3,286 research outputs found
How to distinguish starbursts and quiescently star-forming galaxies: The `bimodal' submillimetre galaxy population as a case study
In recent work (arXiv:1101.0002) we have suggested that the high-redshift (z
~ 2-4) bright submillimetre galaxy (SMG) population is heterogeneous, with
major mergers contributing both at early stages, where quiescently star-forming
discs are blended into one submm source (`galaxy-pair SMGs'), and late stages,
where mutual tidal torques drive gas inflows and cause strong starbursts. Here
we combine hydrodynamic simulations of major mergers with 3-D dust radiative
transfer calculations to determine observational diagnostics that can
distinguish between quiescently star-forming SMGs and starburst SMGs via
integrated data alone. We fit the far-IR SEDs of the simulated galaxies with
the optically thin single-temperature modified blackbody, the full form of the
single-temperature modified blackbody, and a power-law temperature-distribution
model. The effective dust temperature, T_dust, and power-law index of the dust
emissivity in the far-IR, \beta, derived can significantly depend on the
fitting form used, and the intrinsic \beta\ of the dust is not recovered.
However, for all forms used here, there is a T_dust above which almost all
simulated galaxies are starbursts, so a T_dust cut is very effective at
selecting starbursts. Simulated merger-induced starbursts also have higher
L_IR/M_gas and L_IR/L_FUV than quiescently star-forming galaxies and lie above
the star formation rate-stellar mass relation. These diagnostics can be used to
test our claim that the SMG population is heterogeneous and to observationally
determine what star formation mode dominates a given galaxy population. We
comment on applicability of these diagnostics to ULIRGs that would not be
selected as SMGs. These `hot-dust ULIRGs' are typically starburst galaxies
lower in mass than SMGs, but they can also simply be SMGs observed from a
different viewing angle.Comment: 21 pages, 11 figures. Accepted for publication in MNRAS. Minor
changes to text but otherwise identical to v
The Moment of Inertia and the Scissors Mode of a Bose-condensed Gas
We relate the frequency of the scissors mode to the moment of inertia of a
trapped Bose gas at finite temperature in a semi-classical approximation. We
apply these theoretical results to the data obtained in our previous study of
the properties of the scissors mode of a trapped Bose-Einstein condensate of
Rb atoms as a function of the temperature. The frequency shifts that we
measured show quenching of the moment of inertia of the Bose gas at
temperatures below the transition temperature - the system has a lower moment
of inertia that of a rigid body with the same mass distribution, because of
superfluidity.Comment: 14 pages, 5 fig
On the Evolution of the Star Formation Rate Function of Massive Galaxies. Constraints at 0.4<z<1.8 from the GOODS-MUSIC Catalogue
[abridged] We study the evolution of the Star Formation Rate Function (SFRF)
of massive galaxies over the 0.4<z<1.8 redshift range and its implications for
our understanding of the physical processes responsible for galaxy evolution.
We use multiwavelength observations included in the GOODS-MUSIC catalogue,
which provides a suitable coverage of the spectral region from 0.3 to 24 micron
and either spectroscopic or photometric redshifts for each object. Individual
SFRs have been obtained by combining UV and 24 micron observations, when the
latter were available. For all other sources an "SED fitting" SFR estimate has
been considered. We then define a stellar mass limited sample, complete in the
Mstar>1.e10 Msun range and determine the SFRF using the 1/Vmax algorithm. We
define simulated galaxy catalogues based on three different semi-analytical
models of galaxy formation and evolution. We show that the theoretical SFRFs
are well described by a double power law functional form and its redshift
evolution is approximated with high accuracy by a pure evolution of the typical
SFR. We find good agreement between model predictions and the high-SFR end of
the SFRF, when the observational errors on the SFR are taken into account.
