163 research outputs found
On the dependence of galaxy morphologies on galaxy mergers
The distribution of galaxy morphological types is a key test for models of
galaxy formation and evolution, providing strong constraints on the relative
contribution of different physical processes responsible for the growth of the
spheroidal components. In this paper, we make use of a suite of semi-analytic
models to study the efficiency of galaxy mergers in disrupting galaxy discs and
building galaxy bulges. In particular, we compare standard prescriptions
usually adopted in semi-analytic models, with new prescriptions proposed by
Kannan et al., based on results from high-resolution hydrodynamical
simulations, and we show that these new implementations reduce the efficiency
of bulge formation through mergers. In addition, we compare our model results
with a variety of observational measurements of the fraction of
spheroid-dominated galaxies as a function of stellar and halo mass, showing
that the present uncertainties in the data represent an important limitation to
our understanding of spheroid formation. Our results indicate that the main
tension between theoretical models and observations does not stem from the
survival of purely disc structures (i.e. bulgeless galaxies), rather from the
distribution of galaxies of different morphological types, as a function of
their stellar mass.Comment: MNRAS in press, 11 pages, 5 figure
Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study
The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br+ and C2H5I+) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin–orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin–orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC–Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br+ cations undergo prompt C–Br bond fission to form predominantly C2H5+ + Br* products with a near-limiting ‘parallel’ recoil velocity distribution. The observed C2H3+ + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5+ products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I+, along with an additional energetically accessible C–I bond fission channel to form C2H5 + I+ products. HX (X = Br, I) elimination from the highly internally excited C2H5X+ cation is deemed the most probable route to forming the C2H4+ fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C–C bond fission process to form CH2X+ + CH3 products
The Strongly Polarized Afterglow of GRB 020405
We report polarization measurements and photometry for the optical afterglow
of the gamma-ray burst GRB 020405. We measured a highly significant 9.9%
polarization (in V band) 1.3 days after the burst and argue that it is
intrinsic to the GRB. The light curve decay is well fitted by a
power-law; we do not see any evidence for a break between 1.24 and 4.3 days
after the burst. We discuss these measurements in the light of several models
of GRB afterglows.Comment: submitted to ApJ
Ultra-deep catalog of X-ray groups in the Extended Chandra Deep Field South
Ultra-deep observations of ECDF-S with Chandra and XMM-Newton enable a search
for extended X-ray emission down to an unprecedented flux of
ergs s cm. We present the search for the extended emission on
spatial scales of 32 in both Chandra and XMM data, covering
0.3 square degrees and model the extended emission on scales of arcminutes. We
present a catalog of 46 spectroscopically identified groups, reaching a
redshift of 1.6. We show that the statistical properties of ECDF-S, such as
logN-logS and X-ray luminosity function are broadly consistent with LCDM, with
the exception that dn/dz/d test reveals that a redshift range of
in ECDF-S is sparsely populated. The lack of nearby structure,
however, makes studies of high-redshift groups particularly easier both in
X-rays and lensing, due to a lower level of clustered foreground. We present
one and two point statistics of the galaxy groups as well as weak-lensing
analysis to show that the detected low-luminosity systems are indeed low-mass
systems. We verify the applicability of the scaling relations between the X-ray
luminosity and the total mass of the group, derived for the COSMOS survey to
lower masses and higher redshifts probed by ECDF-S by means of stacked weak
lensing and clustering analysis, constraining any possible departures to be
within 30% in mass. Abridged.Comment: 20 pages, 21 figures, 3 tables, to match the journal versio
The Diverse Nature of Optical Emission Lines in Brightest Cluster Galaxies: IFU Observations of the Central Kiloparsecs
We present integral field spectroscopy of the nebular line emission in a
sample of 9 brightest cluster galaxies (BCGs). The sample was chosen to probe
both cooling flow and non-cooling flow clusters, as well as a range of cluster
X-ray luminosities. The line emission morphology and velocity gradients suggest
a great diversity in the properties of the line emitting gas. While some BGCs
show evidence for filamentary or patchy emission (Abell 1060, Abell 1668 and
MKW3s), others have extended emission (Abell 1204, Abell 2199), while still
others have centrally concentrated emission (Abell 2052). We examine diagnostic
line ratios to determine the dominant ionization mechanisms in each galaxy.
