The evolution of submillimetre galaxies: two populations and a redshift cut-off

We explore the epoch dependence of number density and star-formation rate for submillimetre galaxies (SMGs) found at 850 um. The study uses a sample of 38 SMG in the GOODS-N field, for which cross-waveband identifications have been obtained for 35/38 members together with redshift measurements or estimates. A maximum-likelihood analysis is employed, along with the `single-source-survey' technique. We find a diminution in both space density and star formation rate at z > 3, closely mimicking the redshift cut-offs found for QSOs selected in different wavebands. The diminution in redshift is particularly marked, at a significance level too small to measure. The data further suggest, at a significance level of about 0.001, that two separately-evolving populations may be present, with distinct luminosity functions. These results parallel the different evolutionary behaviours of LIRGs and ULIRGs, and represent another manifestation of `cosmic down-sizing', suggesting that differential evolution extends to the most extreme star-forming galaxies.Comment: 12 pages, 11 figures, MNRAS accepted. The new version, as accepted for MNRAS, is substantially revised, with more detail on sample selection as well as extended significance tests of the result

On the X-ray properties of submm-selected galaxies

We present an analysis of the X-ray properties of 35 submm galaxies(SMGs) in the CDF-N. Using a sample of robust 850micron-selected galaxies, with accurate positions from Spitzer and/or radio counterparts, we find 16 objects (45+/-8%) with significant X-ray detections in the 2Ms Chandra data. 6 of these SMGs (~17+/-6%) have measured X-ray luminosities or upper limits consistent with those expected based on the far-infrared (FIR) or radio-derived star formation rate (SFR), and hence with the X-rays coming solely from star formation. In another 7 sources (20+/-7%) a dominant AGN contribution to the X-ray emission is required, while in 3 more it is unclear whether stellar process or accretion are responsible. Stacking of the X-ray undetected SMGs reveals a highly significant detection. If due to star formation, this corresponds to an average X-ray derived SFR of ~150 Msun/yr. We deduce that the AGN fraction in SMGs based on X-ray observations is 20-29 (+/-7) %, which is towards the lower limit of previous estimates. Spectral analysis shows that in general the SMGs are not heavily obscured in the X-ray but most of the SMGs classfied as AGN show absorption with N_H in excess of 10^22 cm^-2. Of the secure AGN, the bolometric luminosity appears to be dominated by the AGN in only 3 cases. In ~85% of the SMGs, the X-ray spectrum effectively rules out an AGN contribution that dominates the bolometric emission, even if the AGN is Compton thick. The evidence therefore suggests that intense star formation accounts for both the FIR and X-ray emission in most SMGs. We argue that, rather than having an especially high AGN fraction or duty cycle, SMGs have a high X-ray detection rate at very faint fluxes partly because of their high star formation rates and, in rarer cases, because the submm emission is from an AGN.(abridged)Comment: 12 pages, 5 figures, accepted for publication in MNRA

The local FIR Galaxy Colour-Luminosity distribution: A reference for BLAST, and Herschel/SPIRE sub-mm surveys

We measure the local galaxy far-infrared (FIR) 60-to-100 um colour-luminosity distribution using an all-sky IRAS survey. This distribution is an important reference for the next generation of FIR--submillimetre surveys that have and will conduct deep extra-galactic surveys at 250--500 um. With the peak in dust-obscured star-forming activity leading to present-day giant ellipticals now believed to occur in sub-mm galaxies near z~2.5, these new FIR--submillimetre surveys will directly sample the SEDs of these distant objects at rest-frame FIR wavelengths similar to those at which local galaxies were observed by IRAS. We have taken care to correct for temperature bias and evolution effects in our IRAS 60 um-selected sample. We verify that our colour-luminosity distribution is consistent with measurements of the local FIR luminosity function, before applying it to the higher-redshift Universe. We compare our colour-luminosity correlation with recent dust-temperature measurements of sub-mm galaxies and find evidence for pure luminosity evolution of the form (1+z)^3. This distribution will be useful for the development of evolutionary models for BLAST and SPIRE surveys as it provides a statistical distribution of rest-frame dust temperatures for galaxies as a function of luminosity.Comment: 12 pages, 7 figures. MNRAS in press. This revision matches final published version. Fixes typos in footnote 1 and equation 8. Minor modifications to text and references. Final results unchange

