arXiv subjects

Alexandra Pope

Publications and source records attributed to Alexandra Pope.

108 records · Page 6Linked to original sources

arXiv2010

Searching for the highest redshift sources in 250-500 micron submillimeter surveys

We explore a technique for identifying the highest redshift (z>4) sources in Herschel/SPIRE and BLAST submillimeter surveys by localizing the position of the far-infrared dust peak. Just as Spitzer/IRAC was used to identify stellar `bump' sources, the far-IR peak is also a redshift indicator; although, the latter also depends on the average dust temperature. We demonstrate the wide range of allowable redshifts for a reasonable range of dust temperatures and show that it is impossible to constraint the redshift of individual objects using solely the position of the far-IR peak. By fitting spectral energy distribution models to simulated Herschel/SPIRE photometry we show the utility of radio and/or far-infrared data in breaking this degeneracy. With prior knowledge of the dust temperature distribution it is possible to obtain statistical samples of high redshift submillimeter galaxy candidates. We apply this technique to the BLAST survey of ECDFS to constrain the number of dusty galaxies at z>4. We find 8 +/- 2 galaxies with flux density ratios of S500>S350; this sets an upper limit of 17 +/- 4 deg-2 if we assume all are at z>4. This is <35% of all 500 micron-selected galaxies down to S500>45 mJy (LIR>2e13Lsun for z>4). Modeling with conventional temperature and redshift distributions estimates the percentage of these 500 micron peak galaxies at z>4 to be between 10-85%. Our results are consistent with other estimates of the number density of very high redshift submillimeter galaxies and follows the decline in the star formation rate density at z>4.

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arXiv2010

New Observational Constraints and Modeling of the Infrared Background: Dust Obscured Star-Formation at z>1 and Dust in the Outer Solar System

We provide measurements of the integrated galaxy light at 70, 160, 250, 350 and 500 micron using deep far-infrared and submillimeter data from space (Spitzer) and balloon platform (BLAST) extragalactic surveys. We use the technique of stacking at the positions of 24 micron sources, to supplement the fraction of the integrated galaxy light that is directly resolved through direct detections. We demonstrate that the integrated galaxy light even through stacking, falls short by factors of 2-3 in resolving the extragalactic far-infrared background. We also show that previous estimates of the integrated galaxy light (IGL) through stacking, have been biased towards high values. This is primarily due to multiple counting of the far-infrared/submillimeter flux from 24 micron sources which are clustered within the large point spread function of a brighter far-infrared source. Using models for the evolution of the luminosity function at z<1.2 which are constrained by observations at 24 micron and 70 micron, and which are consistent with the results from the stacking analysis, we find that galaxies at z<1.2, account for ~95-55% of the extragalactic far-infrared background in the ~70-500 micron range respectively. This places strong upper limits on the fraction of dust obscured star-formation at z>1, which are remarkably, below the values derived from the extinction corrected ultraviolet luminosities of galaxies. We use the results to make predictions for the nature of galaxies that extragalactic surveys with Herschel Space Observatory will reveal. Finally, from our constraints on the far-infrared IGL, we provide evidence for the existence of ice mantle dust, orbiting the sun at a distance of ~40 AU, which is contributing intensity to both the near- and far-infrared background. [ABRIDGED]

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arXiv2010

Starburst or AGN Dominance in Submillimetre-Luminous Candidate AGN?

It is widely believed that ultraluminous infrared (IR) galaxies and active galactic nuclei (AGN) activity are triggered by galaxy interactions and merging, with the peak of activity occurring at z~2, where submillimetre galaxies are thousands of times more numerous than local ULIRGs. In this evolutionary picture, submillimetre galaxies (SMGs) would host an AGN, which would eventually grow a black hole (BH) strong enough to blow off all of the gas and dust leaving an optically luminous QSO. To probe this evolutionary sequence we have focussed on the 'missing link' sources, which demonstrate both strong starburst (SB) and AGN signatures, in order to determine if the SB is the main power source even in SMGs when we have evidence that an AGN is present from their IRAC colours. The best way to determine if a dominant AGN is present is to look for their signatures in the mid-infrared with the Spitzer IRS, since often even deep X-ray observations miss identifying the presence of AGN in heavily dust-obscured SMGs. We present the results of our audit of the energy balance between star-formation and AGN within this special sub-population of SMGs -- where the BH has grown appreciably to begin heating the dust emission.

