365 research outputs found
Searching for life in the Universe: unconventional methods for an unconventional problem
The search for life, on and off our planet, can be done by conventional methods with which we are all familiar. These methods are sensitive and specific, and are often capable of detecting even single cells. However, if the search broadens to include life that may be different (even subtly different) in composition, the methods and even the approach must be altered. Here we discuss the development of what we call non-earthcentric life detection – detecting life with methods that could detect life no matter what its form or composition. To develop these methods, we simply ask, can we define life in terms of its general properties and particularly those that can be measured and quantified? Taking such an approach we can search for life using physics and chemistry to ask questions about structure, chemical composition, thermodynamics, and kinetics. Structural complexity can be searched for using computer algorithms that recognize complex structures. Once identified, these structures can be examined for a variety of chemical traits, including elemental composition, chirality, and complex chemistry. A second approach involves defining our environment in terms of energy sources (i.e., reductants), and oxidants (e.g. what is available to eat and breathe), and then looking for areas in which such phenomena are inexplicably out of chemical equilibrium. These disequilibria, when found, can then be examined in detail for the presence of the structural and chemical complexity that presumably characterizes any living systems. By this approach, we move the search for life to one that should facilitate the detection of any earthly life it encountered, as well as any non-conventional life forms that have structure, complex chemistry, and live via some form of redox chemistry
Simulations of Damped Lyman-Alpha and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift
We use hydrodynamic cosmological simulations to study damped Lyman-alpha
(DLA) and Lyman limit (LL) absorption at redshifts z=2-4 in five variants of
the cold dark matter scenario. Our standard simulations resolve the formation
of dense concentrations of neutral gas in halos with circular velocity v_c
roughly 140 km/s for Omega_m=1 and 90 km/s for Omega_m=0.4, at z=2; an
additional LCDM simulation resolves halos down to v_c approximately 50 km/s at
z=3. We find a clear relation between HI column density and projected distance
to the center of the nearest galaxy, with DLA absorption usually confined to
galactocentric radii less than 10-15 kpc and LL absorption arising out to
projected separations of 30 kpc or more. Detailed examination provides evidence
of non-equilibrium effects on absorption cross-section. If we consider only
absorption in the halos resolved by our standard simulations, then all five
models fall short of reproducing the observed abundance of DLA and LL systems
at these redshifts. If we extrapolate to lower halo masses, we find all four
models are consistent with the observed abundance of DLA systems if the the
extrapolated behavior extends to circular velocities roughly 50-80 km/s, and
they may produce too much absorption if the relation continues to 40 km/s. Our
results suggest that LL absorption is closely akin to DLA absorption, arising
in less massive halos or at larger galactocentric radii but not caused by
processes acting on a radically different mass scale.Comment: 33 pages with 10 embedded EPS figures. Substantially revised and
updated from original version. Includes new high-resolution simulations.
Accepted for publication in the Ap
Spectral Classification of Galaxies
We investigate the integrated spectra of a sample of 24 normal galaxies. A
principal component analysis suggests that most of the variance present in the
spectra is due to the differences in morphology of the galaxies in the sample.
We show that spectroscopic parameters extracted from the spectra, like the
amplitude of the 4000 \AA~ break or of the CN band, correlate well with Hubble
types and are useful for quantitative classification.Comment: 7 pages uuencoded compressed PostScript file. To appear in Vistas in
Astronomy, special issue on Artificial Neural Networks in Astronom
Extremely Red Objects from the NICMOS/HST Parallel Imaging Survey
We present a catalog of extremely red objects discovered using the NICMOS/HST
parallel imaging database and ground-based optical follow-up observations.
