arXiv subjects

Walter Gear

Publications and source records attributed to Walter Gear.

15 recordsLinked to original sources

arXiv2023

The JCMT Nearby Galaxies Legacy Survey: SCUBA-2 observations of nearby galaxies

We present 850$\mu$m observations of a sample of 8 nearby spiral galaxies, made using the SCUBA-2 camera on the James Clerk Maxwell Telescope (JCMT) as part of the JCMT Nearby Galaxies Legacy Survey (NGLS). We corrected our data for the presence of the $^{12}$CO $J=3\to 2$ line in the SCUBA-2 850$\mu$m bandwidth using NGLS HARP data, finding a typical $^{12}$CO contribution of $\sim 20$%. We measured dust column densities, temperatures and opacity indices by fitting spectral energy distributions constructed from SCUBA-2 and archival Herschel observations, and used archival GALEX and Spitzer data to make maps of surface density of star formation ($\Sigma_{\rm SFR}$). Typically, comparing SCUBA-2-derived H$_2$ surface densities ($\Sigma_{\rm H_2}$) to $\Sigma_{\rm SFR}$ gives shallow star formation law indices within galaxies, with SCUBA-2-derived values typically being sublinear and Herschel-derived values typically being broadly linear. This difference is likely due to the effects of atmospheric filtering on the SCUBA-2 data. Comparing the mean values of $\Sigma_{\rm H_2}$ and $\Sigma_{\rm SFR}$ of the galaxies in our sample returns a steeper star formation law index, broadly consistent with both the Kennicutt-Schmidt value of 1.4 and linearity. Our results show that a SCUBA-2 detection is a good predictor of star formation. We suggest that Herschel emission traces gas in regions which will form stars on timescales $\sim 5-100$ Myr, comparable to the star formation timescale traced by GALEX and Spitzer data, while SCUBA-2 preferentially traces the densest gas within these regions, which likely forms stars on shorter timescales.

astro-ph.GA↗

arXiv2021

The HASHTAG project: The First Submillimeter Images of the Andromeda Galaxy from the Ground

Observing nearby galaxies with submillimeter telescopes on the ground has two major challenges. First, the brightness is significantly reduced at long submillimeter wavelengths compared to the brightness at the peak of the dust emission. Second, it is necessary to use a high-pass spatial filter to remove atmospheric noise on large angular scales, which has the unwelcome by-product of also removing the galaxy's large-scale structure. We have developed a technique for producing high-resolution submillimeter images of galaxies of large angular size by using the telescope on the ground to determine the small-scale structure (the large Fourier components) and a space telescope (Herschel or Planck) to determine the large-scale structure (the small Fourier components). Using this technique, we are carrying out the HARP and SCUBA-2 High Resolution Terahertz Andromeda Galaxy Survey (HASHTAG), an international Large Program on the James Clerk Maxwell Telescope, with one aim being to produce the first high-fidelity high-resolution submillimeter images of Andromeda. In this paper, we describe the survey, the method we have developed for combining the space-based and ground-based data, and present the first HASHTAG images of Andromeda at 450 and 850um. We also have created a method to predict the CO(J=3-2) line flux across M31, which contaminates the 850um band. We find that while normally the contamination is below our sensitivity limit, the contamination can be significant (up to 28%) in a few of the brightest regions of the 10 kpc ring. We therefore also provide images with the predicted line emission removed.

astro-ph.GA↗

arXiv2019

The case for a 'sub-millimeter SDSS': a 3D map of galaxy evolution to z~10

The Sloan Digital Sky Survey (SDSS) was revolutionary because of the extraordinary breadth and ambition of its optical imaging and spectroscopy. We argue that a 'sub-millimeter SDSS' - a sensitive large-area imaging+spectroscopic survey in the sub-mm window - will revolutionize our understanding of galaxy evolution in the early Universe. By detecting the thermal dust continuum emission and atomic and molecular line emission of galaxies out to z~10 it will be possible to measure the redshifts, star formation rates, dust and gas content of hundreds of thousands of high-z galaxies down to ~L*. Many of these galaxies will have counterparts visible in the deep optical imaging of the Large Synoptic Survey Telescope. This 3D map of galaxy evolution will span the peak epoch of galaxy formation all the way back to cosmic dawn, measuring the co-evolution of the star formation rate density and molecular gas content of galaxies, tracking the production of metals and charting the growth of large-scale structure.

astro-ph.GA↗

arXiv2014

A stubbornly large mass of cold dust in the ejecta of Supernova 1987A

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 micron [O I] line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 micron flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5+-0.1 Msun of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 Msun of amorphous carbon and 0.5 Msun of silicates, totalling 0.8 Msun of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies.

