2,414 research outputs found
The Luminosity Function of Nearby Galaxy Clusters II: Redshifts and Luminosity Function for Galaxies in the Region of the Centaurus Cluster
We acquired spectra for a random sample of galaxies within a 0.83 square
degree region centered on the core of the Centaurus cluster. Radial velocities
were obtained for 225 galaxies to limiting magnitudes of V < 19.5. Of the
galaxies for which velocities were obtained, we find 35% to be member galaxies.
Of the 78 member galaxies, magnitudes range from 11.8 < V < 18.5 (-21.6 < M_{V}
< -14.9 for H_o = 70 km s^-1 Mpc^-1) with a limiting central surface brightness
of \mu_o < 22.5 mag arcsec^-2. We constructed the cluster galaxy luminosity
function by using these spectroscopic results to calculate the expected
fraction of cluster members in each magnitude bin. The faint-end slope of the
luminosity function using this method is shallower than the one obtained using
a statistical method to correct for background galaxy contamination. We also
use the spectroscopy results to define surface brightness criteria to establish
membership for the full sample. Using these criteria, we find a luminosity
function very similar to the one constructed with the statistical background
correction. For both, we find a faint-end slope alpha ~ -1.4. Adjusting the
surface brightness membership criteria we find that the data are consistent
with a faint-end slope as shallow as -1.22 or as steep as -1.50. We describe in
this paper some of the limitations of using these methods for constructing the
galaxy luminosity function.Comment: 16 pages, 12 figures, accepted by A
Galaxy And Mass Assembly (GAMA) : The mechanisms for quiescent galaxy formation at z<1
© 2016 The Authors. One key problem in astrophysics is understanding how and why galaxies switch off their star formation, building the quiescent population that we observe in the local Universe. From the Galaxy And Mass Assembly and VIsible MultiObject Spectrograph Public Extragalactic Redshift surveys, we use spectroscopic indices to select quiescent and candidate transition galaxies.We identify potentially rapidly transitioning post-starburst (PSB) galaxies and slower transitioning green-valley galaxies. Over the last 8Gyr, the quiescent population has grown more slowly in number density at high masses (M * > 10 11 M ⊙ ) than at intermediate masses (M * > 10 10.6 M ⊙ ). There is evolution in both the PSB and green-valley stellar mass functions, consistent with higher mass galaxies quenching at earlier cosmic times.At intermediatemasses (M * > 10 10.6 M ⊙ ), we find a green-valley transition time-scale of 2.6 Gyr. Alternatively, at z ~ 0.7, the entire growth rate could be explained by fast-quenching PSB galaxies, with a visibility time-scale of 0.5 Gyr. At lower redshift, the number density of PSBs is so low that an unphysically short visibility window would be required for them to contribute significantly to the quiescent population growth. The importance of the fast-quenching route may rapidly diminish at z 10 11 M ⊙ ), there is tension between the large number of candidate transition galaxies compared to the slow growth of the quiescent population. This could be resolved if not all high-mass PSB and green-valley galaxies are transitioning from star forming to quiescent, for example if they rejuvenate out of the quiescent population following the accretion of gas and triggering of star formation, or if they fail to completely quench their star formation
High redshift AGNs from the 1Jy catalogue and the magnification bias
We have found a statistically significant (99.1 \%) excess of red ()
galaxies with photographic magnitudes , taken from the APM Sky
Catalogue around radiosources from the 1Jy catalogue. The amplitude,
scale and dependence on galaxy colours of the observed overdensity are
consistent with its being a result of the magnification bias caused by the weak
gravitational lensing of large scale structures at redshift
and are hardly explained by other causes, as obscuration by dust.Comment: uuencoded file containing 3 ps files: the main text, a table and a
figure. To appear in ApJ Letter
A log-quadratic relation for predicting supermassive black hole masses from the host bulge Sersic index
We reinvestigate the correlation between black hole mass and bulge
concentration. With an increased galaxy sample, updated estimates of galaxy
