arXiv subjects

C. D. Wilson

Publications and source records attributed to C. D. Wilson.

At least 91 records · Page 5Linked to original sources

arXiv2010

Enhanced dust heating in the bulges of early-type spiral galaxies

Stellar density and bar strength should affect the temperatures of the cool (T ~ 20-30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 microns. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (key insights in nearby galaxies: a far-infrared survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 +/- 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23(+/-3)% hotter for morphological types earlier than Sc, and only 9(+/-3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar.

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arXiv2010

The dust morphology of the elliptical Galaxy M86 with SPIRE

We present Herschel-SPIRE observations at 250-500um of the giant elliptical galaxy M86 and examine the distribution of the resolved cold dust emission and its relation with other galactic tracers. The SPIRE images reveal three dust components: emission from the central region; a dust lane extending north-south; and a bright emission feature 10kpc to the south-east. We estimate that approximately 10^6 solar masses of dust is spatially coincident with atomic and ionized hydrogen, originating from stripped material from the nearby spiral NGC4438 due to recent tidal interactions with M86. The gas-to-dust ratio of the cold gas component ranges from ~20-80. We discuss the different heating mechanisms for the dust features.

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arXiv2010

SPIRE imaging of M82: cool dust in the wind and tidal streams

M82 is a unique representative of a whole class of galaxies, starbursts with superwinds, in the Very Nearby Galaxy Survey with Herschel. In addition, its interaction with the M81 group has stripped a significant portion of its interstellar medium from its disk. SPIRE maps now afford better characterization of the far-infrared emission from cool dust outside the disk, and sketch a far more complete picture of its mass distribution and energetics than previously possible. They show emission coincident in projection with the starburst wind and in a large halo, much more extended than the PAH band emission seen with Spitzer. Some complex substructures coincide with the brightest PAH filaments, and others with tidal streams seen in atomic hydrogen. We subtract the far-infrared emission of the starburst and underlying disk from the maps, and derive spatially-resolved far-infrared colors for the wind and halo. We interpret the results in terms of dust mass, dust temperature, and global physical conditions. In particular, we examine variations in the dust physical properties as a function of distance from the center and the wind polar axis, and conclude that more than two thirds of the extraplanar dust has been removed by tidal interaction, and not entrained by the starburst wind.

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arXiv2010

Far-Infrared Line Imaging of the Starburst Ring in NGC 1097 with the Herschel/PACS Spectrometer

NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Telescope. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 $μ$m, [OIII] 88 $μ$m, [NII] 122 $μ$m, [CII] 158 $μ$m and [NII] 205 $μ$m. We observe that in the [OI] 63 $μ$m, [OIII] 88 $μ$m, and [NII] 122 $μ$m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220$ km s$^{-1}$ (inclination uncorrected) measured in all lines. The [OI] 63 $μ$m/[CII] 158 $μ$m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 $μ$m and [OIII] 88 $μ$m maps. Variations of the [NII] 122 $μ$m/[NII] 205 $μ$m ratio correspond to a range in the ionized gas density between 150 and 400 cm$^{-3}$.

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arXiv2010

The JCMT Nearby Galaxies Legacy Survey II: Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies

We present the results of large-area CO J=3-2 emission mapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627, completed at the James Clerk Maxwell Telescope as part of the Nearby Galaxies Legacy Survey. These galaxies all have moderate to strong CO J=3-2 detections over large areas of the fields observed by the survey, showing resolved structure and dynamics in their warm/dense molecular gas disks. All three galaxies were part of the Spitzer Infrared Nearby Galaxies Survey sample, and as such have excellent published multi-wavelength ancillary data. These data sets allow us to examine the star formation properties, gas content, and dynamics of these galaxies on sub-kiloparsec scales. We find that the global gas depletion times for dense/warm molecular gas in these galaxies is consistent with other results for nearby spiral galaxies, indicating this may be independent of galaxy properties such as structures, gas compositions, and environments. Similar to the results from the THINGS HI survey, we do not see a correlation of the star formation efficiency with the gas surface density consistent with the Schmidt-Kennicutt law. Finally, we find that the star formation efficiency of the dense molecular gas traced by CO J=3-2 is potentially flat or slightly declining as a function of molecular gas density, the CO J=3-2/J=1-0 ratio (in contrast to the correlation found in a previous study into the starburst galaxy M83), and the fraction of total gas in molecular form.