Within an area of 16 square arc-minutes, we detect 15 objects with and . We have also obtained K-band photometry for
a subset of the 15 EROs. All of the selected EROs imaged at
K-band have . Our objects have colors in the
range of 1.3 - 2.1, redder than the cluster ellipticals at and
nearly 1 magnitude redder than the average population selected from the F160W
images at the same depth. In addition, among only 22 NICMOS pointings, we
detected two groups or clusters in two fields, each contains 3 or more EROs,
suggesting that extremely red galaxies may be strongly clustered. At bright
magnitudes with , the ERO surface density is similar to what
has been measured by other surveys. At the limit of our sample, F160W = 21.5,
our measured surface density is 0.94 arcmin^{-2}. Excluding the two
possible groups/clusters and the one apparently stellar object, reduces the
surface density to 0.38 arcmin^{-2}.Comment: To appear in the AJ August issue. Replaced with the published versio
High-resolution O VI absorption line observations at 1.2 < z < 1.7 in the bright QSO HE 0515-4414
STIS Echelle observations at a resolution of 10 km/s and UVES/VLT
spectroscopy at a resolution of 7 km/s of the luminous QSO HE 0515-4414 (z_em =
1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshift
range 1.2 < z_abs < 1.7 (1.38503, 1.41601, 1.60175, 1.67359). In addition two
associated systems at z = 1.69707 and z = 1.73585 are present. For the first
time high resolution observations allow to measure radial velocities of H I, C
IV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697)
with the result that significant velocity differences (up to 18 km/s) are
observed between H I and O VI, while smaller differences (up to 5 km/s) are
seen between C IV and O VI. We tentatively conclude that H I, O VI, and C IV
are not formed in the same volumes and that therefore implications on
ionization mechanisms are not possible from observed column density ratios O
VI/H I or O VI/C IV. The number density of O VI absorbers with W_rest > 25 mA
is dN/dz < 10, roughly a factor of 5 less than what has been found by Tripp at
al. (2000) at low redshift. An estimate of the cosmological mass-density of the
O VI-phase yields Omega_b(O VI) = 0.0003 h^{-1}_{75} for [O/H] = -1 and an
assumed ionization fraction O VI/O = 0.2. This corresponds to an increase by
roughly a factor of 15 between z = 1.5 (this work) and the value found by Tripp
et al. (2000) at z = 0.21, if the same oxygen abundance [O/H] = -1 is assumed.
Agreement with the simulations by Dave et al. (2001) can be obtained, if the
oxygen abundance increases by a factor of 3 over the same redshift interval.Comment: 8 pages, 1 figure, accepted for publication in A&
Imaging and spectroscopy of galaxies associated with two z~0.7 damped Lyman-alpha absorption systems
We have identified galaxies near two quasars which are at the redshift of
damped Lyman-alpha (DLA) systems in the UV spectra of the quasars. Both
galaxies are actively forming stars. One galaxy has a luminosity close to the
break in the local galaxy luminosity function, L*, the other is significantly
fainter than L* and appears to be interacting with a nearby companion. Despite
the strong selection effects favoring spectroscopic identification of the most
luminous DLA galaxies, many of the spectroscopically-identified DLA galaxies in
the literature are sub-L*, suggesting that the majority of the DLA population
is probably sub-L*, in contrast to MgII absorbers at similar redshifts whose
mean luminosity is close to L*.Comment: 9 pages, to appear in AJ, November 2003 issu
The Population of Damped Lyman-alpha and Lyman Limit Systems in the Cold Dark Matter Model
Lyman limit and damped Lyman-alpha absorption systems probe the distribution
of collapsed, cold gas at high redshift. Numerical simulations that incorporate
gravity and gas dynamics can predict the abundance of such absorbers in
cosmological models. We develop a semi-analytical method to correct the
numerical predictions for the contribution of unresolved low mass halos, and we
apply this method to the Katz et al. (1996) simulation of the standard cold
dark matter model (, , , ). Using
this simulation and higher resolution simulations of individual low mass
systems, we determine the relation between a halo's circular velocity and
its cross section for producing Lyman limit or damped absorption. We combine
this relation with the Press-Schechter formula for the abundance of halos to
compute the number of absorbers per unit redshift. The resolution correction
increases the predicted abundances by about a factor of two at z=2, 3, and 4,
bringing the predicted number of damped absorbers into quite good agreement
with observations. Roughly half of the systems reside in halos with circular
velocities v_c>100\kms and half in halos with 35\kms. Halos
with v_c>150\kms typically harbor two or more systems capable of producing
damped absorption. Even with the resolution correction, the predicted abundance
of Lyman limit systems is a factor of three below observational estimates,
signifying either a failure of standard CDM or a failure of these simulations
to resolve the systems responsible for most Lyman limit absorption. By
comparing simulations with and without star formation, we find that depletion
of the gas supply by star formation affects absorption line statistics at
only for column densities exceeding .Comment: AASlatex, 17 pages w/ 3 embedded ps figures. Submitted to Ap
Quasars and Ultraluminous Infrared Galaxies: At the Limit?
