arXiv subjects

David Hughes

Publications and source records attributed to David Hughes.

30 records · Page 2Linked to original sources

arXiv2013

X-ray Detections of Sub-millimetre Galaxies: Active Galactic Nuclei Versus Starburst Contribution

We present a large-scale study of the X-ray properties and near-IR-to-radio SEDs of submillimetre galaxies (SMGs) detected at 1.1mm with the AzTEC instrument across a ~1.2 square degree area of the sky. Combining deep 2-4 Ms Chandra data with Spitzer IRAC/MIPS and VLA data within the GOODS-N/S and COSMOS fields, we find evidence for AGN activity in ~14 percent of 271 AzTEC SMGs, ~28 percent considering only the two GOODS fields. Through X-ray spectral modeling and SED fitting using Monte Carlo Markov Chain techniques to Siebenmorgen et al. (2004) (AGN) and Efstathiou et al. (2000) (starburst) templates, we find that while star formation dominates the IR emission, with SFRs ~100-1000 M_sun/yr, the X-ray emission for most sources is almost exclusively from obscured AGNs, with column densities in excess of 10^23 cm^-2. Only for ~6 percent of our sources do we find an X-ray-derived SFR consistent with NIR-to-radio SED derived SFRs. Inclusion of the X-ray luminosities as a prior to the NIR-to-radio SED effectively sets the AGN luminosity and SFR, preventing significant contribution from the AGN template. Our SED modeling further shows that the AGN and starburst templates typically lack the required 1.1 mm emission necessary to match observations, arguing for an extended, cool dust component. The cross correlation function between the full samples of X-ray sources and SMGs in these fields does not indicate a strong correlation between the two populations at large scales, suggesting that SMGs and AGNs do not necessarily trace the same underlying large scale structure. Combined with the remaining X-ray-dim SMGs, this suggests that sub-mm bright sources may evolve along multiple tracks, with X-ray-detected SMGs representing transitionary objects between periods of high star formation and AGN activity while X-ray-faint SMGs represent a brief starburst phase of more normal galaxies.

astro-ph.GA↗

arXiv2010

Detection of Strong Millimeter Emission from the Circumstellar Dust Disk Around V1094 Sco: Cold and Massive Disk around a T Tauri Star in a Quiescent Accretion Phase?

We present the discovery of a cold massive dust disk around the T Tauri star V1094 Sco in the Lupus molecular cloud from the 1.1 millimeter continuum observations with AzTEC on ASTE. A compact ($r\lesssim$320 AU) continuum emission coincides with the stellar position having a flux density of 272 mJy which is largest among T Tauri stars in Lupus. We also present the detection of molecular gas associated with the star in the five-point observations in $^{12}$CO J=3--2 and $^{13}$CO J=3--2. Since our $^{12}$CO and $^{13}$CO observations did not show any signature of a large-scale outflow or a massive envelope, the compact dust emission is likely to come from a disk around the star. The observed SED of V1094 Sco shows no distinct turnover from near infrared to millimeter wavelengths, which can be well described by a flattened disk for the dust component, and no clear dip feature around 10 $\micron$ suggestive of absence of an inner hole in the disk. We fit a simple power-law disk model to the observed SED. The estimated disk mass ranges from 0.03 to $\gtrsim$0.12 $M_\sun$, which is one or two orders of magnitude larger than the median disk mass of T Tauri stars in Taurus.

astro-ph.SR↗

arXiv2010

BLAST Observations of the South Ecliptic Pole field: Number Counts and Source Catalogs

We present results from a survey carried out by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) on a 9 deg^2 field near the South Ecliptic Pole at 250, 350 and 500 μm. The median 1σ depths of the maps are 36.0, 26.4 and 18.4 mJy, respectively. We apply a statistical method to estimate submillimeter galaxy number counts and find that they are in agreement with other measurements made with the same instrument and with the more recent results from Herschel/SPIRE. Thanks to the large field observed, the new measurements give additional constraints on the bright end of the counts. We identify 132, 89 and 61 sources with S/N>4 at 250, 350, 500 μm, respectively and provide a multi-wavelength combined catalog of 232 sources with a significance >4σ in at least one BLAST band. The new BLAST maps and catalogs are available publicly at http://blastexperiment.info.