Medicalisation of Vaping in the UK? E-cigarette users’ perspectives on the merging of commercial and medical routes to vaping

Background: In the UK, most smokers choosing e-cigarettes to quit smoking will access vaping via commercial routes. In recent years, however, a shift towards medicalisation of vaping has become apparent, with public health guidance supporting e-cigarettes for smoking cessation and increased partnership working between healthcare professionals and the vaping industry. To achieve the UK’s Smokefree 2030 target, the UK Government has set out measures to utilise e-cigarettes in NHS settings and to move towards streamlining processes to make e-cigarettes available to a million smokers. This paper aims to understand acceptability of different approaches by seeking perspectives of people with lived experience of e-cigarette use for smoking cessation. Methods: Mixed methods data, collected between March 2018 and March 2019 as part of a broader study of e-cigarette use trajectories (ECtra study). Data here relate to views of partnership working and medicalisation of vaping extracted from 136 interviews/extended surveys of people who had used e-cigarettes to try to stop smoking. Qualitative data were thematically analysed. Participant ratings of interventions were presented descriptively and differences in participant characteristics and ratings were reported. Results: Three qualitative themes were identified: pro-partnership, anti-partnership and medicalisation dissonance. Medicalisation was discussed for its potential to reassure smokers about e-cigarette harms and its potential to reach smokers from disadvantaged backgrounds. Concerns were raised about cost effectiveness, quality of support, conflicts of interest, and limiting product choice. Most participants rated interventions involving partnership working as potentially helpful in switching from smoking to vaping. There were no statistically significant associations between age, gender and socio-economic status and helpfulness ratings. Conclusions: Both commercial and medical routes to vaping offer perceived benefits to vapers and may complement and reinforce each other to support smoking cessation

Searching for massive galaxies at z>=3.5 in GOODS-North

We constrain the space density and properties of massive galaxy candidates at redshifts of z>=3.5 in the GOODS-N field. By selecting sources in the Spitzer+IRAC bands, a highly stellar mass-complete sample is assembled,including massive galaxies which are very faint in the optical/near-IR bands that would be missed by samples selected at shorter wavelengths. The z>=3.5 sample was selected down to 23 mag at 4.5 micron using photometric redshifts that have been obtained by fitting the galaxies SEDs at optical, near-IR and IRAC bands. We also require that the brightest band in which candidates are detected is the IRAC 8 micron band in order to ensure that the near-IR 1.6 micron (rest-frame) peak is falling in or beyond this band. We found 53 z>=3.5 candidates, with masses in the range of M~10^10-10^11M_sun. At least ~81% of these galaxies are missed by traditional Lyman Break selection methods based on UV light. Spitzer+MIPS emission is detected for 60% of the sample of z>=3.5 galaxy candidates. Although in some cases this might suggest a residual contamination from lower redshift star-forming galaxies or AGN, 37% of these objects are also detected in the sub-mm/mm bands in recent SCUBA,AzTEC and MAMBO surveys, and have properties fully consistent with vigorous starburst galaxies at z>=3.5. The comoving number density of galaxies with stellar masses >= 5x10^10M_sun(a reasonable stellar mass completeness limit for our sample) is 2.6x10^-5Mpc^-3 (using the volume within 3.5<z<5), and the corresponding stellar mass density is ~2.9x10^6M_sunMpc^-3, or~3% of the local density above the same stellar mass limit.For the sub-sample of MIPS-undetected galaxies,we find a number density of ~0.97x10^-5Mpc^-3 and a stellar mass density of ~1.15x10^6M_sun Mpc^-3.[abridged]Comment: Accepted by A&A; 35 pages, 15 figures, references update