We have detected the host galaxies of 16 nearby, radio-quiet quasars using
images obtained with the Near-Infrared Camera and Multi-Object Spectrometer
(NICMOS). We confirm that these luminous quasars tend to live in luminous,
early-type host galaxies, and we use the host-galaxy magnitudes to refine the
luminosity/host-mass limit inferred from ground-based studies. If quasars obey
the relation found for massive dark
objects in nonactive galaxies, then our analysis implies that they radiate at
up to of the Eddington rate. An analogous analysis for ultraluminous
infrared galaxies shows them to accrete at up to similar Eddington fractions,
consistent with the hypothesis that some of them are powered by embedded
quasars.Comment: 9 pages, includes 2 eps figs, accepted to ApJLet
Planetary Science Goals for the Spitzer Warm Era
The overarching goal of planetary astronomy is to deduce how the present collection of objects found in our Solar System were formed from the original material present in the proto-solar nebula. As over two hundred exo-planetary systems are now known, and multitudes more are expected, the Solar System represents the closest and best system which we can study, and the only one in which we can clearly resolve individual bodies other than planets. In this White Paper we demonstrate how to use Spitzer Space Telescope InfraRed Array Camera Channels 1 and 2 (3.6 and 4.5 µm) imaging photometry with large dedicated surveys to advance our knowledge of Solar System formation and evolution. There are a number of vital, key projects to be pursued using dedicated large programs that have not been pursued during the five years of Spitzer cold operations. We present a number of the largest and most important projects here; more will certainly be proposed once the warm era has begun, including important observations of newly discovered objects
Object Classification in Astronomical Multi-Color Surveys
We present a photometric method for identifying stars, galaxies and quasars
in multi-color surveys, which uses a library of >65000 color templates. The
method aims for extracting the information content of object colors in a
statistically correct way and performs a classification as well as a redshift
estimation for galaxies and quasars in a unified approach. For the redshift
estimation, we use an advanced version of the MEV estimator which determines
the redshift error from the redshift dependent probability density function.
The method was originally developed for the CADIS survey, where we checked
its performance by spectroscopy. The method provides high reliability (6 errors
among 151 objects with R<24), especially for quasar selection, and redshifts
accurate within sigma ~ 0.03 for galaxies and sigma ~ 0.1 for quasars.
We compare a few model surveys using the same telescope time but different
sets of broad-band and medium-band filters. Their performance is investigated
by Monte-Carlo simulations as well as by analytic evaluation in terms of
classification and redshift estimation. In practice, medium-band surveys show
superior performance. Finally, we discuss the relevance of color calibration
and derive important conclusions for the issues of library design and choice of
filters. The calibration accuracy poses strong constraints on an accurate
classification, and is most critical for surveys with few, broad and deeply
exposed filters, but less severe for many, narrow and less deep filters.Comment: 21 pages including 10 figures. Accepted for publication in Astronomy
& Astrophysic
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