astro-ph.CO↗

arXiv2010

An Investigation of the Dust Content in the Galaxy pair NGC 1512/1510 from Near-Infrared to Millimeter Wavelengths

We combine new ASTE/AzTEC 1.1 mm maps of the galaxy pair NGC 1512/1510 with archival Spitzer IRAC and MIPS images covering the wavelength range 3.6--160 um from the SINGS project to derive accurate dust masses in each galaxy, and in sub--galactic regions in NGC 1512. The two galaxies form a pair consisting of a large, high--metallicity spiral (NGC 1512) and a low metallicity, blue compact dwarf (NGC 1510). The derived total dust masses are (2.4+/-0.6) 10^7 Msun and (1.7+/-3.6) 10^5 Msun for NGC 1512 and NGC 1510, respectively. The derived ratio of dust mass to H I gas mass for the galaxy pair (0.0034) is much lower than expected, while regions within NGC 1512, specifically the central region and the arms, do not show such unusually low ratios; furthermore, the dust--to--gas ratio is within expectations for NGC 1510. These results suggest that a fraction of the H I included in the determination of the M_d/M_HI ratio of the NGC 1512/1510 pair is not associated with the star forming disks/regions of either galaxy. We also perform simple two--temperature (warm + cold) modified--blackbody fits to the far--infrared/mm data. The warm dust temperature of the low--metallicity NGC 1510 (~36 K) is substantially higher than that of the high--metallicity NGC 1512 (~24 K). In both galaxies, a substantial fraction (>93%) of the total dust mass is in a cool dust component, with temperatures 14--16 K for NGC 1512 and 15--24 K for NGC 1510. In contrast, the warm dust in the arms and central regions of NGC 1512 represents a much larger fraction of the total dust content, in agreement with the fact that all three regions have higher specific star formation rates than the average in the galaxy.

astro-ph.CO↗

arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) -- VI. 350 micron mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 microns has been carried out to map the regions around several 850 micron-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared luminosities and hence star-formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350 micron flux densities for 24 850 micron sources, seven of which are detected at >2.5-sigma within a 10 arcsec search radius of the 850 micron positions. When results from the literature are included the total number of 350 micron flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-infrared (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich (Mdust~9x10^8 Msun), luminous (Lfir~2x10^12 Lsun), star-forming galaxies with intrinsic dust temperatures of ~35 K and star-formation rates of ~400 Msun/yr. We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350 micron source counts, N350(>25mJy)~200-500 deg^-2.

astro-ph↗

arXiv2003

A Heavenly Example of Scale Free Networks and Self-Organized Criticality

The sun provides an explosive, heavenly example of self-organized criticality. Sudden bursts of intense radiation emanate from rapid rearrangements of the magnetic field network in the corona. Avalanches are triggered by loops of flux that reconnect or snap into lower energy configurations when they are overly stressed. Our recent analysis of observational data reveals that the loops (links) and footpoints (nodes), where they attach on the photosphere, embody a scale free network. The statistics of the avalanches and of the network structure are unified through a simple dynamical model where the avalanches and network co-generate each other into a complex, critical state. This particular example points toward a general dynamical mechanism for self-generation of complex networks.

cond-mat.stat-mech↗

arXiv2003

Scale-Free Magnetic Networks: Comparing Observational Data with a Self-Organizing Model of the Coronal Field