However, the observational SFRF is characterised by a double peaked structure,
which is absent in its theoretical counterparts. At z>1.0 the observed SFRF
shows a relevant density evolution, which is not reproduced by SAMs, due to the
well known overprediction of intermediate mass galaxies at z~2. The agreement
at the low-SFR end is poor: all models overpredict the space density of SFR~1
Msun/yr and no model reproduces the double peaked shape of the observational
SFRF. If confirmed by deeper IR observations, this discrepancy will provide a
key constraint on theoretical modelling of star formation and stellar feedback.Comment: 12 pages, 4 figures and 3 table. Accepted for publication by MNRAS -
updated reference
Metallicity Gradients at Large Galactocentric Radii Using the Near-infrared Calcium Triplet
We describe a new spectroscopic technique for measuring radial metallicity
gradients out to large galactocentric radii. We use the DEIMOS multi-object
spectrograph on the Keck telescope and the galaxy spectrum extraction technique
of Proctor et al. (2009). We also make use of the metallicity sensitive
near-infrared (NIR) Calcium triplet (CaT) features together with single stellar
population models to obtain metallicities. Our technique is applied as a pilot
study to a sample of three relatively nearby (<30 Mpc) intermediate-mass to
massive early-type galaxies. Results are compared with previous literature
inner region values and generally show good agreement. We also include a
comparison with profiles from dissipational disk-disk major merger simulations.
Based on our new extended metallicity gradients combined with other
observational evidence and theoretical predictions, we discuss possible
formation scenarios for the galaxies in our sample. The limitations of our new
technique are also discussed.Comment: 13 Pages, 9 Figures, 7 Tables, Accepted for publication in MNRA
Galaxy Properties from the Ultra-violet to the Far-Infrared: Lambda-CDM models confront observations
We combine a semi-analytic model of galaxy formation with simple analytic
recipes describing the absorption and re-emission of starlight by dust in the
interstellar medium of galaxies. We use the resulting models to predict galaxy
counts and luminosity functions from the far-ultraviolet to the sub-mm, from
redshift five to the present, and compare with an extensive compilation of
observations. We find that in order to reproduce the rest-UV and optical
luminosity functions at high redshift, we must assume an evolving normalization
in the dust-to-metal ratio, implying that galaxies of a given bolometric
luminosity (or metal column density) must be less extinguished than their local
counterparts. In our best-fit model, we find remarkably good agreement with
observations from rest-frame 1500 Angstroms to 250 microns. At longer
wavelengths, most dramatically in the sub-mm, our models underpredict the
number of bright galaxies by a large factor. The models reproduce the observed
total IR luminosity function fairly well. We show the results of varying
several ingredients of the models, including various aspects of the dust
attenuation recipe, the dust emission templates, and the cosmology. We use our
models to predict the integrated Extragalactic Background Light (EBL), and
compare with an observationally-motivated EBL model and with other available
observational constraints.Comment: 27 pages, 17 figures, 1 table, accepted to MNRAS, this version
matches accepted manuscrip
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
The mode of gas accretion onto star-forming galaxies
It is argued that galaxies like ours sustain their star formation by
transferring gas from an extensive corona to the star-forming disc. The
transfer is effected by the galactic fountain -- cool clouds that are shot up
from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz
instability strips gas from these clouds. If the pressure and the the
metallicity of the corona are high enough, the stripped gas causes a similar
mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations
of cloud-corona interaction are presented. These confirm the existence of a
critical ablation rate above which the corona is condensed, and imply that for
the likely parameters of the Galactic corona this rate lies near the actual
ablation rate of clouds. In external galaxies trails of HI behind individual
clouds will not be detectable, although the integrated emission from all such
trails should be significant. Parts of the trails of the clouds that make up
the Galaxy's fountain should be observable and may account for features in
targeted 21-cm observations of individual high-velocity clouds and surveys of
Galactic HI emission. Taken in conjunction with the known decline in the
availability of cold infall with increasing cosmic time and halo mass, the
proposed mechanism offers a promising explanation of the division of galaxies
between the blue cloud to the red sequence in the colour-luminosity plane.Comment: 12 pages, 8 figures, 1 table. Accepted for publication in MNRA
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