astro-ph.SR↗

arXiv2012

Can dust emission be used to map the interstellar medium in high-redshift galaxies? Results from the Herschel Reference Survey

It has often been suggested that an alternative to the standard CO/21-cm method for estimating the mass of the interstellar medium (ISM) in a galaxy might be to estimate the mass of the ISM from the continuum dust emission. In this paper, we investigate the potential of this technique using Herschel observations of ten galaxies in the Herschel Reference Survey and in the Herschel Virgo Cluster Survey. We show that the emission detected by Herschel is mostly from dust that has a temperature and emissivity index similar to that of dust in the local ISM in our galaxy, with the temperature generally increasing towards the centre of each galaxy. We calibrate the dust method using the CO and 21-cm observations to provide an independent estimate of the mass of hydrogen in each galaxy, solving the problem of the uncertain `X factor' for the molecular gas by minimizing the dispersion in the ratio of the masses estimated using the two methods. With the calibration for the dust method and the estimate of the X-factor produced in this way, the dispersion in the ratio of the two gas masses is 30%, which gives an upper limit on the fundamental accuracy of the dust method. The calibration we obtain for the dust method is very similar to an independent Herschel measurement for M31 and to the calibration for the Milky Way from Planck measurements.

astro-ph.CO↗

arXiv2009

Observing the Evolution of the Universe

How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass. We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages.

astro-ph.CO↗

arXiv2003

The Canada-UK Deep Submillimetre Survey VIII: Source Identifications in the 3-hour field

We present optical, near-infrared and radio observations of the 3-hour field of the Canada-UK Deep Submillimetre Survey. Of the 27 submillimetre sources in the field, nine have secure identifications with either a radio source or a near-IR source. We show that the percentage of sources with secure identifications in the CUDSS is consistent with that found for the bright `8 mJy' submillimetre survey, once allowance is made for the different submillimetre and radio flux limits. Of the 14 secure identifications in the two CUDSS fields, eight are VROs or EROs, five have colours typical of normal galaxies, and one is a radio source which has not yet been detected at optical/near-IR wavelengths. Eleven of the identifications have optical/near-IR structures which are either disturbed or have some peculiarity which suggests that the host galaxy is part of an interacting system. One difference between the CUDSS results and the results from the 8-mJy survey is the large number of low-redshift objects in the CUDSS; we give several arguments why these are genuine low-redshift submillimetre sources rather than being gravitational lenses which are gravitationally amplifying a high-$z$ submillimetre source. We construct a $K-z$ diagram for various classes of high-redshift galaxy and show that the SCUBA galaxies are on average less luminous than classical radio galaxies, but are very similar in both their optical/IR luminosities and their colours to the host galaxies of the radio sources detected in $μ$Jy radio surveys.

astro-ph↗

arXiv2003

The Canada-UK Deep Submillimeter Survey VII: Optical and near-infrared identifications for the 14h field

We present the multi-wavelength identifications for 23 sources in the Canada-UK Deep Submillimeter Survey (CUDSS) 14h field. The identifications have been selected on the basis of radio and near-infrared data and we argue that, to our observational limits, both are effective at selecting the correct counterparts of the SCUBA sources. We discuss the properties of these identifications and find that they are very red in near-infrared color, with many classified as Extremely Red Objects, and show disturbed morphologies. Using the entire CUDSS catalogue of 50 sources we use a combination of spectroscopic redshifts (4 objects), 1.4GHz-to-850um flux ratio redshift estimates (10 objects), and redshift lower-limits based on non-detections at 1.4GHz (the rest of the sample) to estimate a lower-limit on the median redshift of the population of z_med > 1.4. Working from simple models and using the properties of the secure identifications, we discuss general and tentative constraints on the redshift distribution and the expected colors and magnitudes of the entire population.