We propose that the coronal magnetic field, linking concentrations on the photosphere through an interwoven web of flux, embodies a scale-free network. It arises from a self-organized critical dynamics including flux emergence, the diffusion and merging of magnetic concentrations, as well as avalanches of reconnecting flux tubes. Magnetic concentrations such as fragments, pores and sunspots, are `nodes' joined by flux tubes or `links'. The number of links emanating from a node is scale-free. We reanalyze the quiet-Sun data of Close et al and show that the distribution of magnetic concentration strengths is a power law with an index $γ= 1.7 \pm 0.3$, over the entire range of the measurement, about $(2-500)\times 10^{17}$ Mx. This distribution is compatible with that for the sizes of active regions reported by Harvey and Schwaan. Thus magnetic concentrations may be scale-free from the smallest measurable fragments to the large active regions. Numerical simulations of a self-organized critical model give the same index $γ$, within statistical uncertainty. The exponential distribution of flux tube lengths also agrees quantitatively with results from the model. Calibration with the measured diffusion constant of magnetic concentrations allows us to calculate a flux turnover time in the model to be of order 10 hours and the total solar flux to be of order $10^{23}$Mx, agreeing with observations. We introduce two other statistical quantities to characterize scale-free networks. The probability distribution for the amount of flux connecting a pair of concentrations, and the number of distinct concentrations linked to a given one are predicted to be scale-free, with different indices. Our approach unifies the observation of scale free flare energies with the coronal magnetic field structure.

astro-ph↗

arXiv2002

Deep radio imaging of the SCUBA 8-mJy survey fields: sub-mm source identifications and redshift distribution

The SCUBA 8-mJy survey is the largest submm extragalactic mapping survey undertaken to date, centred on the Lockman Hole and ELAIS N2 regions. Here, we present new 1.4-GHz imaging of the depth and resolution necessary to reliably identify radio counterparts for 18 of 30 submm sources. Armed with this greatly improved positional information, we present and analyse new optical, near-IR and XMM-Newton X-ray imaging to identify optical/IR host galaxies to half of the submm-selected sources in those fields. As many as 15% of the submm sources detected at 1.4 GHz are resolved by the 1.4'' beam and a further 25% have more than one radio counterpart, suggesting that radio and submm emission arise from extended starbursts and that interactions are common. We note that less than a quarter of the submm-selected sample would have been recovered by targeting optically faint radio sources, underlining the selective nature of such surveys. At least 60% of the radio-confirmed optical/IR host galaxies appear to be morphologically distorted; many are composite systems - red galaxies with relatively blue companions; just over half are found to be very red or extremely red; contrary to popular belief, most are sufficiently bright to be tackled with spectrographs on 8-m telescopes. We estimate the median redshift of the S(850um) ~ 8 mJy submm galaxy population: if the radio/far-IR correlation holds at high redshift, and our sample is unbiased, we derive a conservative median z >= 2.0, or >= 2.4 using spectral templates more representative of known submm galaxies [abridged].

astro-ph↗

arXiv2001

Large Scale Structures, Symmetry, and Universality in Sandpiles

We introduce a sandpile model where, at each unstable site, all grains are transferred randomly to downstream neighbors. The model is local and conservative, but not Abelian. This does not appear to change the universality class for the avalanches in the self-organized critical state. It does, however, introduce long-range spatial correlations within the metastable states. We find large scale networks of occupied sites whose density vanishes in the thermodynamic limit, for d>1.

cond-mat.stat-mech↗

arXiv1998

Unveiling Dust-enshrouded Star Formation in the Early Universe: a Sub-mm Survey of the Hubble Deep Field

The advent of sensitive sub-mm array cameras now allows a proper census of dust-enshrouded massive star-formation in very distant galaxies, previously hidden activity to which even the faintest optical images are insensitive. We present the deepest sub-mm survey of the sky to date, taken with the SCUBA camera on the James Clerk Maxwell Telescope and centred on the Hubble Deep Field. The high source density found in this image implies that the survey is confusion-limited below a flux density of 2 mJy. However, within the central 80 arcsec radius independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which simulations indicate can be ascribed to individual galaxies. We give positions and flux densities for these, and furthermore show using multi-frequency photometric data that the brightest sources in our map lie at redshifts z~3. These results lead to integral source counts which are completely inconsistent with a no-evolution model, and imply that massive star-formation activity continues at redshifts > 2. The combined brightness of the 5 most secure sources in our map is sufficient to account for 30 - 50% of the previously unresolved sub-mm background, and we estimate statistically that the entire background is resolved at about the 0.3 mJy level. Finally we discuss possible optical identifications and redshift estimates for the brightest sources. One source appears to be associated with an extreme starburst galaxy at z~1, whilst the remaining four appear to lie in the redshift range 2 < z < 4. This implies a star-formation density over this redshift range that is at least five times higher than that inferred from the ultraviolet output of HDF galaxies.

