite galaxies. Finally, we present a
simple recipe for the stripping of gas and dark matter in satellites that can
be used in models where subhalo evolution is not followed in detail.Comment: Models of ram-pressure stripping and some extra discussion added,
references added. Conclusions unchanged. 20 pages, 15 figures. Accepted for
publication in MNRAS
On the puzzling plateau in the specific star formation rate at z=2-7
The observational indications for a constant specific star-formation rate
(sSFR) in the redshift range z=2-7 are puzzling in the context of current
galaxy-formation models. Despite the tentative nature of the data, their marked
conflict with theory motivates a study of the possible implications. The
plateau at sSFR ~ 2 Gyr^-1 is hard to reproduce because (a) its level is low
compared to the cosmological specific accretion rate at z > 6, (b) it is higher
than the latter at z ~ 2, (c) the natural correlation between SFR and stellar
mass makes it difficult to manipulate their ratio, and (d) a low SFR at high z
makes it hard to produce enough massive galaxies by z ~ 2. Using a flexible
semi-analytic model, we explore ad-hoc modifications to the standard physical
recipes trying to obey the puzzling observational constraints. Successful
models involve non-trivial modifications, such as (a) a suppressed SFR at z > 4
in galaxies of all masses, by enhanced feedback or reduced SFR efficiency,
following an initial active phase at z > 7, (b) a delayed gas consumption into
stars, allowing the gas that was prohibited from forming stars or ejected at
high z to form stars later in more massive galaxies, and (c) enhanced growth of
massive galaxies, in terms of either faster assembly or more efficient
starbursts in mergers, or by efficient star formation in massive haloes.Comment: 17 pages, 11 figures. MNRAS accepted. References added, small changes
to text after referee report. Results and conclusions unchange
CMB-HD: an Ultra-Deep, High-Resolution Millimeter-Wave Survey over Half the Sky
A millimeter-wave survey over half the sky, that spans frequencies in the range of 30 to 350 gigahertz, and that is both an order of magnitude deeper and of higher-resolution than currently funded surveys would yield an enormous gain in understanding of both fundamental physics and astrophysics. By providing such a deep, high-resolution millimeter-wave survey (about 0.5 microK-arcminutes noise and 15 arcseconds resolution at 150 gigahertz), CMB-HD (Cosmic Microwave Background - Henry Draper catalog entry) will enable major advances. It will allow 1) the use of gravitational lensing of the primordial microwave background to map the distribution of matter on small scales (k approximately equal to 10 h per megaparsec), which probes dark matter particle properties. It will also allow 2) measurements of the thermal and kinetic Sunyaev-Zeldovich effects on small scales to map the gas density and gas pressure profiles of halos over a wide field, which probes galaxy evolution and cluster astrophysics. In addition, CMB-HD would allow us to cross critical thresholds in fundamental physics: 3) ruling out or detecting any new, light (less than 0.1 electronvolts), thermal particles, which could potentially be the dark matter, and 4) testing a wide class of multi-field models that could explain an epoch of inflation in the early Universe. Such a survey would also 5) monitor the transient sky by mapping the full observing region every few days, which opens a new window on gamma-ray bursts, novae, fast radio bursts, and variable active galactic nuclei. Moreover, CMB-HD would 6) provide a census of planets, dwarf planets, and asteroids in the outer Solar System, and 7) enable the detection of exo-Oort clouds around other solar systems, shedding light on planet formation. The combination of CMB-HD with contemporary ground and space-based experiments will also provide powerful synergies. CMB-HD will deliver this survey in 5 years of observing 20,000 square degrees, using two new 30-meter-class off-axis cross-Dragone telescopes to be located at Cerro Toco in the Atacama Desert. The telescopes will field about 2.4 million detectors (600,000 pixels) in total. The CMB-HD survey will be made publicly available, with usability and accessibility a priority
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