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arXiv2009

The JCMT Nearby Galaxies Legacy Survey III: Comparisons of cold dust, polycyclic aromatic hydrocarbons, molecular gas, and atomic gas in NGC 2403

We used 3.6, 8.0, 70, 160 micron Spitzer Space Telescope data, James Clerk Maxwell Telescope HARP-B CO J=(3-2) data, National Radio Astronomy Observatory 12 meter telescope CO J=(1-0) data, and Very Large Array HI data to investigate the relations among PAHs, cold (~20 K) dust, molecular gas, and atomic gas within NGC 2403, an SABcd galaxy at a distance of 3.13 Mpc. The dust surface density is mainly a function of the total (atomic and molecular) gas surface density and galactocentric radius. The gas-to-dust ratio monotonically increases with radius, varying from ~100 in the nucleus to ~400 at 5.5 kpc. The slope of the gas-to-dust ratio is close to that of the oxygen abundance, suggesting that metallicity strongly affects the gas-to-dust ratio within this galaxy. The exponential scale length of the radial profile for the CO J=(3-2) emission is statistically identical to the scale length for the stellar continuum-subtracted 8 micron (PAH 8 micron) emission. However, CO J=(3-2) and PAH 8 micron surface brightnesses appear uncorrelated when examining sub-kpc sized regions.

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arXiv2009

Rotation of the Warm Molecular Gas Surrounding Ultracompact HII Regions

We present molecular line and 1.4 mm continuum observations towards five massive star forming regions at arcsecond resolution using the Submillimeter Array (SMA). We find that the warm molecular gas surrounding each HII region (as traced by SO_2 and OCS) appears to be undergoing bulk rotation. From the molecular line emission and thermal component of the continuum emission, we independently derived gas masses for each region which are consistent with each other. From the free-free component of the continuum emission we estimate the minimum stellar mass required to power the HII region and find that this mass, when added to the derived gas mass, is a significant fraction of the dynamical mass for that region.

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arXiv2008

The JCMT Nearby Galaxies Legacy Survey I. Star Forming Molecular Gas in Virgo Cluster Spiral Galaxies

We present large-area maps of the CO J=3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO J=1-0, 24 micron, and Halpha images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC4321, and NGC 4569), we obtain molecular gas masses of 7E8-3E9 Msun and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO J=3-2 line is a better tracer of the dense star forming molecular gas than the CO J=1-0 line, as it shows a better correlation with the star formation rate surface density both within and between galaxies. NGC 4254 appears to have a larger star formation efficiency(smaller gas depletion time), perhaps because it is on its first passage through the Virgo Cluster. NGC 4569 shows a large-scale gradient in the gas properties traced by the CO J=3-2/J=1-0 line ratio, which suggests that its interaction with the intracluster medium is affecting the dense star-forming portion of the interstellar medium directly. The fourth galaxy in our sample, NGC 4579, has weak CO J=3-2 emission despite having bright 24 micron emission; however, much of the central luminosity in this galaxy may be due to the presence of a central AGN.

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arXiv2008

Clarifying the nature of the brightest submillimetre sources: interferometric imaging of LH850.02

We present high-resolution interferometric imaging of LH850.02, the brightest 850- and 1200-micron submillimetre (submm) galaxy in the Lockman Hole. Our observations were made at 890 micron with the Submillimetre Array (SMA). Our high-resolution submm imaging detects LH850.02 at >6-sigma as a single compact (size < 1 arcsec or < 8 kpc) point source and yields its absolute position to ~0.2-arcsec accuracy. LH850.02 has two alternative radio counterparts within the SCUBA beam (LH850.02N & S), both of which are statistically very unlikely to be so close to the SCUBA source position by chance. However, the precise astrometry from the SMA shows that the submm emission arises entirely from LH850.02N, and is not associated with LH850.02S (by far the brighter of the two alternative identifications at 24-micron). Fits to the optical-infrared multi-colour photometry of LH850.02N & S indicate that both lie at z~3.3, and are therefore likely to be physically associated. At these redshifts, the 24 micron--to--submm flux density ratios suggest that LH850.02N has an Arp220-type starburst-dominated far-IR SED, while LH850.02S is more similar to Mrk231, with less dust-enshrouded star-formation activity, but a significant contribution at 24-micron (rest-frame ~5-6 micron) from an active nucleus. This complex mix of star-formation and AGN activity in multi-component sources may be common in the high redshift ultraluminous galaxy population, and highlights the need for precise astrometry from high resolution interferometric imaging for a more complete understanding.

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arXiv2007

Outflow and Accretion in Massive Star Forming Regions

In order to distinguish between the various components of massive star forming regions (i.e. infalling, outflowing and rotating gas structures) within our own Galaxy, we require high angular resolution observations which are sensitive to structures on all size scales. To this end, we present observations of the molecular and ionized gas towards massive star forming regions at 230 GHz from the SMA (with zero spacing from the JCMT) and at 22 and 23 GHz from the VLA at arcsecond or better resolution. These observations (of sources such as NGC7538, W51e2 and K3-50A) form an integral part of a multi-resolution study of the molecular and ionized gas dynamics of massive star forming regions (i.e. Klaassen & Wilson 2007). Through comparison of these observations with 3D radiative transfer models, we hope to be able to distinguish between various modes of massive star formation, such as ionized or halted accretion (i.e Keto 2003 or Klaassen et al. 2006 respectively).