astro-ph↗

arXiv1997

Dust, gas and the evolutionary status of the radio galaxy, 8C1435+635, at z = 4.25

We present the results of new rest-frame far-IR observations of the z = 4.25 radio galaxy, 8C1435+635, which not only confirm that it contains an enormous quantity of dust (as first inferred from its mm-wave detection by Ivison 1995) but also allow the first meaningful constraints to be placed on the mass of this dust and associated gas. The new measurements consist of: (i) clear detections of sub-mm continuum emission at 450 and 850um obtained with the new sub-mm bolometer array, SCUBA, on the JCMT, (ii) continuum upper limits at 350, 750 and 175um obtained with SCUBA and the PHT far-IR camera aboard ISO, and (iii) a sensitive upper limit on the CO(4-3) line flux obtained with the IRAM 30-m MRT. The resulting rest-frame 33- to 238-um continuum coverage allows us to deduce that 2E8 Msun of dust at a temperature of 40 +/- 5 K is responsible for the observed mm/sub-mm emission. Using our CO upper limit, which constrains M(H2)/M(dust) to <950, we go on to calculate robust limits on the total gas reserves (H2 + HI) which are thereby constrained to between 4E10 and 1.2E12 Msun. The sub-mm properties of 8C1435+635 are thus strikingly similar to those of the z = 3.80 radio galaxy, 4C41.17, the only other high-redshift galaxy detected to date at sub-mm wavelengths whose properties appear not to be exaggerated by gravitational lensing (Dunlop et al. 1994; Hughes, Dunlop & Rawlings 1997). The inferred gas masses of both objects are sufficiently large to suggest that the formative starbursts of massive elliptical galaxies are still in progress at around z = 4. Observations of complete samples of radio galaxies spanning a range of redshifts and radio luminosities will be required to determine if the spectacular far-IR properties of 8C1435+635 and 4C41.17 are primarily due to their extreme redshifts or their extreme radio luminosities.

astro-ph↗

arXiv1997

High-redshift radio galaxies and quasars at sub-millimetre wavelengths: assessing their evolutionary status

We present new results of a study of the sub-millimetre continuum emission from a sample of 9 radio galaxies and 4 radio-quiet quasars at redshifts $z = 0.75 - 4.26$. The observations were made at 800$μm$, using the single-element bolometer UKT14 on the James Clerk Maxwell Telescope (JCMT), reaching a typical r.m.s. sensitivity of $σ_{rms} \sim 4$ mJy and represent some of the deepest submillimetre extragalactic measurements made to date. Three detections were achieved, of which two are secure (4C41.17, Dunlop et al. (1994) and H1413$+$117) and one (53W002) is tentative, whilst comparable upper-limits were obtained for 7 of the 10 remaining sources. We use these data as the motivation for a detailed discussion of the conversion from submillimetre and millimetre continuum fluxes to dust/gas masses and star formation rates at high-redshift, and determine these quantities from our own and other data on high-redshift radio-galaxies and quasars. In particular we have investigated the impact of the four main sources of uncertainty in deriving physical quantities from such data, namely i) potential contamination by galactic cirrus, ii) uncertainty in the value of the dust rest-frequency mass-absorption coefficient, iii) difficulty in constraining the dust temperature, and iv) estimation of the appropriate gas:dust ratio in high-redshift objects. Our discussion emphasises how important it will be to quantify and, where possible, minimize such uncertainties (via, for example, appropriate observational strategies) in order to fully capitalize on the ten-fold improvement in sensitivity offered by the imminent arrival of the next generation of bolometer arrays, such as SCUBA on the JCMT.

astro-ph↗