Tracy Margaret Anne Webb

The Canada-UK Deep Submillimeter Survey. IV. The Survey of the 14 Hour Field

We have used SCUBA to survey an area of simeq 50 arcmin2, detecting 19 sources down to a 3 σ sensitivity limit of ~3.5 mJy at 850 μm. Monte Carlo simulations have shown that the fluxes of sources in this and similar SCUBA surveys are biased upward by the effects of source confusion and noise, leading to an overestimate by a factor of ~1.4 in the fraction of the 850 μm background that has been resolved by SCUBA. Once a correction is made for this effect, about 20% of the background has been resolved. The simulations have also been used to quantify the effects of confusion on source positions. Of the 19 SCUBA sources, five are microjansky radio sources, and two are ISO 15 μm sources. The radio/submillmeter flux ratios imply that the dust in these galaxies is being heated by young stars rather than active galactic nuclei. The upper limit to the average 450 μm/850 μm flux ratio implies either that the SCUBA galaxies are at z G 2 or, if they are at lower redshifts, that the dust is generally colder than in ULIRGs. We have used simple evolution models 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 submillimeter luminosity density reach a maximum at some redshift? (3) If the SCUBA sources are protoellipticals, when exactly did ellipticals form? We show, however, that the observations are not yet good enough to answer 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 3 (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.

The Canada-UK Deep Submillimeter Survey. VI. The 3 Hour Field

We present the complete submillimeter data for the Canada-UK Deep Submillimeter Survey (CUDSS) 3h field. The observations were taken with the Submillimeter Common-User Bolometric Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) on Mauna Kea. The 3h field is one of two main fields in our survey and covers 60 arcmin2 to a 3 ? depth of ~3 mJy. In this field we have detected 27 sources above 3 ? and 15 above 3.5 ?. We assume that the differential source counts follow the form N(S) S-? and measure ? = 3.3, in good agreement with previous studies. We estimate that SCUBA sources brighter than 3 mJy are responsible for ~13% of the extragalactic background at 850 ?m (after correcting for flux boosting, 20% with no correction), which is in general agreement with previous estimates made by other groups. Using preliminary Infrared Space Observatory 15 ?m maps and VLA 1.4 GHz data, we have identified counterparts for 10 objects and have detected two sources at 450 ?m. With this information we estimate a lower limit on the median redshift of the sample of z > 1.4 with 6%-10% lying at z 3 mJy sources using the source catalogs from the CUDSS two main fields, the 3h and 14h fields, and find ?(?) = 4.4

A spatially resolved map of the kinematics, star formation and stellar mass assembly in a star-forming galaxy at z = 4.9

We present a detailed study of the spatially resolved kinematics, star formation and stellar mass in a highly amplified galaxy at z = 4.92 behind the lensing cluster MS 1358+62. We use the observed optical, near- and mid-infrared imaging from Hubble Space Telescope ACS & NICMOS and Spitzer IRAC to derive the stellar mass and the Gemini/NIFS IFU to investigate the velocity structure of the galaxy from the nebular [O II]λλ3726.8,3728.9 emission. Using a detailed gravitational lens model, we account for lensing amplification factor 12.5 ± 2.0 and find that this intrinsically L^* galaxy has a stellar mass of M_★= 7 ± 2 × 10^8 M_⊙, a dynamical mass of M_(dyn)= 3 ± 1 × 10^(9)csc^2(i) M_⊙ (within of 2 kpc) and a star formation rate of 42 ± 8 M_⊙ yr^(−1). The source-plane UV/optical morphology of this galaxy is dominated by five discrete star-forming regions. Exploiting the dynamical information we derive masses for individual star-forming regions of M_(cl)~ 10^(8−9) M_⊙ with sizes of ~200 pc. We find that, at a fixed size, the star formation rate density within these H ii regions is approximately two orders of magnitude greater than those observed in local spiral/starburst galaxies, but consistent with the most massive H II regions in the local Universe such as 30 Doradus. Finally, we compare the spatially resolved nebular emission-line velocity with the Lyα and UV interstellar medium (ISM) lines and find that this galaxy is surrounded by a galactic scale outflow in which the Lyα appears redshifted by ~150 km s^(−1) and the UV-ISM lines blueshifted by ~−200 km s^(−1) from the (systemic) nebular emission. The velocity structure of the outflow mirrors that of the nebular emission suggesting the outflow is young (≲15 Myr), and has yet to burst out of the system. Taken together, these results suggest that this young galaxy is undergoing its first major epoch of mass assembly.

