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