1,939 research outputs found
Cluster detection from surface-brightness fluctuations in SDSS data
Galaxy clusters can be detected as surface brightness enhancements in
smoothed optical surveys. This method does not require individual galaxies to
be identifiable, and enables clusters to be detected out to surprisingly high
redshifts, as recently demonstrated by the Las Campanas Distant Cluster Survey
(LCDCS). Here, we investigate redshift limits for cluster detection in the
Sloan Digital Sky Survey (SDSS). Calibrating assumptions about the surface
brightness profile, the mass-to-light ratio, and the spectral energy
distribution of galaxy clusters using available observational data, we show
that it should be possible to detect galaxy groups out to redshifts of ~0.5,
and massive galaxy clusters out to redshifts of ~1.2 in summed r'+i'+z' SDSS
data. Redshift estimates can be derived from the SDSS magnitudes of brightest
cluster members out to redshifts near unity. Over the area of sky it covers,
SDSS should find >~98% of the clusters detectable by the Planck satellite
through the thermal Sunyaev-Zel'dovich effect. The few Planck clusters not
detected in SDSS will almost all be at z>~1.2.Comment: 7 pages, submitted to Astronomy & Astrophysic
Formation and Disruption of Cosmological Low Mass Objects
We investigate the evolution of cosmological low mass (low virial
temperature) objects and the formation of the first luminous objects. First,
the `cooling diagram' for low mass objects is shown. We assess the cooling rate
taking into account the contribution of H_2, which is not in chemical
equilibrium generally, with a simple argument of time scales. The reaction
rates and the cooling rate of H_2 are taken from the recent results by Galli &
Palla (1998). Using this cooling diagram, we also estimate the formation
condition of luminous objects taking into account the supernova (SN) disruption
of virialized clouds. We find that the mass of the first luminous object is
several times 10^7 solar mass, because smaller objects may be disrupted by the
SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds
also discussed. The resultant metallicity of the clouds is about 1/1000 of the
solar metallicity.Comment: 11 pages, 2 figures, To appear in ApJ
Is there Evidence for a Hubble bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies
We examine recent evidence from the luminosity-redshift relation of Type Ia
Supernovae (SNe Ia) for the detection of a ``Hubble bubble'' --
a departure of the local value of the Hubble constant from its globally
averaged value \citep{Jha:07}. By comparing the MLCS2k2 fits used in that study
to the results from other light-curve fitters applied to the same data, we
demonstrate that this is related to the interpretation of SN color excesses
(after correction for a light-curve shape-color relation) and the presence of a
color gradient across the local sample. If the slope of the linear relation
() between SN color excess and luminosity is fit empirically, then the
bubble disappears. If, on the other hand, the color excess arises purely from
Milky Way-like dust, then SN data clearly favors a Hubble bubble. We
demonstrate that SN data give , instead of the
one would expect from purely Milky-Way-like dust. This suggests that either SN
intrinsic colors are more complicated than can be described with a single
light-curve shape parameter, or that dust around SN is unusual. Disentangling
these possibilities is both a challenge and an opportunity for large-survey SN
Ia cosmology.Comment: Further information and data at
http://qold.astro.utoronto.ca/conley/bubble/ Accepted for publication in ApJ
First Assessment of Mountains on Northwestern Ellesmere Island, Nunavut, as Potential Astronomical Observing Sites
Ellesmere Island, at the most northerly tip of Canada, possesses the highest
mountain peaks within 10 degrees of the pole. The highest is 2616 m, with many
summits over 1000 m, high enough to place them above a stable low-elevation
thermal inversion that persists through winter darkness. Our group has studied
four mountains along the northwestern coast which have the additional benefit
of smooth onshore airflow from the ice-locked Arctic Ocean. We deployed small
robotic site testing stations at three sites, the highest of which is over 1600
m and within 8 degrees of the pole. Basic weather and sky clarity data for over
three years beginning in 2006 are presented here, and compared with available
nearby sea-level data and one manned mid-elevation site. Our results point to
coastal mountain sites experiencing good weather: low median wind speed, high
clear-sky fraction and the expectation of excellent seeing. Some practical
aspects of access to these remote locations and operation and maintenance of
equipment there are also discussed.Comment: 21 pages, 2 tables, 15 figures; accepted for publication in PAS
Formation of Primordial Protostars
The evolution of collapsing metal free protostellar clouds is investigated
for various masses and initial conditions.
