ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer

Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largest mm/submm interferometer, and currently the Early Science is ongoing, together with the commissioning and science verification (CSV). Here we present a study of the temporal phase stability of the entire ALMA system from antennas to the correlator. We verified the temporal phase stability of ALMA using data, taken during the last two years of CSV activities. The data consist of integrations on strong point sources (i.e., bright quasars) at various frequency bands, and at various baseline lengths (up to 600 m). From the observations of strong quasars for a long time (from a few tens of minutes, up to an hour), we derived the 2-point Allan Standard Deviation after the atmospheric phase correction using the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna, and confirmed that the phase stability of all the baselines reached the ALMA specification. Since we applied the WVR phase correction to all the data mentioned above, we also studied the effectiveness of the WVR phase correction at various frequencies, baseline lengths, and weather conditions. The phase stability often improves a factor of 2 - 3 after the correction, and sometimes a factor of 7 improvement can be obtained. However, the corrected data still displays an increasing phase fluctuation as a function of baseline length, suggesting that the dry component (e.g., N2 and O2) in the atmosphere also contributes the phase fluctuation in the data, although the imperfection of the WVR phase correction cannot be ruled out at this moment.Comment: Proc. SPIE 8444-125, in press (7 pages, 4 figures, 1 table

Star-forming Cloud Complexes in the Central Molecular Zone of NGC 253

We report 350 and 230 GHz observations of molecular gas and dust in the starburst nucleus of NGC 253 at 20-40 pc (1"-2") resolution. The data contain CO(3-2), HCN(4-3), CO(2-1), 13CO(2-1), C18O(2-1), and continuum at 0.87 mm and 1.3 mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central molecular zone (CMZ) is measured to be about 300 pc x 100 pc at the half maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at arcsecond resolution, and they are associated with compact radio sources due to recent massive star formation. They contribute one third of the CO emission in the central 300 pc and have 12CO peak brightness temperatures around 50 K, molecular gas column densities on the order of 10^{4} Msun pc^{-2}, gas masses on the order of 10^{7} Msun in the size scale of 20 pc, volume-averaged gas densities of n(H2) ~ 4000 cm^{-3}, and high HCN-to-CO ratios suggestive of higher fractions of dense gas than in the surrounding environment. It is suggested that these are natal molecular cloud complexes of massive star formation. The CMZ of NGC 253 is also compared with that of our Galaxy in CO(2-1) at the same 20 pc resolution. Their overall gas distributions are strikingly similar. The five molecular cloud complexes appear to be akin to such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l=1.3deg cloud in the Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher temperatures and higher surface (and presumably volume) densities than its non-starburst cousin.Comment: ApJ in press, 18 page

XMM-Newton Observations of NGC 507: Super-solar Metal Abundances in the Hot ISM

We present the results of the X-ray XMM-Newton observations of NGC 507, a dominant elliptical galaxy in a small group of galaxies, and report 'super-solar' metal abundances of both Fe and a-elements in the hot ISM of this galaxy. We find Z_Fe = 2-3 times solar inside the D25 ellipse of NGC 507. This is the highest Z_Fe reported so far for the hot halo of an elliptical galaxy; this high Iron abundance is fully consistent with the predictions of stellar evolution models, which include the yield of both type II and Ia supernovae. The spatially resolved, high quality XMM spectra provide enough statistics to formally require at least three emission components: two soft thermal components indicating a range of temperatures in the hot ISM, plus a harder component, consistent with the integrated output of low mass X-ray binaries (LMXBs). The abundance of a-elements (most accurately determined by Si) is also found to be super-solar. The a-elements to Fe abundance ratio is close to the solar ratio, suggesting that ~70% of the Iron mass in the hot ISM was originated from SNe Type Ia. The a-element to Fe abundance ratio remains constant out to at least 100 kpc, indicating that SNe Type II and Ia ejecta are well mixed in a scale much larger than the extent of the stellar body.Comment: 29 pages, 6 figures, Accepted in ApJ (v613, Oct. 1, 2004); Minor revisions after referee's comments; A high-resolution pdf file available at http://hea-www.harvard.edu/~kim/pap/N507_XMM.pd

