arXiv subjects

C. Darren Dowell

Publications and source records attributed to C. Darren Dowell.

49 records · Page 3Linked to original sources

arXiv2010

Absolute polarization angle calibration using polarized diffuse Galactic emission observed by BICEP

We present a method of cross-calibrating the polarization angle of a polarimeter using BICEP Galactic observations. \bicep\ was a ground based experiment using an array of 49 pairs of polarization sensitive bolometers observing from the geographic South Pole at 100 and 150 GHz. The BICEP polarimeter is calibrated to +/-0.01 in cross-polarization and less than +/-0.7 degrees in absolute polarization orientation. BICEP observed the temperature and polarization of the Galactic plane (R.A= 100 degrees ~ 270 degrees and Dec. = -67 degrees ~ -48 degrees). We show that the statistical error in the 100 GHz BICEP Galaxy map can constrain the polarization angle offset of WMAP Wband to 0.6 degrees +\- 1.4 degrees. The expected 1 sigma errors on the polarization angle cross-calibration for Planck or EPIC are 1.3 degrees and 0.3 degrees at 100 and 150 GHz, respectively. We also discuss the expected improvement of the BICEP Galactic field observations with forthcoming BICEP2 and Keck observations.

astro-ph.IM↗

arXiv2010

350 micron Polarimetry from the Caltech Submillimeter Observatory

We present a summary of data obtained with the 350 micron polarimeter, Hertz, at the Caltech Submillimeter Observatory. We give tabulated results and maps showing polarization vectors and flux contours. The summary includes over 4300 individual measurements in 56 Galactic sources and 2 galaxies. Of these measurements, 2153 have P >= 3σ_p statistical significance. The median polarization of the entire data set is 1.46%.

astro-ph.GA↗

arXiv2009

Anchoring Magnetic Field in Turbulent Molecular Clouds

One of the key problems in star formation research is to determine the role of magnetic fields. Starting from the atomic inter-cloud medium (ICM) which has density nH ~ 1 per cubic cm, gas must accumulate from a volume several hundred pc across in order to form a typical molecular cloud. Star formation usually occurs in cloud cores, which have linear sizes below 1 pc and densities nH2 > 10^5 per cubic cm. With current technologies, it is hard to probe magnetic fields at scales lying between the accumulation length and the size of cloud cores, a range corresponds to many levels of turbulent eddy cascade, and many orders of magnitude of density amplification. For field directions detected from the two extremes, however, we show here that a significant correlation is found. Comparing this result with molecular cloud simulations, only the sub-Alfvenic cases result in field orientations consistent with our observations.

astro-ph.GA↗

arXiv2009

Magnetic Fields and Infall Motions in NGC 1333 IRAS 4

We present single-dish 350 micron dust continuum polarimetry as well as HCN and HCO+ J=4-3 rotational emission spectra obtained on NGC 1333 IRAS 4. The polarimetry indicates a uniform field morphology over a 20" radius from the peak continuum flux of IRAS 4A, in agreement with models of magnetically supported cloud collapse. The field morphology around IRAS 4B appears to be quite distinct however, with indications of depolarization observed towards the peak flux of this source. Inverse P-Cygni profiles are observed in the HCN J=4-3 line spectra towards IRAS 4A, providing a clear indication of infall gas motions. Taken together, the evidence gathered here appears to support the scenario that IRAS 4A is a cloud core in a critical state of support against gravitational collapse.

astro-ph.SR↗

arXiv2009

A complete view of galaxy evolution: panchromatic luminosity functions and the generation of metals

When and how did galaxies form and their metals accumulate? Over the last decade, this has moved from an archeological question to a live investigation: there is now a broad picture of the evolution of galaxies in dark matter halos: their masses, stars, metals and supermassive blackholes. Galaxies have been found and studied in which these formation processes are taking place most vigorously, all the way back in cosmic time to when the intergalactic medium (IGM) was still largely neutral. However, the details of how and why the interstellar medium (ISM) in distant galaxies cools, is processed, recycled and enriched in metals by stars, and fuels active galactic nuclei (AGNs) remain uncertain. In particular, the cooling of gas to fuel star formation, and the chemistry and physics of the most intensely active regions is hidden from view at optical wavelengths, but can be seen and diagnosed at mid- & far-infrared (IR) wavelengths. Rest-frame IR observations are important first to identify the most luminous, interesting and important galaxies, secondly to quantify accurately their total luminosity, and finally to use spectroscopy to trace the conditions in the molecular and atomic gas out of which stars form. In order to map out these processes over the full range of environments and large-scale structures found in the universe - from the densest clusters of galaxies to the emptiest voids - we require tools for deep, large area surveys, of millions of galaxies out to z~5, and for detailed follow-up spectroscopy. The necessary tools can be realized technically. Here, we outline the requirements for gathering the crucial information to build, validate and challenge models of galaxy evolution.