The contribution of AGN to the sub-mm population

We find that X-ray sources in the Extended Chandra Deep Field South are strongly spatially correlated with LABOCA 870 micron sources. We investigate the dependence of this correlation on X-ray flux, hardness ratio and column density, finding that specifically faint and absorbed X-ray sources are significant sub-mm emitters. In the X-ray source redshift subsample we confirm the previous result that higher luminosity sources (L_X>10^44 ergs/s) have greater 870um fluxes but we also find that this subsample selects against absorbed sources, faint in X-ray flux. Overall, we find that X-ray sources contribute 1.5 \pm 0.1 Jy/deg^2 to the sub-mm background, ~3% of the total, in agreement with the prediction of an obscured AGN model which also gives a reasonable fit to the bright sub-mm source counts. This non-unified model also suggests that when Compton-thick, X-ray-undetected sources are included, then the fractional AGN contribution to the sub-mm background would rise from ~3% to a total of 25-40%, although in a unified model the AGN contribution would only reach ~13%, because the sub-mm flux of the X-ray sources is then more representative of the whole AGN population. Measurements of the dependence of sub-mm flux on X-ray flux, luminosity and column density all agree well with the predictions of the non-unified AGN model. Heavily absorbed, X-ray-undetected AGN could explain the further cross-correlation we find between sub-mm sources and z > 0.5 red galaxies. We conclude that sub-mm galaxies may contain the long-sought absorbed AGN population needed to explain the X-ray background.Comment: 15 pages, 13 figures; submitted to MNRA

Coastal Tropical Convection in a Stochastic Modeling Framework

Recent research has suggested that the overall dependence of convection near coasts on large-scale atmospheric conditions is weaker than over the open ocean or inland areas. This is due to the fact that in coastal regions convection is often supported by meso-scale land-sea interactions and the topography of coastal areas. As these effects are not resolved and not included in standard cumulus parametrization schemes, coastal convection is among the most poorly simulated phenomena in global models. To outline a possible parametrization framework for coastal convection we develop an idealized modeling approach and test its ability to capture the main characteristics of coastal convection. The new approach first develops a decision algorithm, or trigger function, for the existence of coastal convection. The function is then applied in a stochastic cloud model to increase the occurrence probability of deep convection when land-sea interactions are diagnosed to be important. The results suggest that the combination of the trigger function with a stochastic model is able to capture the occurrence of deep convection in atmospheric conditions often found for coastal convection. When coastal effects are deemed to be present the spatial and temporal organization of clouds that has been documented form observations is well captured by the model. The presented modeling approach has therefore potential to improve the representation of clouds and convection in global numerical weather forecasting and climate models.Comment: Manuscript submitted for publication in Journal of Advances in Modeling Earth System

The Hubble Deep Field North SCUBA Super-map III - Optical and near-infrared properties of submillimetre galaxies

We present a new sub-mm Super-map in the HDF-North region (GOODS-North field), containing 40 statistically robust sources at 850 microns. This map contains additional data, and several new sources, including one of the brightest blank-sky extragalactic sub-mm sources ever detected. We have used the ACS HST images and ground-based near-IR observations from GOODS to develop a systematic approach for counterpart identification. 72 per cent of our sources with optical coverage have a unique optical counterpart using our new techniques for counterpart identification, and an additional 18 per cent have more than one possibility that meet our criteria in the ACS images. We have found a much higher ERO rate than other sub-mm surveys, due to the increased depth in the optical images. The median photometric redshift (and quartile range), from optical and near-infrared data, is 1.7 (1.3-2.5) for the radio-detected sub-mm sources, and rises to 2.3 (1.3-2.7) for the radio-undetected sub-sample. We find interesting correlations between the 850 micron flux and both the i magnitude and the photometric redshift, from which there appears to be an absence of high redshift faint counterparts to the lower flux density SCUBA sources. While the quantitative morphologies span a range of values, in general the sub-mm galaxies show larger sizes and a higher degree of asymmetry than other galaxy populations at the same redshifts.Comment: 20 pages, 11 eps figures. Accepted for publication in MNRAS December 17, 200