THE CANADA-UNITED KINGDOM DEEP SUBMILLIMETER SURVEY. V. THE SUBMILLIMETER PROPERTIES OF LYMAN BREAK GALAXIES

We have used 850 μm maps obtained as part of the Canada-UK Deep Submillimeter Survey (CUDSS) to investigate the submillimeter properties of Lyman break galaxies (LBGs). We used three samples of LBGs: two from the Canada-France Deep Fields (CFDF) survey covering CUDSS 14 and CUDSS 03, and one from Steidel and collaborators also covering CUDSS 14. We measure a mean flux from both CFDF LBG samples at a level of ~2 σ of 0.414 ± 0.263 mJy for CUDSS 03 and 0.382 ± 0.206 mJy for CUDSS 14, but the Steidel et al. sample is consistent with zero flux. From this we place upper limits on the Lyman break contribution to the 850 μm background of ~20%. We have also measured the cross-clustering between the LBGs and the SCUBA sources. From this measurement we infer a large clustering amplitude of r0 = 11.5 ± 3.0 ± 3.0 h-1 Mpc for the Steidel et al. sample (where the first error is statistical and the second systematic), r0 = 4.5 ± 7.0 ± 5.0 h-1 Mpc for CFDF 14, and r0 = 7.5 ± 7.0 ± 5.0 h-1 Mpc for CFDF 03. The Steidel et al. sample, for which we have the most significant detection of clustering, is also the largest of the three samples and has spectroscopically confirmed redshifts

The importance of major mergers in the build up of stellar mass in brightest cluster galaxies at z = 1

Recent independent results from numerical simulations and observations have shown that brightest cluster galaxies (BCGs) have increased their stellar mass by a factor of almost 2 between z ∼ 0.9 and z ∼ 0.2. The numerical simulations further suggest that more than half this mass is accreted through major mergers. Using a sample of 18 distant galaxy clusters with over 600 spectroscopically confirmed cluster members between them, we search for observational evidence that major mergers do play a significant role. We find a major merger rate of 0.38 ± 0.14 mergers per Gyr at z ∼ 1. While the uncertainties, which stem from the small size of our sample, are relatively large, our rate is consistent with the results that are derived from numerical simulations. If we assume that this rate continues to the present day and that half of the mass of the companion is accreted on to the BCG during these mergers, then we find that this rate can explain the growth in the stellar mass of the BCGs that is observed and predicted by simulations. Major mergers therefore appear to be playing an important role, perhaps even the dominant one, in the build up of stellar mass in these extraordinary galaxies.

The SCUBA-2 Cosmology Legacy Survey: blank-field number counts of 450-μm-selected galaxies and their contribution to the cosmic infrared background

The first deep blank-field 450 mu m map (1 sigma approximate to 1.3 mJy) from the Submillimetre Common-User Bolometer Array-2 SCUBA-2 Cosmology Legacy Survey (S2CLS), conducted with the James Clerk Maxwell Telescope (JCMT) is presented. Our map covers 140 arcmin(2) of the Cosmological Evolution Survey field, in the footprint of the Hubble Space Telescope (HST) Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. Using 60 submillimetre galaxies detected at >= 3.75s, we evaluate the number counts of 450-mu m-selected galaxies with flux densities S-450 > 5 mJy. The 8 arcsec JCMT beam and high sensitivity of SCUBA-2 now make it possible to directly resolve a larger fraction of the cosmic infrared background (CIB, peaking at. similar to 200 mu m) into the individual galaxies responsible for its emission than has previously been possible at this wavelength. At S450 > 5 mJy, we resolve (7.4 +/- 0.7) x 10(-2) MJy sr(-1) of the CIB at 450 mu m (equivalent to 16 +/- 7 per cent of the absolute brightness measured by the Cosmic Background Explorer at this wavelength) into point sources. A further similar to 40 per cent of the CIB can be recovered through a statistical stack of 24 mu m emitters in this field, indicating that the majority (approximate to 60 per cent) of the CIB at 450 mu m is emitted by galaxies with S450 > 2 mJy. The average redshift of 450 mu m emitters identified with an optical/near-infrared counterpart is estimated to be = 1.3, implying that the galaxies in the sample are in the ultraluminous class (LIR approximate to 1.1 x 1012 L approximate to). If the galaxies contributing to the statistical stack lie at similar redshifts, then the majority of the CIB at 450 mu m is emitted by galaxies in the luminous infrared galaxy (LIRG) class with LIR > 3.6 x 1011 L-circle dot.