astro-ph.CO↗

arXiv2008

CMB polarimetry with BICEP: instrument characterization, calibration, and performance

BICEP is a ground-based millimeter-wave bolometric array designed to target the primordial gravity wave signature on the polarization of the cosmic microwave background (CMB) at degree angular scales. Currently in its third year of operation at the South Pole, BICEP is measuring the CMB polarization with unprecedented sensitivity at 100 and 150 GHz in the cleanest available 2% of the sky, as well as deriving independent constraints on the diffuse polarized foregrounds with select observations on and off the Galactic plane. Instrument calibrations are discussed in the context of rigorous control of systematic errors, and the performance during the first two years of the experiment is reviewed.

astro-ph↗

arXiv2008

Spatially Resolved Imaging at 350um of Cold Dust in Nearby Elliptical Galaxies

Continuum observations at 350um are presented of seven nearby elliptical galaxies, for which CO-gas disks have recently been resolved with interferometry mapping. These SHARCII mapping results provide the first clearly resolved far-infrared(FIR) to submillimeter(submm) continuum emission from cold dust (with temperatures 32K > T > 22K) of any elliptical galaxy at a distance >40Mpc. The measured FIR excess shows that the most likely and dominant heating source of this dust is not dilute stellar radiation or cooling flows, but rather star-formation, that could have been triggered by an accretion or merger event and fueled by dust-rich material that has settled in a dense region co-spatial with the central CO-gas disks. The dust is detected even in two cluster ellipticals that are deficient in HI, showing that, unlike the HI, cold dust and CO in ellipticals can survive in the presence of hot X-ray gas, even in galaxy clusters. No dust cooler than 20K, either distributed outside the CO disks, or co-spatial with and heated by the entire dilute stellar optical galaxy (or very extended HI), is currently evident.

astro-ph↗

arXiv2007

SHARC-II 350 micron Observations of Thermal Emission from Warm Dust in z>=5 Quasars

We present observations of four z>= SDSS quasars at 350 micron with the SHARC-II bolometer camera on the Caltech Submillimeter Observatory. These are among the deepest observations that have been made by SHARC-II at 350 micron, and three quasars are detected at >=3 sigma significance, greatly increasing the sample of 350 micron (corresponds to rest frame wavelengths of <60 micron at z>=5), detected high-redshift quasars. The derived rest frame far-infrared (FIR) emission in the three detected sources is about five to ten times stronger than that expected from the average SED of the local quasars given the same 1450A luminosity. Combining the previous submillimeter and millimeter observations at longer wavelengths, the temperatures of the FIR-emitting warm dust from the three quasar detections are estimated to be in the range of 39 to 52 K. Additionally, the FIR-to-radio SEDs of the three 350 micron detections are consistent with the emission from typical star forming galaxies. The FIR luminosities are ~10^{13} L_solar and the dust masses are >= 10^{8}M_solar. These results confirm that huge amounts of warm dust can exist in the host galaxies of optically bright quasars as early as z~6. The universe is so young at these epochs (~1 Gyr) that a rapid dust formation mechanism is required. We estimate the size of the FIR dust emission region to be about a few kpc, and further provide a comparison of the SEDs among different kinds of dust emitting sources to investigate the dominant dust heating mechanism.

astro-ph↗

arXiv2007

First Constraints on Source Counts at 350 Microns

We have imaged a $\sim$6 arcminute$^2$ region in the Boötes Deep Field using the 350 $μ$m-optimised second generation Submillimeter High Angular Resolution Camera (SHARC II), achieving a peak 1$σ$ sensitivity of $\sim$5 mJy. We detect three sources above 3$σ$, and determine a spurious source detection rate of 1.09 in our maps. In the absence of $5σ$ detections, we rely on deep 24 $μ$m and 20 cm imaging to deduce which sources are most likely to be genuine, giving two real sources. From this we derive an integral source count of 0.84$^{+1.39}_{-0.61}$ sources arcmin$^{-2}$ at $S>13$ mJy, which is consistent with 350 $μ$m source count models that have an IR-luminous galaxy population evolving with redshift. We use these constraints to consider the future for ground-based short-submillimetre surveys.