distances, black hole masses, and Sersic indices `n' - a measure of
concentration - we perform a least-squares regression analysis to obtain a
relation suitable for the purpose of predicting black hole masses in other
galaxies. In addition to the linear relation, log(M_bh) = 7.81(+/-0.08) +
2.69(+/-0.28)[log(n/3)] with epsilon_(intrin)=0.31 dex, we investigated the
possibility of a higher order M_bh-n relation, finding the second order term in
the best-fitting quadratic relation to be inconsistent with a value of zero at
greater than the 99.99% confidence level. The optimal relation is given by
log(M_bh) = 7.98(+/-0.09) + 3.70(+/-0.46)[log(n/3)] -
3.10(+/-0.84)[log(n/3)]^2, with epsilon_(intrin)=0.18 dex and a total absolute
scatter of 0.31 dex. Extrapolating the quadratic relation, it predicts black
holes with masses of ~10^3 M_sun in n=0.5 dwarf elliptical galaxies, compared
to ~10^5 M_sun from the linear relation, and an upper bound on the largest
black hole masses in the local universe, equal to 1.2^{+2.6}_{-0.4}x10^9
M_sun}. In addition, we show that the nuclear star clusters at the centers of
low-luminosity elliptical galaxies follow an extrapolation of the same
quadratic relation. Moreover, we speculate that the merger of two such
nucleated galaxies, accompanied by the merger and runaway collision of their
central star clusters, may result in the late-time formation of some
supermassive black holes. Finally, we predict the existence of, and provide
equations for, a relation between M_bh and the central surface brightness of
the host bulge
Can the Future Influence the Present?
One widely accepted model of classical electrodynamics assumes that a moving charged particle produces both retarded and advanced fields. This formulation first appeared at least 75 years ago. It was popularized in the 1940\u27s by work of Wheeler and Feynman. But the most fundamental question associated with the model has remained unanswered: When (if ever) does the two-body problem have a unique solution? The present paper gives an answer in one special case. Imagine two identical charged particles alone in the universe moving symmetrically along the x axis. One is at x(t) and the other is at −x(t). Their motion is then governed by a system of functional differential equations involving both retarded and advanced arguments. This system together with the Newtonian initial data x(0)=x0\u3e0 and x′(0)=0 has a unique solution for all time provided x0 is sufficiently large. Perhaps the existence and uniqueness proof given for this special case will pave the way for more general results on this curious two-body problem
Phase transformation in Si from semiconducting diamond to metallic beta-Sn phase in QMC and DFT under hydrostatic and anisotropic stress
Silicon undergoes a phase transition from the semiconducting diamond phase to
the metallic beta-Sn phase under pressure. We use quantum Monte Carlo
calculations to predict the transformation pressure and compare the results to
density functional calculations employing the LDA, PBE, PW91, WC, AM05, PBEsol
and HSE06 exchange-correlation functionals. Diffusion Monte Carlo predicts a
transition pressure of 14.0 +- 1.0 GPa slightly above the experimentally
observed transition pressure range of 11.3 to 12.6 GPa. The HSE06 hybrid
functional predicts a transition pressure of 12.4 GPa in excellent agreement
with experiments. Exchange-correlation functionals using the local-density
approximation and generalized-gradient approximations result in transition
pressures ranging from 3.5 to 10.0 GPa, well below the experimental values. The
transition pressure is sensitive to stress anisotropy. Anisotropy in the stress
along any of the cubic axes of the diamond phase of silicon lowers the
equilibrium transition pressure and may explain the discrepancy between the
various experimental values as well as the small overestimate of the quantum
Monte Carlo transition pressure
Strong asymptotics for Jacobi polynomials with varying nonstandard parameters
Strong asymptotics on the whole complex plane of a sequence of monic Jacobi
polynomials is studied, assuming that with and satisfying , , . The
asymptotic analysis is based on the non-Hermitian orthogonality of these
polynomials, and uses the Deift/Zhou steepest descent analysis for matrix
Riemann-Hilbert problems. As a corollary, asymptotic zero behavior is derived.
