73 research outputs found
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
The mass-metallicity gradient relation of early-type galaxies
We present a newly observed relation between galaxy mass and radial
metallicity gradients of early-type galaxies. Our sample of 51 early-type
galaxies encompasses a comprehensive mass range from dwarf to brightest cluster
galaxies. The metallicity gradients are measured out to one effective radius by
comparing nearly all of the Lick absorption-line indices to recent models of
single stellar populations. The relation shows very different behaviour at low
and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10
M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a
tight relation with mass, such that metallicity gradients become shallower with
decreasing mass and positive at the very low-mass end. Above the mass
transition point several massive galaxies have steeper gradients, but a clear
downturn is visible marked by a broad scatter. The results are interpreted in
comparison with competing model predictions. We find that an early star-forming
collapse could have acted as the main mechanism for the formation of low-mass
galaxies, with star formation efficiency increasing with galactic mass. The
high-mass downturn could be a consequence of merging and the observed larger
scatter a natural result of different merger properties. These results suggest
that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed
initially by mergers of gas-rich disc galaxies and then subsequently evolved
via dry merger events. The varying efficiency of the dissipative merger-induced
starburst and feedback processes have shaped the radial metallicity gradients
in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette
An Imaging and Spectroscopic Study of the z=3.38639 Damped Lyman Alpha System in Q0201+1120: Clues to Star Formation Rate at High Redshift
We present the results of a series of imaging and spectroscopic observations
aimed at identifying and studying the galaxy responsible for the z = 3.38639
damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part
of a concentration of matter which includes at least four galaxies (probably
many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber
may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up
spectroscopy is still required for positive identification. The gas is
turbulent, with many absorption components distributed over approximately 270
km/s and a large spin temperature, T_s greater than 4000K. The metallicity is
relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From
consideration of the relative ratios of elements which have different
nucleosynthetic timescales, it would appear that the last major episode of star
formation in this DLA occurred at z greater than 4.3, more than approximately
500 Myr prior to the time when we observe it.Comment: Accepted for publication in Ap
Metallicity Gradients at Large Galactocentric Radii Using the Near-infrared Calcium Triplet
We describe a new spectroscopic technique for measuring radial metallicity
gradients out to large galactocentric radii. We use the DEIMOS multi-object
spectrograph on the Keck telescope and the galaxy spectrum extraction technique
of Proctor et al. (2009). We also make use of the metallicity sensitive
near-infrared (NIR) Calcium triplet (CaT) features together with single stellar
population models to obtain metallicities. Our technique is applied as a pilot
study to a sample of three relatively nearby (<30 Mpc) intermediate-mass to
massive early-type galaxies. Results are compared with previous literature
inner region values and generally show good agreement. We also include a
comparison with profiles from dissipational disk-disk major merger simulations.
Based on our new extended metallicity gradients combined with other
observational evidence and theoretical predictions, we discuss possible
formation scenarios for the galaxies in our sample. The limitations of our new
technique are also discussed.Comment: 13 Pages, 9 Figures, 7 Tables, Accepted for publication in MNRA
X-shooter Observations of the Gravitational Lens System CASSOWARY 5
We confirm an eighth gravitational lens system in the CASSOWARY catalogue.
Exploratory observations with the X-shooter spectrograph on the VLT show the
system CSWA5 to consist of at least three images of a blue star-forming galaxy
at z = 1.0686, lensed by an apparent foreground group of red galaxies one of
which is at z = 0.3877. The lensed galaxy exhibits a rich spectrum with broad
interstellar absorption lines and a wealth of nebular emission lines.