Most of the galaxies show regions with AGN-like spectra, however for two BCGs,
Abell 1060 and Abell 1204, the emission line diagnostics suggest regions which
can be described by the emission from young stellar populations. The diversity
of emission line properties in our sample of BCGs suggests that the emission
mechanism is not universal, with different ionization processes dominating
different systems. Given this diversity, there is no evidence for a clear
distinction of the emission line properties between cooling flow and
non-cooling flow BCGs. It is not always cooling flow BCGs which show emission
(or young stellar populations), and non-cooling flow BCGs which do not.Comment: 23 pages, 23 figures, accepted for publication in MNRAS. Full
resolution images are online http://web.ipac.caltech.edu/staff/louis
Galaxy properties from Voids to Clusters in the SDSS-DR4
We investigate the environmental dependence of galaxy population properties
in a complete volume-limited sample of 91566 galaxies in the redshift range
0.05 <= z <= 0.095 and with M_r <= -20.0, selected from the Sloan Digital Sky
Survey (SDSS) Data Release 4. Our aim is to search for systematic variations in
the properties of galaxies with the local galaxy density. In particular, we
analize how the (u - r) color index and the morphological type of galaxies (the
latter evaluated through the SDSS Eclass and FracDev parameters) are related to
the environment and to the luminosity of galaxies, in order to find hints that
can be related to the presence of a ''void'' galaxy population. Void galaxies
are identified through a highly selective criterion, which takes also into
account redshift and allows us to exclude from the sample all the galaxies that
are not really close to the center of underdense regions. We study the (u - r)
color distribution for galaxies in different luminosity bins, and we look for
correlations of color with the environment, the luminosity, and the
morphological type of the galaxies. We find that galaxies in underdense regions
(voids) have lower luminosity (M_r > -21) and are bluer than cluster galaxies.
The transition from overdense to underdense environments is smooth, the
fraction of late-type galaxies decreases while the fraction of early-type
galaxies increases smoothly from underdense to dense environments. We do not
find any sudden transition in the galaxy properties with density, which,
according to a suggestion by Peebles (2001), should mark the transition to a
population of "void" galaxies in LCDM models. On the contrary, our results
suggest a continuity of galaxy properties, from voids to clusters.Comment: A&A, accepted for publication. 17 pages, 9 figures. Constructive
comments welcom
Structure of the Galaxies in the NGC 80 Group
BV-bands photometric data obtained at the 6-m telescope of the Special
Astrophysical Observatory are used to analyze the structure of 13 large disk
galaxies in the NGC 80 group. Nine of the 13 galaxies under consideration are
classified by us as lenticular galaxies. The stellar populations in the
galaxies are very different, from old ones with ages of T>10 Gyrs (IC 1541) to
relatively young, with the ages of T<2-3 Gyr (IC 1548, NGC 85). In one case,
current star formation is known (UCM 0018+2216). In most of the galaxies, more
precisely in all of them more luminous than M(B) -18, two-tiered
(`antitruncated') stellar disks are detected, whose radial surface brightness
profiles can be fitted by two exponential segments with different scalelengths
-- shorter near the center and longer at the periphery. All dwarf S0 galaxies
with single-scalelength exponential disks are close companions to giant
galaxies. Except for this fact, no dependence of the properties of S0 galaxies
on distance from the center of the group is found. Morphological traces of
minor merger are found in the lenticular galaxy NGC 85. Basing on the last two
points, we conclude that the most probable mechanisms for the transformation of
spirals into lenticular galaxies in groups are gravitational ones, namely,
minor mergers and tidal interactions.Comment: 24 pages, 9 figures, slightly improved version of the paper published
in the December, 2009, issue of the Astronomy Report
Synchronous Evolution of Galaxies in Groups: NGC 524 Group
By means of panoramic spectroscopy at the SAO RAS BTA telescope, we
investigated the properties of stellar populations in the central regions of
five early-type galaxies -- the NGC 524 group members. The evolution of the
central regions of galaxies looks synchronized: the average age of stars in the
bulges of all the five galaxies lies in the range of 3--6 Gyr. Four of the five
galaxies revealed synchronized bursts of star formation in the nuclei 1--2 Gyr
ago. The only galaxy, in which the ages of stellar population in the nucleus
