5,303 research outputs found
Stellar populations across the NGC4244 truncated galactic disk
We use HST/ACS to study the resolved stellar populations of the nearby,
nearly edge-on galaxy NGC4244 across its outer disk surface density break. The
stellar photometry allows us to study the distribution of different stellar
populations and reach very low equivalent surface brightnesses. We find that
the break occurs at the same radius for young, intermediate age, and old stars.
The stellar density beyond the break drops sharply by a factor of at least 600
in 5 kpc. The break occurs at the same radius independent of height above the
disk, but is sharpest in the midplane and nearly disappears at large heights.
These results make it unlikely that truncations are caused by a star formation
threshold alone: the threshold would have to keep the same radial position from
less than 100 Myr to 10 Gyr ago, in spite of potential disturbances such as
infall and redistribution of gas by internal processes. A dynamical
interpretation of truncation formation is more likely such as due to angular
momentum redistribution by bars or density waves, or heating and stripping of
stars caused by the bombardment of dark matter sub-halos. The latter
explanation is also in quantitative agreement with the small diffuse component
we see around the galaxy.Comment: ApJ Letters, in press. Five pages, 2 figure
Discovery of a cluster of galaxies behind the Milky Way: X-ray and optical observations
We report the discovery of Cl 2334+48, a rich cluster of galaxies in the Zone
of Avoidance, identified in public images from the XMM-Newton archive. We
present the main properties of this cluster using the XMM-Newton X-ray data,
along with new optical spectroscopic and photometric observations. Cl 2334+48
is located at z = 0.271 +/- 0.001, as derived from the optical spectrum of the
brightest member galaxy. Such redshift agrees with a determination from the
X-ray spectrum (z = 0.263 (+0.012/-0.010)), in which an intense emission line
is matched to the rest wavelength of the Fe Kalpha complex. Its intracluster
medium has a plasma temperature of 4.92 (+0.50/-0.48) keV, sub-solar abundance
(0.38 +/- 0.12 Zsun), and a bolometric luminosity of 3.2 x 10^44 erg/s. A
density contrast delta = 2500 is obtained in a radius of 0.5 Mpc/h70, and the
corresponding enclosed mass is 1.5 x 10^14 Msun. Optical images show an
enhancement of g'-i' > 2.5 galaxies around the central galaxy, as expected if
these were cluster members. The central object is a luminous E-type galaxy,
which is displaced ~ 40 kpc/h70 from the cluster X-ray center. In addition, it
has a neighbouring arc-like feature (~ 22" or 90 kpc/h70 from it), probably due
to strong gravitational lensing. The discovery of Cl 2334+48 emphasises the
remarkable capability of the XMM-Newton to reveal new clusters of galaxies in
the Zone of Avoidance.Comment: 9 pages, 11 figures, Accepted for publication in A&A (on July 12,
2006
Quantified HI Morphology I: Multi-Wavelengths Analysis of the THINGS Galaxies
Galaxy evolution is driven to a large extent by interactions and mergers with
other galaxies and the gas in galaxies is extremely sensitive to the
interactions. One method to measure such interactions uses the quantified
morphology of galaxy images. Well-established parameters are Concentration,
Asymmetry, Smoothness, Gini, and M20 of a galaxy image. Thus far, the
application of this technique has mostly been restricted to restframe
ultra-violet and optical images. However, with the new radio observatories
being commissioned (MeerKAT, ASKAP, EVLA, WSRT/APERTIF, and ultimately SKA), a
new window on the neutral atomic hydrogen gas (HI) morphology of a large
numbers of galaxies will open up. The quantified morphology of gas disks of
spirals can be an alternative indicator of the level and frequency of
interaction. The HI in galaxies is typically spatially more extended and more
sensitive to low-mass or weak interactions. In this paper, we explore six
morphological parameters calculated over the extent of the stellar (optical)
disk and the extent of the gas disk for a range of wavelengths spanning UV,
Optical, Near- and Far-Infrared and 21 cm (HI) of 28 galaxies from The HI
Nearby Galaxy Survey (THINGS). Though the THINGS sample is small and contains
only a single ongoing interaction, it spans both non-interacting and
post-interacting galaxies with a wealth of multi-wavelength data. We find that
the choice of area for the computation of the morphological parameters is less
of an issue than the wavelength at which they are measured. The signal of
interaction is as good in the HI as in any of the other wavelengths in which
morphology has been used to trace the interaction rate to date, mostly
star-formation dominated ones (near- and far-ultraviolet). The Asymmetry and
M20 parameters are the ones which show the most promise as tracers of
interaction in 21 cm line observations.Comment: 16 pages, 11 figure, table 1, accepted by MNRAS, appendix not
include
Evolution of Galactic Discs: Multiple Patterns, Radial Migration and Disc Outskirts
We investigate the evolution of galactic disks in N-body Tree-SPH
simulations. We find that disks, initially truncated at three scale-lengths,
can triple their radial extent, solely driven by secular evolution. Both Type I
(single exponential) and Type II (down-turning) observed disk
surface-brightness profiles can be explained by our findings. We relate these
results to the strong angular momentum outward transfer, resulting from torques
and radial migration associated with multiple patterns, such as central bars
and spiral waves of different multiplicity. We show that even for stars ending
up on cold orbits, the changes in angular momentum exhibit complex structure as
a function of radius, unlike the expected effect of transient spirals alone.
Focussing on one of our models, we find evidence for non-linear coupling among
m=1, 2, 3 and 4 density waves, where m is the pattern multiplicity. We suggest
that the naturally occurring larger resonance widths at galactic radii beyond
four scale-lengths may have profound consequences on the formation and location
of breaks in disk density profiles, provided spirals are present at such large
distances. We also consider the effect of gas inflow and show that when
in-plane smooth gas accretion of ~5 M_sun/yr is included, the outer disks
become more unstable, leading to a strong increase in the stellar velocity
dispersion. This, in turn, causes the formation of a Type III (up-turning)
profile in the old stellar population. We propose that observations of Type III
surface brightness profiles, combined with an up-turn in the stellar velocity
dispersions beyond the disk break, could be a signature of ongoing
gas-accretion. The results of this study suggest that disk outskirts comprised
of stars migrated from the inner disk would have relatively large radial
velocity dispersions, and significant thickness when seen edge-on. [Abridged]Comment: Replaced with accepted version. New Fig. 5 added, Section 10
decreased in size, old Fig. 17 removed. Conclusions remain the same.
High-resolution version can be found at http://www.ivanminchev.co
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
- âŚ