Preliminary analysis of these features shows the galaxy to be young, with an
age of 25-50 Myr. With a star-formation rate of approximately 20 solar
masses/yr, the galaxy has already assembled a stellar mass of 3 x 10^9 solar
masses and reached half-solar metallicity. Its blue spectral energy
distribution and Balmer line ratios suggest negligible internal dust
extinction. A more in-depth analysis of the properties of this system is
currently hampered by the lack of a viable lensing model. However, it is
already clear that CSWA5 shares many of its physical characteristics with the
general population of UV-selected galaxies at redshifts z = 1-3, motivating
further study of both the source and the foreground mass concentration
responsible for the gravitational lensing.Comment: 12 pages; Accepted for publication in MNRA
Stellar Masses and Star Formation Histories for 80,000 Galaxies from the Sloan Digital Sky Survey
SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
NGC6240: Merger-Induced Star Formation & Gas Dynamics
We present spatially resolved integral field spectroscopic K-band data at a
resolution of 0.13" (60pc) and interferometric CO(2-1) line observations of the
prototypical merging system NGC6240. Despite the clear rotational signature,
the stellar kinematics in the two nuclei are dominated by dispersion. We use
Jeans modelling to derive the masses and the mass-to-light ratios of the
nuclei. Combining the luminosities with the spatially resolved Br-gamma
equivalent width shows that only 1/3 of the K-band continuum from the nuclei is
associated with the most recent star forming episode; and that less than 30% of
the system's bolometric luminosity and only 9% of its stellar mass is due to
this starburst. The star formation properties, calculated from typical merger
star formation histories, demonstrate the impact of different assumptions about
the star formation history. The properties of the nuclei, and the existence of
a prominent old stellar population, indicate that the nuclei are remnants of
the progenitor galaxies' bulges.Comment: 18 pages, 14 figures. Accepted for publication in A&
The Intermediate-Scale Clustering of Luminous Red Galaxies
We report the intermediate-scale (0.3 to 40 Mpc/h) clustering of 35,000
luminous early-type galaxies at redshifts 0.16 to 0.44 from the Sloan Digital
Sky Survey. We present the redshift-space two-point correlation function
\xi(s), the projected correlation function w_p(r_p), and the deprojected
real-space correlation function \xi(r), for approximately volume-limited
samples. As expected, the galaxies are highly clustered, with the correlation
length varying from 9.8 +/- 0.2 Mpc/h to 11.2 +/- 0.2 Mpc/h, dependent on the
specific luminosity range. For the -23.2 < Mg < -21.2 sample, the inferred bias
relative to that of L* galaxies is 1.84 +/- 0.11 for 1 Mpc/h < r_p < 10 Mpc/h,
with yet stronger clustering on smaller scales. We detect luminosity-dependent
bias within the sample but see no evidence for redshift evolution between z=0.2
and z=0.4. We find a clear indication for deviations from a power-law in the
real-space correlation function, with a dip at ~ 2 Mpc/h scales and an upturn
on smaller scales. The precision measurements of these clustering trends offer
new avenues for the study of the formation and evolution of these massive
galaxies.Comment: 11 pages, 14 figures. Accepted to the Astrophysical Journa
The early-type galaxies NGC 1407 and NGC 1400 - II: star formation and chemical evolutionary history
We present a possible star formation and chemical evolutionary history for
two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest
galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as
part of the Group Evolution Multi-wavelength Study (GEMS). Our analysis is
based on new high signal-to-noise spatially resolved integrated spectra
obtained at the ESO 3.6m telescope, out to 0.6 (NGC 1407) and 1.3 (NGC 1400)
effective radii. Using Lick/IDS indices we estimate luminosity-weighted ages,
metallicities and -element abundance ratios. Colour radial
distributions from HST/ACS and Subaru Suprime-Cam multi-band wide-field imaging
are compared to colours predicted from spectroscopically determinated ages and
metallicities using single stellar population models. The galaxies formed over
half of their mass in a single short-lived burst of star formation (> 100
M(sun)/year) at redshift z>5. This likely involved an outside-in mechanism with
supernova-driven galactic winds, as suggested by the flatness of the
alpha-element radial profiles and the strong negative metallicity gradients.
Our results support the predictions of the revised version of the monolithic
collapse model for galaxy formation and evolution. We speculate that, since
formation the galaxies have evolved quiescently and that we are witnessing the
first infall of NGC 1400 in the group.Comment: 14 pages, 9 tables, 6 figures, Accepted for publication in MNRA
Three-Dimensional Reconstruction of the Giant Mimivirus Particle with an X-Ray Free-Electron Laser
Citation: Ekeberg, T., Svenda, M., Abergel, C., Maia, F., Seltzer, V., Claverie, J. M., . . . Hajdu, J. (2015). Three-Dimensional Reconstruction of the Giant Mimivirus Particle with an X-Ray Free-Electron Laser. Physical Review Letters, 114(9), 6. doi:10.1103/PhysRevLett.114.098102We present a proof-of-concept three-dimensional reconstruction of the giant mimivirus particle from experimentally measured diffraction patterns from an x-ray free-electron laser. Three-dimensional imaging requires the assembly of many two-dimensional patterns into an internally consistent Fourier volume. Since each particle is randomly oriented when exposed to the x-ray pulse, relative orientations have to be retrieved from the diffraction data alone. We achieve this with a modified version of the expand, maximize and compress algorithm and validate our result using new methods.Additional Authors: Andersson, I.;Loh, N. D.;Martin, A. V.;Chapman, H.;Bostedt, C.;Bozek, J. D.;Ferguson, K. R.;Krzywinski, J.;Epp, S. W.;Rolles, D.;Rudenko, A.;Hartmann, R.;Kimmel, N.;Hajdu, J
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