astro-ph↗

arXiv2000

The Canada-UK Deep Submillimetre Survey: The Survey of the 14-hour field

We have used SCUBA to survey an area of 50 square arcmin, detecting 19 sources down to a 3sigma sensitivity limit of 3.5 mJy at 850 microns. We have used Monte-Carlo simulations to assess the effect of source confusion and noise on the SCUBA fluxes and positions, finding that the fluxes of sources in the SCUBA surveys are significantly biased upwards and that the fraction of the 850 micron background that has been resolved by SCUBA has been overestimated. The radio/submillmetre flux ratios imply that the dust in these galaxies is being heated by young stars rather than AGN. We have used simple evolution models based on our parallel SCUBA survey of the local universe to address the major questions about the SCUBA sources: (1) what fraction of the star formation at high redshift is hidden by dust? (2) Does the submillimetre luminosity density reach a maximum at some redshift? (3) If the SCUBA sources are proto-ellipticals, when exactly did ellipticals form? However, we show that the observations are not yet good enough for definitive answers to these questions. There are, for example, acceptable models in which 10 times as much high-redshift star formation is hidden by dust as is seen at optical wavelengths, but also acceptable ones in which the amount of hidden star formation is less than that seen optically. There are acceptable models in which very little star formation occurred before a redshift of three (as might be expected in models of hierarchical galaxy formation), but also ones in which 30% of the stars have formed by this redshift. The key to answering these questions are measurements of the dust temperatures and redshifts of the SCUBA sources.

astro-ph↗

arXiv1999

Deep sub-mm surveys: High redshift ULIRGs and the formation of the metal-rich spheroids

Deep surveys of the sky at millimeter wavelengths have revealed a population of ultra-luminous infrared galaxies (ULIRGs) at high redshifts. These appear similar to local objects of similar luminosities (such as Arp220) but are much more ``important'' at high redshift than at low reshift, in the sense that they represent a much larger fraction of the total luminous output of the distant Universe than they do locally. In fact the ULIRGs at high redshift are producing a significant fraction (>= 15%) of the total luminous output of the Universe averaged over all wavelengths and all epochs. The high z ULIRGs could plausibly be responsible for producing the metal-rich spheroidal components of galaxies, including the bulges that are the subject of this conference. In this case we would infer from the redshift distribution of the sources that much of this activity is probably happening relatively recently at z <= 2.

astro-ph↗

arXiv1998

A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe

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 deepest optical images are insensitive. We present the deepest sub-mm survey, taken with the SCUBA camera on the James Clerk Maxwell Telescope (JCMT) and centred on the Hubble Deep Field (HDF). The high source density on 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. These data lead to integral source counts which are completely inconsistent with a no evolution model, whilst 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 statistically the entire background is resolved at about the 0.3 mJy level. Four of the five brightest sources appear to be associated with galaxies which lie in the redshift range 2 < z < 4. With the caveat that this is a small sample of sources detected in a small survey area, these submm data imply a star-formation density over this redshift range that is at least five times higher than that inferred from the rest-frame ultraviolet output of HDF galaxies.

astro-ph↗

arXiv1998

The Canada-UK Deep Submillimetre Survey: First Submillimetre Images, the Source Counts, and Resolution of the Background

We present the first results of a deep unbiased submillimetre survey carried out at 450 and 850 microns. We detected 12 sources at 850 microns, giving a surface density of sources with 850-micron flux densities > 2.8mJy of of 0.49+-0.16 per square arcmin. The sources constitute 20-30% of the background radiation at 850 microns and thus a significant fraction of the entire background radiation produced by stars. This implies, through the connection between metallicity and background radiation, that a significant fraction of all the stars that have ever been formed were formed in objects like those detected here. The combination of their large contribution to the background radiation and their extreme bolometric luminosities make these objects excellent candidates for being proto-ellipticals. Optical astronomers have recently shown that the UV-luminosity density of the universe increases by a factor of about 10 between z=0 and z=1 and then decreases again at higher redshifts. Using the results of a parallel submillimetre survey of the local universe, we show that both the submillimetre source density and background can be explained if the submillimetre luminosity density evolves in a similar way to the UV-luminosity density. Thus, if these sources are ellipticals in the process of formation, they may be forming at relatively modest redshifts.

astro-ph↗

arXiv1998

Deep Sub-millimeter Surveys and the Hidden Phases of Galaxy Formation

Deep surveys in the far-infrared and sub-millimeter wavebands are revealing a new phase of galactic evolution hidden by dust. Observations with SCUBA on the JCMT show that 25% of the COBE/FIRAS background at 850 microns is being produced by high luminosity sources (L ~ 3x10^12 L_sun) at high redshifts 0.5 < z < 3+. These sources have an estimated redshift distribution that is broadly consistent with a global star-formation history that is similar to that inferred from optical observations. The sub-mm galaxies and optically selected galaxies are producing comparable quantities of stars. However, the sub-mm sources are doing so in systems that have luminosities that are an order of magnitude higher, and comoving densities an order of magnitude lower, then the optically selected galaxies. These high luminosity sources are plausibly responsible for producing the spheroidal components of massive galaxies at z ~ 2.

astro-ph↗

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↗