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arXiv2007

Upper limits to the water abundance in starburst galaxies

We have searched for emission from the 557 GHz ortho-water line in the interstellar medium of six nearby starburst galaxies. We used the Odin satellite to observe the 1_10-1_01 transition of o-H2O in the galaxies NGC253, IC342, M82, NGC4258, CenA, and M51. None of the galaxies in our sample was detected. We derive three sigma upper limits to the H2O abundance relative to H2 ranging from 2e-9 to 1e-8. The best of these upper limits are comparable to the measured abundance of H$_2$O in the Galactic star forming region W3. However, if only 10% of the molecular gas is in very dense cores, then the water abundance limits in the cores themselves would be larger by a factor of 10 i.e. 2e-8 to 1e-7. These observations suggest that detections of H2O emission in galaxies with the upcoming Herschel Space Observatory are likely to require on-source integration times of an hour or more except in the very brightest extragalactic targets such as M82 and NGC253.

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arXiv2007

Outflow and Infall in a Sample of Massive Star Forming Regions

We present single pointing observations of SiO, HCO$^+$ and H$^{13}$CO$^+$ from the James Clerk Maxwell Telescope towards 23 massive star forming regions previously known to contain molecular outflows and ultracompact HII regions. We detected SiO towards 14 sources and suggest that the non-detections in the other nine sources could be due to those outflows being older and without ongoing shocks to replenish the SiO. We serendipitously detected SO$_2$ towards 17 sources in the same tuning as HCO$^+$. We detected HCO$^+$ towards all sources, and suggest that it is tracing infall in nine cases. For seven infall candidates, we estimate mass infall rates between 1$\times10^{-2}$ and 2$\times10^{-5}$ M$_{\odot}$ yr$^{-1}$. Seven sources show both SiO detections (young outflows) and HCO$^+$ infall signatures. We also find that the abundance of H$^{13}$CO$^+$ tends to increase along with the abundance of SiO in sources for which we could determine abundances. We discuss these results with respect to current theories of massive star formation via accretion. From this survey, we suggest that perhaps both models of ionized accretion and halted accretion may be important in describing the evolution of a massive protostar (or protostars) beyond the formation of an HII region.

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arXiv2007

Molecular oxygen in the rho Ophiuchi cloud

Molecular oxygen, O2 has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core rho Oph A was observed with Odin for 34 days during several observing runs. We detect a spectral line at v(LSR) = 3.5 km/s with dv(FWHM) = 1.5 km/s, parameters which are also common to other species associated with rho Ohp A. This feature is identified as the O2 (N_J = 1_1 - 1_0) transition at 118 750.343 MHz. The abundance of molecular oxygen, relative to H2,, is 5E-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

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arXiv2006

High Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17

We have mapped an approximately 5.5 by 5.5 pc portion of the M17 massive star-forming region in both 850 and 450 micron dust continuum emission using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The maps reveal more than 100 dusty clumps with deconvolved linear sizes of 0.05--0.2 pc and masses of 0.8--120 solar masses, most of which are not associated with known mid-infrared point sources. Fitting the clump mass function with a double power law gives a mean power law exponent of alpha_high = -2.4 +/- 0.3 for the high-mass power law, consistent with the exponent of the Salpeter stellar mass function. We show that a lognormal clump mass distribution with a peak at about 4 solar masses produces as good a fit to the clump mass function as does a double power law. This 4 solar mass peak mass is well above the peak masses of both the stellar initial mass function and the mass function of clumps in low-mass star-forming regions. Despite the difference in intrinsic mass scale, the shape of the M17 clump mass function appears to be consistent with the shape of the core mass function in low-mass star-forming regions. Thus, we suggest that the clump mass function in high-mass star-forming regions may be a scaled-up version of that in low-mass regions, instead of its extension to higher masses.