and in the bulge coincide (i.e. the nuclear burst of star formation did not
take place) is NGC 502, the farthest from the center of the group of all the
galaxies studied.Comment: Slightly edited version of the paper to appear in the Astrophysical
Bulletin, 67(3); 24 pages including 8 figure
Pore dimensions and the role of occupancy in unitary conductance of Shaker K channels
Indexación: Web of Science; Scopus.K channels mediate the selective passage of K+ across the plasma membrane by means of intimate interactions with ions at the pore selectivity filter located near the external face. Despite high conservation of the selectivity filter, the K+ transport properties of different K channels vary widely, with the unitary conductance spanning a range of over two orders of magnitude. Mutation of Pro475, a residue located at the cytoplasmic entrance of the pore of the small-intermediate conductance K channel Shaker (Pro475Asp (P475D) or Pro475Gln (P475Q)), increases Shaker's reported. 20-pS conductance by approximately six-and approximately threefold, respectively, without any detectable effect on its selectivity. These findings suggest that the structural determinants underlying the diversity of K channel conductance are distinct from the selectivity filter, making P475D and P475Q excellent probes to identify key determinants of the K channel unitary conductance. By measuring diffusion-limited unitary outward currents after unilateral addition of 2 M sucrose to the internal solution to increase its viscosity, we estimated a pore internal radius of capture of 0.82 for all the three Shaker variants (wild type, P475D, and P475Q). This estimate is consistent with the internal entrance of the Kv1.2/2.1 structure if the effective radius of hydrated K+ is set to 4 A. Unilateral exposure to sucrose allowed us to estimate the internal and external access resistances together with that of the inner pore. We determined that Shaker resistance resides mainly in the inner cavity, whereas only similar to 8% resides in the selectivity filter. To reduce the inner resistance, we introduced additional aspartate residues into the internal vestibule to favor ion occupancy. No aspartate addition raised the maximum unitary conductance, measured at saturating [K+], beyond that of P475D, suggesting an similar to 200-pS conductance ceiling for Shaker. This value is approximately one third of the maximum conductance of the large conductance K (BK) channel (the K channel of highest conductance), reducing the energy gap between their K+ transport rates to similar to 1 kT. Thus, although Shaker's pore sustains ion translocation as the BK channel's does, higher energetic costs of ion stabilization or higher friction with the ion's rigid hydration cage in its narrower aqueous cavity may entail higher resistance.http://jgp.rupress.org/content/146/2/13
Repeatability and reproducibility of multiparametric magnetic resonance imaging of the liver
As the burden of liver disease reaches epidemic levels, there is a high unmet medical need to develop robust, accurate and reproducible non-invasive methods to quantify liver tissue characteristics for use in clinical development and ultimately in clinical practice. This prospective cross-sectional study systematically examines the repeatability and reproducibility of iron-corrected T1 (cT1), T2*, and hepatic proton density fat fraction (PDFF) quantification with multiparametric MRI across different field strengths, scanner manufacturers and models. 61 adult participants with mixed liver disease aetiology and those without any history of liver disease underwent multiparametric MRI on combinations of 5 scanner models from two manufacturers (Siemens and Philips) at different field strengths (1.5T and 3T). We report high repeatability and reproducibility across different field strengths, manufacturers, and scanner models in standardized cT1 (repeatability CoV: 1.7%, bias -7.5ms, 95% LoA of -53.6 ms to 38.5 ms; reproducibility CoV 3.3%, bias 6.5 ms, 95% LoA of -76.3 to 89.2 ms) and T2* (repeatability CoV: 5.5%, bias -0.18 ms, 95% LoA -5.41 to 5.05 ms; reproducibility CoV 6.6%, bias -1.7 ms, 95% LoA -6.61 to 3.15 ms) in human measurements. PDFF repeatability (0.8%) and reproducibility (0.75%) coefficients showed high precision of this metric. Similar precision was observed in phantom measurements. Inspection of the ICC model indicated that most of the variance in cT1 could be accounted for by study participants (ICC = 0.91), with minimal contribution from technical differences. We demonstrate that multiparametric MRI is a non-invasive, repeatable and reproducible method for quantifying liver tissue characteristics across manufacturers (Philips and Siemens) and field strengths (1.5T and 3T)
- …