7,187 research outputs found
Hipparcos distances of Ophiuchus and Lupus cloud complexes
We combine extinction maps from the Two Micron All Sky Survey (2MASS) with
Hipparcos and Tycho parallaxes to obtain reliable and high-precision estimates
of the distance to the Ophiuchus and Lupus dark complexes. Our analysis, based
on a rigorous maximum-likelihood approach, shows that the rho-Ophiuchi cloud is
located at (119 +/- 6) pc and the Lupus complex is located at (155 +/- 8) pc;
in addition, we are able to put constraints on the thickness of the clouds and
on their orientation on the sky (both these effects are not included in the
error estimate quoted above). For Ophiuchus, we find some evidence that the
streamers are closer to us than the core. The method applied in this paper is
currently limited to nearby molecular clouds, but it will find many natural
applications in the GAIA-era, when it will be possible to pin down the distance
and three-dimensional structure of virtually every molecular cloud in the
Galaxy.Comment: A&A in press - Corrected typo (Lupus distance) in the electronic
abstrac
Larson's third law and the universality of molecular cloud structure
Larson (1981) first noted a scaling relation between masses and sizes in
molecular clouds that implies that these objects have approximately constant
column densities. This original claim, based upon millimeter observations of
carbon monoxide lines, has been challenged by many theorists, arguing that the
apparent constant column density observed is merely the result of the limited
dynamic range of observations, and that in reality clouds have column density
variations over two orders of magnitudes. In this letter we investigate a set
of nearby molecular clouds with near-infrared excess methods, which guarantee
very large dynamic ranges and robust column density measurements, to test the
validity of Larson's third law. We verify that different clouds have almost
identical average column densities above a given extinction threshold; this
holds regardless of the extinction threshold, but the actual average surface
mass density is a function of the specific threshold used. We show that a
second version of Larson's third law, involving the mass-radius relation for
single clouds and cores, does not hold in our sample, indicating that
individual clouds are not objects that can be described by constant column
density. Our results instead indicate that molecular clouds are characterized
by a universal structure. Finally we point out that this universal structure
can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter
APM z>4 QSO Survey: Distribution and Evolution of High Column Density HI Absorbers
Eleven candidate damped Lya absorption systems were identified in 27 spectra
of the quasars from the APM z>4 survey covering the redshift range
2.83.5). High resolution echelle spectra (0.8A FWHM)
have been obtained for three quasars, including 2 of the highest redshift
objects in the survey. Two damped systems have confirmed HI column densities of
N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold.
We have discovered the highest redshift damped Lya absorber known at z=4.383 in
QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift
path available for damped surveys for z>3. We combine this high redshift sample
with other quasar samples covering the redshift range 0.008 < z < 4.7 to study
the redshift evolution and the column density distribution function for
absorbers with log N(HI)>=17.2. In the HI column density distribution
f(N)=kN^-beta we find evidence for breaks in the power law, flattening for
17.221.2. The column density
distribution function for the data with log N(HI)>=20.3 is better fit with the
form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the
number density per unit redshift is evident in the higher column density
systems with an apparent decline in N(z) for z>3.5.Comment: To appear in MNRAS. Latex file (10 pages of text) plus 14 separate
postscript figure files. Requires mn.sty. Postscript version with figures
embedded is available at http://www.ociw.edu/~lisa/publications.htm
Evolution of Neutral Gas at High Redshift -- Implications for the Epoch of Galaxy Formation
Though observationally rare, damped Lya absorption systems dominate the mass
density of neutral gas in the Universe. Eleven high redshift damped Lya systems
covering 2.84 QSO Survey,
extending these absorption system surveys to the highest redshifts currently
possible. Combining our new data set with previous surveys we find that the
cosmological mass density in neutral gas, omega_g, does not rise as steeply
prior to z~2 as indicated by previous studies. There is evidence in the
observed omega_g for a flattening at z~2 and a possible turnover at z~3. When
combined with the decline at z>3.5 in number density per unit redshift of
damped systems with column densities log N(HI)>21 atoms cm^-2, these results
point to an epoch at z>3 prior to which the highest column density damped
systems are still forming. We find that over the redshift range 2<z<4 the total
mass in neutral gas is marginally comparable with the total visible mass in
stars in present day galaxies. However, if one considers the total mass visible
in stellar disks alone, ie excluding galactic bulges, the two values are
comparable. We are observing a mass of neutral gas comparable to the mass of
visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5)
was twice omega_g(z~2), implying a much larger amount of star formation must
have taken place between z=3.5 and z=2 than is indicated by metallicity
studies. This created a `cosmic G-dwarf problem'. The more gradual evolution of
omega_g we find alleviates this. These results have profound implications for
theories of galaxy formation.Comment: To appear in MNRAS. Latex file (4 pages of text) plus 3 separate
postscript figure files. Requires mn.sty. Postscript version with figures
embedded is available at http://www.ociw.edu/~lisa/publications.htm
On the fidelity of the core mass functions derived from dust column density data
Aims: We examine the recoverability and completeness limits of the dense core
mass functions (CMFs) derived for a molecular cloud using extinction data and a
core identification scheme based on two-dimensional thresholding.
Methods: We performed simulations where a population of artificial cores was
embedded into the variable background extinction field of the Pipe nebula. We
extracted the cores from the simulated extinction maps, constructed the CMFs,
and compared them to the input CMFs. The simulations were repeated using a
variety of extraction parameters and several core populations with differing
input mass functions and differing degrees of crowding.