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arXiv2005

High Mass Star Formation I: The Mass Distribution of Submillimeter Clumps in NGC 7538

We present submillimeter continuum maps at 450 and 850 microns of a 12 x 8 arcminute region of the NGC 7538 high-mass star-forming region, made using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We used an automated clump-finding algorithm to identify 67 clumps in the 450 micron image and 77 in the 850 micron image. Contrary to previous studies, we find a positive correlation between high spectral index, alpha, and high submillimeter flux, with the difference being accounted for by different treatments of the error beam. We interpret the higher spectral index at submillimeter peaks as a reflection of elevated dust temperature, particularly when there is an embedded infrared source, though it may also reflect changing dust properties. The clump mass-radius relationship is well-fit by a power law of the form M \propto R^(-x) with x = 1.5-2.1, consistent with theories of turbulently-supported clumps. According to our most reliable analysis, the high-mass end (approx. 100-2700 M_sun) of the submillimeter clump mass function in NGC 7538 follows a Salpeter-like power law with index 2.0 +/- 0.3. This result agrees well with similar studies of lower-mass regions rho Oph and Orion B. We interpret the apparent invariance of the shape of the clump mass function over a broad range of parent cloud masses as evidence for the self-similarity of the physical processes which determine it. This result is consistent with models which suggest that turbulent fragmentation, acting at early times, is sufficient to set the clump mass function.

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arXiv2005

Searching for O$_2$ in the SMC:Constraints on Oxygen Chemistry at Low Metallicities

We present a 39 h integration with the Odin satellite on the ground-state 118.75 GHz line of O2 towards the region of strongest molecular emission in the Small Magellanic Cloud. Our 3sigma upper limit to the O2 integrated intensity of <0.049 K km/s in a 9'(160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of <1.3E-6. Although a factor of 20 above the best limit on the O2 abundance obtained for a Galactic source, our result has interesting implications for understanding oxygen chemistry at sub-solar metal abundances. We compare our abundance limit to a variety of astrochemical models and find that, at low metallicities, the low O2 abundance is most likely produced by the effects of photo-dissociation on molecular cloud structure. Freeze-out of molecules onto dust grains may also be consistent with the observed abundance limit, although such models have not yet been run at sub-solar initial metallicities.

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arXiv2005

ISM Properties in Low-Metallicity Environments III. The Dust Spectral Energy Distributions of II Zw 40, He 2-10 and NGC 1140

We present new 450 and 850 micron SCUBA data and 1.3 mm MAMBO data of the dwarf galaxies II Zw 40, He 2-10 and NGC 1140. Additional ISOCAM, IRAS as well as ground based data are used to construct the observed mid-infrared to millimeter spectral energy distribution of these galaxies. These spectral energy distributions are modeled in a self-consistent way, as was achieved with NGC 1569 (Galliano et al., 2003), synthesizing both the global stellar radiation field and the dust emission, with further constraints provided by the photoionisation of the gas. Our study shows that low-metallicity galaxies have very different dust properties compared to the Galaxy. Our main results are: (i) a paucity of PAHs which are likely destroyed by the hard penetrating radiation field, (ii) a very small (3-4 nm) average size of grains, consistent with the fragmentation and erosion of dust particles by the numerous shocks, (iii) a significant millimetre excess in the dust spectral energy distribution which can be explained by the presence of ubiquitous very cold dust (T=5-9 K) accounting for 40 to 80 % of the total dust mass, probably distributed in small clumps. We derive a range of gas-to-dust mass ratios between 300 and 2000, larger than the Galactic values and dust-to-metals ratios of 1/30 to 1/2. The modeled dust size distributions are used to synthesize an extinction curve for each galaxy. The UV slopes of the extinction curves resemble that observed in some regions in the Large Magellanic Cloud. The 2175 angstrom bumps of the modeled extinction curves are weaker than that of the Galaxy, except in the case of II Zw 40, where we are unable to accurately constrain the 2175 angstrom bump carrier.

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arXiv2004

Extended Molecular Gas in the Nearby Starburst Galaxy Maffei 2

We present a 9'x9' fully-sampled map of the CO J=1-0 emission in the nearby starburst galaxy Maffei 2 obtained at the Five College Radio Astronomy Observatory. The map reveals previously known strong CO emission in the central starburst region as well as an extended asymmetric distribution with bright CO lines at the ends of the bar and in a feature at the north-east edge of the molecular disk. This northern feature, proposed previously to be an interacting companion galaxy, could be a dwarf irregular galaxy, although the CO data are also consistent with the feature being simply an extension of one of the spiral arms. We estimate the total molecular gas mass of Maffei 2 to be (1.4-1.7)x10^9 Mo or ~3-4% of its dynamical mass. Adopting the recently determined lower value for the CO-to-H2 conversion factor in the central region, our data lead to the surprising result that the largest concentrations of molecular gas in Maffei 2 lie at the bar ends and in the putative dwarf companion rather than in the central starburst. A gravitational stability analysis reveals that the extended disk of Maffei 2 lies above the critical density for star formation; however, whether the central region is also gravitationally unstable depends both on the details of the rotation curve and the precise value of the CO-to-H2 conversion factor in this region.

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