We show that in a generic case the zeros distribute on the set of critical
trajectories of a certain quadratic differential according to the
equilibrium measure on in an external field. However, when either
, or are geometrically close to ,
part of the zeros accumulate along a different trajectory of the same quadratic
differential.Comment: 31 pages, 12 figures. Some references added. To appear in Journal
D'Analyse Mathematiqu
On the effect of hydrogen on the elastic moduli and acoustic loss behaviour of Ti-6Al-4V
The elastic moduli and acoustic loss behaviour of Ti-6Al-4V (wt.%) in the temperature range 5–298 K have been studied using Resonant Ultrasound Spectroscopy. A peak in the acoustic dissipation was observed at 160 K within the frequency range 250–1000 kHz. Analysis of the data acquired in this study, coupled with complementary data from the literature, showed that this was consistent with a Snoek-like relaxation process with an associated activation energy of 23
3 kJ mol. However, the loss peak was broader than would be expected for a Snoek-like relaxation, and the underlying process was shown to have a spread of relaxation times. It is suggested that this effect arises as a result of variations in the strain experienced by the phase due to different local microstructural constraint by the bounding secondary phase
Galaxy And Mass Assembly (GAMA): a deeper view of the mass, metallicity and SFR relationships
A full appreciation of the role played by gas metallicity (Z), star formation rate (SFR) and stellar mass (M*) is fundamental to understanding how galaxies form and evolve. The connections between these three parameters at different redshifts significantly affect galaxy evolution, and thus provide important constraints for galaxy evolution models. Using data from the Sloan Digital Sky Survey–Data Release 7 (SDSS–DR7) and the Galaxy and Mass Assembly (GAMA) surveys, we study the relationships and dependences between SFR, Z and M*, as well as the Fundamental Plane for star-forming galaxies. We combine both surveys using volume-limited samples up to a redshift of z ≈ 0.36. The GAMA and SDSS surveys complement each other when analysing the relationships between SFR, M* and Z. We present evidence for SFR and metallicity evolution to z ∼ 0.2. We study the dependences between SFR, M*, Z and specific SFR (SSFR) on the M*–Z, M*–SFR, M*–SSFR, Z–SFR and Z–SSFR relations, finding strong correlations between all. Based on those dependences, we propose a simple model that allows us to explain the different behaviour observed between low- and high-mass galaxies. Finally, our analysis allows us to confirm the existence of a Fundamental Plane, for which M* = f(Z, SFR) in star-forming galaxies
The Luminosity Distribution in Galaxy Clusters: A Dwarf Population - Density Relation?
Recent work suggests that rich clusters of galaxies commonly have large
populations of dwarf (i.e. low luminosity) members, that is their luminosity
function (LF) turns up to a steep slope at the faint end. This population, or
more particularly the relative numbers of dwarfs to giants, appears to be very
similar for clusters of similar morphology, but may vary between cluster types.
We have previously suggested that dwarfs may be more common in less compact,
spiral rich clusters. Similarly we have found evidence for population gradients
across clusters, in that the dwarf population appears more spatially extended.
In the present paper we summarise the current evidence and propose, in analogy
to the well-known morphology - density relation, that what we are seeing is a
dwarf population - density relation; dwarfs are more common in lower density
environments. Finally we discuss recent semi-analytic models of galaxy
formation in the hierarchical clustering picture, which may give clues as to
the origin of our proposed relation.Comment: 9 pages, LateX (uses AASTeX aas2pp4 style file, included), with two
embedded postscript figures, also available at
http://WWW.star.bris.ac.uk/publs_preprints/preprints.html, accepted for
publication in the Astrophysical Journal Letter
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