Infrared Spectrograph Spectroscopy and Multi-Wavelength Study of Luminous Star-Forming Galaxies at z ≃ 1.9

We analyze a sample of galaxies chosen to have F_(24μm) > 0.5 mJy and satisfy a certain IRAC color criterion. Infrared Spectrograph (IRS) spectra yield redshifts, spectral types, and polycyclic aromatic hydrocarbons (PAH) luminosities, to which we add broadband photometry from optical through IRAC wavelengths, MIPS from 24-160 μm, 1.1 mm, and radio at 1.4 GHz. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z ~ 1.9. This is the first starburst (SB)-dominated ultraluminous infrared galaxies (ULIRG) sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband spectral energy distributions for SB galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples—local SB galaxies, local active galactic nucleus (AGN)/ULIRGs, and a second 24 μm-luminous z ~ 2 sample dominated by AGN. L_(PAH)/L_(IR) for the new z ~ 2 SB sample is the highest ever seen, some three times higher than in local SBs, whereas in AGNs this ratio is depressed below the SB trend, often severely. Several pieces of evidence imply that AGNs exist in this SB-dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The Advanced Camera for Surveys images show that most objects have very extended morphologies in the rest-frame ultraviolet band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by SB; and the total infrared luminosity density contributed by this type of objects is 0.9-2.6 × 10^7 L_☉ Mpc^(–3).

High spatial resolution observations of CUDSS14A: a SCUBA-selected ultraluminous galaxy at high redshift

We present a high-resolution millimetre interferometric image of the brightest SCUBAselected galaxy from the Canada-UK deep SCUBA survey (CUDSS). We make a very clear detection at 1.3 mm, but fail to resolve any structure in the source. The interferometric position is within 1.5 arcsec of the SCUBA 850 � m centroid, and also within 1.5 arcsec of a 44 � Jy radio source and a very faint, extremely red galaxy which we had previously identified as the submillimetre source. We also present new optical and infrared imaging, and infrared spectroscopy of this source. We model the overall spectral energy distribution and conclude that it lies within the redshift range 2< z < 4.5. The submm/FIR luminosity of CUDSS14A is very weakly dependent on redshift within the constrained range, and is roughly 4×10 12 L⊙ (for H0=75 and an assumed Arp220-like spectrum), which implies a star-formation rate �1000 M⊙ yr −1 . We derive an approximate gas mass of � 10 10 M⊙ which would imply the current starforming activity cannot be sustained for longer than about 10 million years. With the present data however we are unable to rule out a significant AGN contribution to the total luminosity.

The Variable Stars and Blue Horizontal Branch of the Metal-rich Globular Cluster NGC 6441

We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53) globular cluster NGC 6441. Our color-magnitude diagram shows that the extended blue horizontal branch seen in Hubble Space Telescope data exists in the outermost reaches of the cluster. About 17% of the horizontal-branch stars lie blueward and brightward of the red clump. The red clump itself slopes nearly parallel to the reddening vector. A component of this slope is due to differential reddening, but part is intrinsic. The blue horizontal-branch stars are more centrally concentrated than the red clump stars, suggesting mass segregation and a possible binary origin for the blue horizontal-branch stars. We have discovered ~50 new variable stars near NGC 6441, among them eight or more RR Lyrae stars that are highly probable cluster members. Comprehensive period searches over the range 0.2?1.0 days yielded unusually long periods (0.5?0.9 days) for the fundamental pulsators compared with field RR Lyrae of the same metallicity. Three similar long-period RR Lyrae are known in other metal-rich globular clusters. With over 10 examples in hand, it seems that a distinct subclass of long-period, metal-rich RR Lyrae stars is emerging. It appears that these stars have the same intrinsic colors as normal RR Lyrae. Using the minimum-light color of the RR Lyrae, we determine the mean cluster reddening to be E(B - V) = 0.45 ? 0.03 mag, with a significant variation in reddening across the face of the cluster. The observed properties of the horizontal-branch stars are in reasonable agreement with recent models that invoke deep mixing to enhance the atmospheric helium abundance, while they conflict with models that assume high initial helium abundance. The light curves of the c-type RR Lyrae seem to have unusually long rise times and sharp minima. Reproducing these light curves in stellar pulsation models may provide another means of constraining the physical variables responsible for the anomalous blue horizontal-branch extension and sloped red clump observed in NGC 6441.

The SCUBA-2 Cosmology Legacy Survey: 850 μm maps, catalogues and number counts

We present a catalogue of similar to 3000 submillimetre sources detected (>= 3.5 sigma) at 850 mu m over similar to 5 deg(2) surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850 mu m, increasing the sample size of 850 mu m selected submillimetre galaxies by an order of magnitude. The wide 850 mu m survey component of S2CLS covers the extragalactic fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1s depth of S2CLS is 1.2 mJy beam(-1), approaching the SCUBA-2 850 mu m confusion limit, which we determine to be sigma(c) approximate to 0.8 mJy beam(-1). We measure the 850 mu m number counts, reducing the Poisson errors on the differential counts to approximately 4 per cent at S-850 approximate to 3 mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5 degrees-1 degrees scales are generally within 50 per cent of the S2CLS mean for S-850 > 3 mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2 sigma) density enhancement in GOODS-North. The observed counts are in reasonable agreement with recent phenomenological and semi-analytic models, although determining the shape of the faint-end slope (S-850 10 mJy there are approximately 10 sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850 mu m emission