Results: The fidelity of the observed CMF depends on the parameters selected
for the core extraction algorithm for our background. More importantly, it
depends on how crowded the core population is. We find that the observed CMF
recovers the true CMF reliably when the mean separation of cores is larger than
their mean diameter (f>1). If this condition holds, the derived CMF is accurate
and complete above M > 0.8-1.5 Msun, depending on the parameters used for the
core extraction. In the simulations, the best fidelity was achieved with the
detection threshold of 1 or 2 times the rms-noise of the extinction data, and
with the contour level spacings of 3 times the rms-noise. Choosing larger
threshold and wider level spacings increases the limiting mass. The simulations
show that when f>1.5, the masses of individual cores are recovered with a
typical uncertainty of 25-30 %. When f=1 the uncertainty is ~60 %. In very
crowded cases where f<1 the core identification algorithm is unable to recover
the masses of the cores adequately. For the cores of the Pipe nebula f~2.0 and
therefore the use of the method in that region is justified.Comment: 9 pages, 6 figures, accepted for publication in A&
HST/ACS weak lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24
We present a weak lensing analysis of one of the most distant massive galaxy
cluster known, RDCS 1252.9-2927 at z=1.24, using deep images from the Advanced
Camera for Survey (ACS) on board the Hubble Space Telescope (HST). By taking
advantage of the depth and of the angular resolution of the ACS images, we
detect for the first time at z>1 a clear weak lensing signal in both the i
(F775W) and z (F850LP) filters. We measure a 5-\sigma signal in the i band and
a 3-\sigma signal in the shallower z band image. The two radial mass profiles
are found to be in very good agreement with each other, and provide a
measurement of the total mass of the cluster inside a 1Mpc radius of M(<1Mpc) =
(8.0 +/- 1.3) x 10^14 M_\odot in the current cosmological concordance model h
=0.70, \Omega_m=0.3, \Omega_\Lambda=0.7, assuming a redshift distribution of
background galaxies as inferred from the Hubble Deep Fields surveys. A weak
lensing signal is detected out to the boundary of our field (3' radius,
corresponding to 1.5Mpc at the cluster redshift). We detect a small offset
between the centroid of the weak lensing mass map and the brightest cluster
galaxy, and we discuss the possible origin of this discrepancy. The cumulative
weak lensing radial mass profile is found to be in good agreement with the
X-ray mass estimate based on Chandr and XMM-Newton observations, at least out
to R_500=0.5Mpc.Comment: 38 pages, ApJ in press. Full resolution images available at
http://www.eso.org/~prosati/RDCS1252/Lombardi_etal_accepted.pd
The mid-infrared extinction law in the darkest cores of the Pipe Nebula
Context. The properties of dust grains, in particular their size
distribution, are expected to differ from the interstellar medium to the
high-density regions within molecular clouds. Aims. We measure the mid-infrared
extinction law produced by dense material in molecular cloud cores. Since the
extinction at these wavelengths is caused by dust, the extinction law in cores
should depart from that found in low-density environments if the dust grains
have different properties. Methods. We use the unbiased LINES method to measure
the slope of the reddening vectors in color-color diagrams. We derive the
mid-infrared extinction law toward the dense cores B59 and FeSt 1-457 in the
Pipe Nebula over a range of visual extinction between 10 and 50 magnitudes,
using a combination of Spitzer/IRAC, and ESO NTT/VLT data. Results. The
mid-infrared extinction law in both cores departs significantly from a
power-law between 3.6 and 8 micron, suggesting that these cores contain dust
with a considerable fraction of large dust grains. We find no evidence for a
dependence of the extinction law with column density up to 50 magnitudes of
visual extinction in these cores, and no evidence for a variation between our
result and those for other clouds at lower column densities reported elsewhere
in the literature. This suggests that either large grains are present even in
low column density regions, or that the existing dust models need to be revised
at mid-infrared wavelengths. We find a small but significant difference in the
extinction law of the two cores, that we tentatively associate with the onset
of star formation in B59.Comment: 8 pages, 6 figures. Accepted to A&
The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud
The Orion cloud complex presents a variety of star formation mechanisms and
properties and it is still one of the most intriguing targets for star
formation studies. We present VISTA/VIRCAM near-infrared observations of the
L1630N star forming region, including the stellar clusters NGC 2068 and NGC
2071, in the Orion molecular cloud B and discuss them in combination with
Spitzer data. We select 186 young stellar object (YSO) candidates in the region
on the basis of multi-colour criteria, confirm the YSO nature of the majority
of them using published spectroscopy from the literature, and use this sample
to investigate the overall star formation properties in L1630N. The K-band
luminosity function of L1630N is remarkably similar to that of the Trapezium
cluster, i.e., it presents a broad peak in the range 0.3-0.7 M and a
fraction of sub-stellar objects of 20%. The fraction of YSOs still
surrounded by disk/envelopes is very high (85%) compared to other star
forming regions of similar age (1-2 Myr), but includes some uncertain
corrections for diskless YSOs. Yet, a possibly high disk fraction together with
the fact that 1/3 of the cloud mass has a gas surface density above the
threshold for star formation (129 M pc), points towards a
still on-going star formation activity in L1630N. The star formation efficiency
(SFE), star formation rate (SFR) and density of star formation of L1630N are
within the ranges estimated for galactic star forming regions by the Spitzer
"core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower
than the average value measured in the Orion A cloud and, in particular, lower
than that in the southern regions of L1630. This might suggest different star
formation mechanisms within the L1630 cloud complex.Comment: 22 pages, 9 figure
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