astro-ph↗

arXiv2005

The Dusty Disk of the Early Galaxy NGC 3656

SHARC II, 350-micron continuum and archival HST J-H band maps are presented of NGC 3656, the brightest of our sample of six elliptical galaxies for which resolved CO gas disks have recently been detected with 7''-spatial-resolution, interferometry mapping. These gas disks confirm the conclusions of earlier results showing optical dust lanes and unresolved CO that implied the common existence of molecular gas in ellipticals and the disk-like structure of this gas. The presented SHARC II mapping results provide the best to date resolved FIR-submm extent of NGC 3656 and of any elliptical galaxy > 40 Mpc, showing that dust of 29 K exists out to at least 1.8 kpc in this galaxy. These new data are used in conjunction with the archival HST maps and other published data to determine dust properties and associations with galactic structures, including dominant heating sources such as nuclear-activity, star-formation or diffuse-stellar radiation.

astro-ph↗

arXiv2003

Tracing the Magnetic Field in Orion A

We use extensive 350 um polarimetry and continuum maps obtained with Hertz and SHARC II along with HCN and HCO+ spectroscopic data to trace the orientation of the magnetic field in the Orion A star-forming region. Using the polarimetry data, we find that the direction of the projection of the magnetic field in the plane of the sky relative to the orientation of the integral-shaped filament varies considerably as one moves from north to south. While in IRAS 05327-0457 and OMC-3 MMS 1-6 the projection of the field is primarily perpendicular to the filament it becomes better aligned with it at OMC-3 MMS 8-9 and well aligned with it at OMC-2 FIR 6. The OMC-2 FIR 4 cloud, located between the last two, is a peculiar object where we find almost no polarization. The projected angle of the field is more complicated in OMC-1 where it exhibits smooth variations in its orientation across the face of this massive complex. By combining the polarimetry and spectroscopic data we were able to measure a set of average values for the inclination angle of the magnetic field relative to the line of sight. We find that the field is oriented quite close to the plane of the sky in most places. More precisely, the inclination of the magnetic field is ~73 deg. around OMC-3 MMS 6, ~74 deg. at OMC-3 MMS 8-9, ~80 deg. at OMC-2 FIR 4, ~65 deg. in the northeastern part of OMC-1, and ~49 deg. in the Bar. We also present polarimetry data for the OMC-4 region located some 13 arcminutes south of OMC-1.

astro-ph↗

arXiv2003

On the Measurement of the Magnitude and Orientation of the Magnetic Field in Molecular Clouds

We demonstrate that the combination of Zeeman, polarimetry and ion-to-neutral molecular line width ratio measurements permits the determination of the magnitude and orientation of the magnetic field in the weakly ionized parts of molecular clouds. Zeeman measurements provide the strength of the magnetic field along the line of sight, polarimetry measurements give the field orientation in the plane of the sky and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. We apply the technique to the M17 star-forming region using a HERTZ 350 um polarimetry map and HCO+-to-HCN molecular line width ratios to provide the first three-dimensional view of the magnetic field in M17.

astro-ph↗

arXiv2003

Mapping Magnetic Fields in the Cold Dust at the Galactic Center

We report the detection of polarized emission in the vicinity of the Galactic center for 158 positions within eight different pointings of the Hertz polarimeter operating on the Caltech Submillimeter Observatory. These pointings include positions 2 arcminutes offset to the E, NE, and NW of M-0.02-0.07; positions to the SE and NW of the 20 km/s cloud (M-0.13-0.08), CO+0.02-0.02, M+0.07-0.08, and M+0.11-0.08. We use these data in conjunction with previous far-infrared and submillimeter polarization results to find that the direction of the inferred magnetic field is related to the density of the molecular material in the following way: in denser regions, the field is generally parallel to the Galactic plane, whereas in regions with lower densities, the field is generally perpendicular to the plane. This finding is consistent with a model in which an initially poloidal field has been sheared into a toroidal configuration in regions that are dense enough such that the gravitational energy density is greater than the energy density of the magnetic field. Using this model, we estimate the characteristic strength of the magnetic field in the central 30 pc of our Galaxy to be a few mG.

astro-ph↗