Distance traveled by random walkers before absorption in a random medium

We consider the penetration length $l$ of random walkers diffusing in a medium of perfect or imperfect absorbers of number density $\rho$. We solve this problem on a lattice and in the continuum in all dimensions $D$, by means of a mean-field renormalization group. For a homogeneous system in $D>2$, we find that $l\sim \max(\xi,\rho^{-1/2})$, where $\xi$ is the absorber density correlation length. The cases of D=1 and D=2 are also treated. In the presence of long-range correlations, we estimate the temporal decay of the density of random walkers not yet absorbed. These results are illustrated by exactly solvable toy models, and extensive numerical simulations on directed percolation, where the absorbers are the active sites. Finally, we discuss the implications of our results for diffusion limited aggregation (DLA), and we propose a more effective method to measure $l$ in DLA clusters.Comment: Final version: also considers the case of imperfect absorber

O, Ne, Mg, and Fe abundances in hot X-ray emitting halos of galaxy clusters, groups, and giant early-type galaxies with XMM-Newton RGS spectroscopy

Chemical elements in the hot medium permeating early-type galaxies, groups, and clusters make them an excellent laboratory for studying metal enrichment and cycling processes in the largest scales of the Universe. Here, we report the XMM-Newton RGS analysis of 14 early-type galaxies, including the well-known brightest cluster galaxies of Perseus, for instance. The spatial distribution of the O/Fe, Ne/Fe, and Mg/Fe ratios is generally flat at the central 60 arcsecond regions of each object, irrespective of whether or not a central Fe abundance drop has been reported. Common profiles between noble gas and normal metal suggest that the dust depletion process does not work predominantly in these systems. Therefore, observed abundance drops are possibly attributed to other origins, like systematics in the atomic codes. Giant systems of high gas mass-to-luminosity ratio tend to hold a hot gas ($\sim$ 2 keV) yielding the solar N/Fe, O/Fe, Ne/Fe, Mg/Fe, and Ni/Fe ratios. Contrarily, light systems at a subkiloelectronvolt temperature regime, including isolated or group-centered galaxies, generally exhibit super-solar N/Fe, Ni/Fe, Ne/O, and Mg/O ratios. We find that the latest supernova nucleosynthesis models fail to reproduce such a super-solar abundance pattern. Possible systematic uncertainties contributing to these high abundance ratios of cool objects are also discussed in tandem with the crucial role of future X-ray missions.Comment: submitted to Ap

High Density Molecular Gas in the IR-bright Galaxy System VV114

New high resolution CO(3-2) interferometric map of the IR-bright interacting galaxy system VV114 observed with the Submillimeter Array (SMA) reveal a substantial amount of warm and dense gas in the IR-bright but optically obscured galaxy, VV114E, and the overlap region connecting the two nuclei. A 1.8 x 1.4 kpc concentration of CO(3-2) emitting gas with a total mass of 4 x 10^9 Msun coincides with the peaks of NIR, MIR, and radio continuum emission found previously by others, identifying the dense fuel for the AGN and/or the starburst activity there. Extensive CO(2-1) emission is also detected, revealing detailed distribution and kinematics that are consistent with the earlier CO(1-0) results. The widely distributed molecular gas traced in CO(2-1) and the distributed discrete peaks of CO(3-2) emission suggest that a spatially extended intense starbursts may contribute significantly to its large IR luminosity. These new observations further support the notion that VV114 is approaching its final stage of merger, when violent central inflow of gas triggers intense starburst activity possibly boosting the IR luminosity above the ultraluminous threshold.Comment: 11 pages, 2 figures, accepted for publication in ApJ