We perform hydrodynamical calculations for spherically symmetric clouds
taking account of radiative transfer of the molecular hydrogen lines and the
continuum, as well as of chemistry of the molecular hydrogen.
The collapse is found to proceed almost self-similarly like Larson-Penston
similarity solution.
In the course of the collapse, efficient three-body processes transform
atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular
form.
However, hydrogen in the outer part remains totally atomic although there is
an intervening transitional layer of several solar masses, where hydrogen is in
partially molecular form.
No opaque transient core is formed although clouds become optically thick to
H collision-induced absorption continuum, since H dissociation
follows successively.
When the central part of the cloud reaches stellar densities (), a very small hydrostatic core (\sim
5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the
ambient gas accretes onto it.
The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun}
{\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where is instantaneous
mass of the central core, by using a similarity solution which reproduces the
evolution of the cloud before the core formation.Comment: 20 pages, 5 Postscript figures, uses AAS LaTe
Dwarf Galaxy Clustering and Missing Satellites
At redshifts around 0.1 the CFHT Legacy Survey Deep fields contain some
6x10^4 galaxies spanning the mass range from 10^5 to 10^12 Msun. We measure the
stellar mass dependence of the two point correlation using angular measurements
to largely bypass the errors, approximately 0.02 in the median, of the
photometric redshifts. Inverting the power-law fits with Limber's equation we
find that the auto-correlation length increases from a very low 0.4hMpc at
10^5.5 Msun to the conventional 4.5hMpc at 10^10.5 Msun. The power law fit to
the correlation function has a slope which increases from gamma approximately
1.6 at high mass to gamma approximately 2.3 at low mass. The spatial
cross-correlation of dwarf galaxies with more massive galaxies shows fairly
similar trends, with a steeper radial dependence at low mass than predicted in
numerical simulations of sub-halos within galaxy halos. To examine the issue of
missing satellites we combine the cross-correlation measurements with our
estimates of the low mass galaxy number density. We find on the average there
are 60+/-20 dwarfs in sub-halos with M(total) > 10^7 Msun for a typical Local
Group M(total)/M(stars)=30, corresponding to M/L_V approximately 100 for a
galaxy with no recent star formation. The number of dwarfs per galaxy is about
a factor of two larger than currently found for the Milky Way. Nevertheless,
the average dwarf counts are about a factor of 30 below LCDM simulation
results. The divergence from LCDM predictions is one of slope of the relation,
approximately dN/dlnM approximately -0.5 rather than the predicted -0.9, not
sudden onset at some characteristic scale. The dwarf galaxy star formation
rates span the range from passive to bursting, which suggests that there are
few completely dark halos.Comment: revised version submitted to Astrophysical Journa
New Exoplanet Surveys in the Canadian High Arctic at 80 Degrees North
Observations from near the Eureka station on Ellesmere Island, in the
Canadian High Arctic at 80 degrees North, benefit from 24-hour darkness
combined with dark skies and long cloud-free periods during the winter. Our
first astronomical surveys conducted at the site are aimed at transiting
exoplanets; compared to mid-latitude sites, the continuous darkness during the
Arctic winter greatly improves the survey's detection efficiency for
longer-period transiting planets. We detail the design, construction, and
testing of the first two instruments: a robotic telescope, and a set of very
wide-field imaging cameras. The 0.5m Dunlap Institute Arctic Telescope has a
0.8-square-degree field of view and is designed to search for potentially
habitable exoplanets around low-mass stars. The very wide field cameras have
several-hundred-square-degree fields of view pointed at Polaris, are designed
to search for transiting planets around bright stars, and were tested at the
site in February 2012. Finally, we present a conceptual design for the Compound
Arctic Telescope Survey (CATS), a multiplexed transient and transit search
system which can produce a 10,000-square-degree snapshot image every few
minutes throughout the Arctic winter.Comment: 11 pages, 6 figures, SPIE vol 8444, 201
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