13,362 research outputs found
Physical Properties of Galactic Planck Cold Cores revealed by the Hi-GAL survey
Previous studies of the initial conditions of massive star formation have
mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is
due to the fact that IRDCs were first detected in absorption against the bright
mid-IR background, requiring a favourable location to be observed. By
selection, IRDCs represent only a fraction of the Galactic clouds capable of
forming massive stars and star clusters. Due to their low dust temperatures,
IRDCs are bright in the far-IR and millimeter and thus, observations at these
wavelengths have the potential to provide a complete sample of star-forming
massive clouds across the Galaxy. Our aim is to identify the clouds at the
initial conditions of massive star formation across the Galaxy and compare
their physical properties as a function of their Galactic location. We have
examined the physical properties of a homogeneous galactic cold core sample
obtained with the Planck satellite across the Galactic Plane. With the use of
Herschel Hi-GAL observations, we have characterized the internal structure of
them. By using background-subtracted Herschel images, we have derived the H2
column density and dust temperature maps for 48 Planck clumps. Their basic
physical parameters have been calculated and analyzed as a function of location
within the Galaxy. These properties have also been compared with the empirical
relation for massive star formation derived by Kauffmann & Pillai (2010). Most
of the Planck clumps contain signs of star formation. About 25% of them are
massive enough to form high mass stars. Planck clumps toward the Galactic
center region show higher peak column densities and higher average dust
temperatures than those of the clumps in the outer Galaxy. Although we only
have seven clumps without associated YSOs, the Hi-GAL data show no apparent
differences in the properties of Planck cold clumps with and without star
formation.Comment: 22 pages, 11 figures, accepted for publication in A&
Environment and the cosmic evolution of star formation
We present a mark correlation analysis of the galaxies in the Sloan Digital
Sky Survey using weights provided by MOPED. The large size of the sample
permits statistically significant statements about how galaxies with different
metallicities and star formation histories are spatially correlated. Massive
objects formed a larger fraction of their stars at higher redshifts and over
shorter timescales than did less massive objects (sometimes called
down-sizing). We find that those galaxies which dominated the cosmic star
formation at z~3 are predominantly in clusters today, whereas galaxies which
dominate the star formation at z~0 inhabit substantially lower mass objects in
less dense regions today. Hence, our results indicate that star formation and
chemical enrichment occured first in the denser regions of the Universe, and
moved to less dense regions at later times.Comment: 4 pages, 4 figures, submitted to ApJ
Constraints on the equation of state of dark energy and the Hubble constant from stellar ages and the CMB
We place tight constraints on the redshift-averaged, effective value of the
equation of state of dark energy, w, using only the absolute ages of Galactic
stars and the observed position of the first peak in the angular power spectrum
of the CMB. We find w<-0.8 at the 68% confidence level. If we further consider
that w > -1, this finding suggests that within our uncertainties, dark energy
is indistinguishable from a classical vacuum energy term.
We detect a correlation between the ages of the oldest galaxies and their
redshift. This opens up the possibility of measuring w(z) by computing the
relative ages of the oldest galaxies in the universe as a function of redshift,
dz/dt. We show that this is a realistic possibility by computing dz/dt at z~0
from SDSS galaxies and obtain an independent estimate for the Hubble constant,
H_0 = 69 \pm 12 km s-1 Mpc-1. The small number of galaxies considered at z>0.2
does not yield, currently, a precise determination of w(z), but shows that the
age--redshift relation is consistent with a Standard LCDM universe with .Comment: Submitted to Ap
Dark energy, non-minimal couplings and the origin of cosmic magnetic fields
In this work we consider the most general electromagnetic theory in curved
space-time leading to linear second order differential equations, including
non-minimal couplings to the space-time curvature. We assume the presence of a
temporal electromagnetic background whose energy density plays the role of dark
energy, as has been recently suggested. Imposing the consistency of the theory
in the weak-field limit, we show that it reduces to standard electromagnetism
in the presence of an effective electromagnetic current which is generated by
the momentum density of the matter/energy distribution, even for neutral
sources. This implies that in the presence of dark energy, the motion of
large-scale structures generates magnetic fields. Estimates of the present
amplitude of the generated seed fields for typical spiral galaxies could reach
G without any amplification. In the case of compact rotating objects,
the theory predicts their magnetic moments to be related to their angular
momenta in the way suggested by the so called Schuster-Blackett conjecture.Comment: 5 pages, no figure
The Cosmic Neutrino Background and the Age of the Universe
We discuss the cosmological degeneracy between the age of the Universe, the
Hubble parameter and the effective number of relativistic particles N_eff. We
show that independent determinations of the Hubble parameter H(z) as those
recently provided by Simon,Verde, Jimenez (2006), combined with other
cosmological data sets can provide the most stringent constraint on N_eff,
yielding N_eff=3.7 (-1.2) (+1.1) at 95% confidence level. A neutrino background
is detected with high significance: N_eff >1.8 at better than 99% confidence
level. Constraints on the age of the universe in the framework of an extra
background of relativistic particles are improved by a factor 3.Comment: JCAP, in pres
Collapsing molecular clouds with tracer particles: Part II, Collapse Histories
In order to develop a complete theory of star formation, one essentially
needs to know two things: what collapses, and how long it takes. This is the
second paper in a series, where we query how long a parcel of gas takes to
collapse and the process it undergoes. We embed pseudo-Lagrangian tracer
particles in simulations of collapsing molecular clouds, identify the particles
that end in dense knots, and then examine the collapse history of the gas. We
find a nearly universal behavior of cruise-then-collapse. We identify gas the
moment before it collapses, , and examine how it transitions to
high density. We find that the time to collapse is uniformly distributed
between and the end of the simulation at , and that the collapse duration is universally short, . We find that the collapse of each core happens by a
process akin to violent relaxation, wherein a fast reordering of the potential
and kinetic energies occurs, in , after which a virialized
object remains. We describe the collapse in four stages; collection, hardening,
singularity, and mosh. Collection sweeps low density gas into moderate density.
Hardening brings kinetic and gravitational energies into quasi-equipartition.
Singularity is the free-fall collapse, forming a virialized object in . Mosh encompasses tidal dynamics of sub clumps and nearby cores
during the collapse. In this work we focus primarily on isolated clumps. With
this novel lens we can observe the details of collapse
Integral-field spectroscopy of the quadruple QSO HE 0435-1223: Evidence for microlensing
We present the first spatially resolved spectroscopic observations of the
recently discovered quadruple QSO and gravitational lens HE0435-1223. Using the
Potsdam Multi-Aperture Spectrophotometer (PMAS), we show that all four QSO
components have very similar but not identical spectra. In particular, the
spectral slopes of components A, B, and D are indistinguishable, implying that
extinction due to dust plays no major role in the lensing galaxy. While also
the emission line profiles are identical within the error bars, as expected
from lensing, the equivalent widths show significant differences between
components. Most likely, microlensing is responsible for this phenomenon. This
is also consistent with the fact that component D, which shows the highest
relative continuum level, has brightened by 0.07 mag since Dec 2001. We find
that the emission line flux ratios between the components are in better
agreement with simple lens models than broad band or continuum measurements,
but that the discrepancies between model and data are still unacceptably large.
Finally, we present a detection of the lensing galaxy, although this is close
to the limits of the data. Comparing with a model galaxy spectrum, we obtain a
redshift estimate of z_lens=0.44+-0.02.Comment: 9 pages, 7 figures, accepted for publication in A&
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