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arXiv2009

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)

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arXiv2008

Clustering of Dust-Obscured Galaxies at z ~ 2

We present the angular autocorrelation function of 2603 dust-obscured galaxies (DOGs) in the Bootes field of the NOAO Deep Wide-Field Survey. DOGs are red, obscured galaxies, defined as having R-[24] \ge 14 (F_24/F_R \ga 1000). Spectroscopy indicates that they are located at 1.5 \la z \la 2.5. We find strong clustering, with r_0 = 7.40^{+1.27}_{-0.84} Mpc/h for the full F_24 > 0.3 mJy sample. The clustering and space density of the DOGs are consistent with those of submillimeter galaxies, suggestive of a connection between these populations. We find evidence for luminosity-dependent clustering, with the correlation length increasing to r_0 = 12.97^{+4.26}_{-2.64} Mpc/h for brighter (F_24 > 0.6 mJy) DOGs. Bright DOGs also reside in richer environments than fainter ones, suggesting these subsamples may not be drawn from the same parent population. The clustering amplitudes imply average halo masses of log M = 12.2^{+0.3}_{-0.2} Msun for the full DOG sample, rising to log M = 13.0^{+0.4}_{-0.3} Msun for brighter DOGs. In a biased structure formation scenario, the full DOG sample will, on average, evolve into ~ 3 L* present-day galaxies, whereas the most luminous DOGs may evolve into brightest cluster galaxies.

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arXiv2008

The nature of faint Spitzer-selected dust-obscured galaxies

We use deep far-IR, submm, radio and X-ray imaging and mid-IR spectroscopy to explore the nature of a sample of Spitzer-selected dust-obscured galaxies (DOGs) in GOODS-N. A sample of 79 galaxies satisfy the criteria R-[24]>14 (Vega) down to S24>100 microJy. Twelve of these galaxies have IRS spectra available which we use to measure redshifts and classify these objects as being dominated by star formation or active galactic nuclei (AGN) activity in the mid-IR. The IRS spectra and Spitzer photometric redshifts confirm that the DOGs lie in a tight redshift distribution around z~2. Based on mid-IR colors, 80% of DOGs are likely dominated by star formation; the stacked X-ray emission from this sub-sample of DOGs is also consistent with star formation. Since only a small number of DOGs are individually detected at far-IR and submm wavelengths, we use a stacking analysis to determine the average flux from these objects and plot a composite IR (8-1000 microns) spectral energy distribution (SED). The average luminosity of these star forming DOGs is LIR~1e12 Lsun. We compare the average star forming DOG to the average bright (S850>5 mJy) submillimeter galaxy (SMG); the S24>100 microJy DOGs are 3 times more numerous but 8 times less luminous in the IR. The far-IR SED shape of DOGs is similar to that of SMGs (average dust temperature of around 30 K) but DOGs have a higher mid-IR to far-IR flux ratio. The average star formation-dominated DOG has a star formation rate of 200 Msun/yr which, given their space density, amounts to a contribution of 0.01 Msun/yr/Mpc3 (or 5-10%) to the star formation rate density at z~2. We use the composite SED to predict the average flux of DOGs in future Herschel/PACS 100 micron and SCUBA-2 450 micron surveys and show that the majority of them will be detected.

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arXiv2008

Molecular Gas in the z=1.2 Ultraluminous Merger GOODS J123634.53+621241.3

We report the detection of CO(2-1) emission from the z=1.2 ultraluminous infrared galaxy (ULIRG) GOODS J123634.53+621241.3 (also known as the sub-millimeter galaxy GN26). These observations represent the first discovery of high-redshift CO emission using the new Combined Array for Research in Millimeter-Wave Astronomy (CARMA). Of all high-redshift (z>1) galaxies within the GOODS-North field, this source has the largest far-infrared (FIR) flux observed in the Spitzer 70um and 160um bands. The CO redshift confirms the optical identification of the source, and the bright CO(2-1) line suggests the presence of a large molecular gas reservoir of about 7x10^10 M(sun). The infrared-to-CO luminosity ratio of L(IR)/L'(CO) = 80+/-30 L(sun) (K Km/s pc^2)^-1 is slightly smaller than the average ratio found in local ULIRGs and high-redshift sub-millimeter galaxies. The short star-formation time scale of about 70 Myr is consistent with a starburst associated with the merger event and is much shorter than the time scales for spiral galaxies and estimates made for high-redshift galaxies selected on the basis of their B-z and z-K colors.

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arXiv2007

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.

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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.

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arXiv2007

Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies

We present deep mid-infrared spectroscopy with the Spitzer Space Telescope of 13 submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North (GOODS-N) field. We find strong polycyclic aromatic hydrocarbon (PAH) emission in all of our targets which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and active galactic nuclei (AGN) activity to the mid-IR emission. In the high signal-to-noise ratio composite spectrum of SMGs, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. We find that the mid-IR properties of SMGs are distinct from those of 24 micron -selected ULIRGs at z~2; the former are predominantly dominated by star formation while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template spectral energy distributions to determine the best estimate of the total infrared luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multi-wavelength analysis shows that the total infrared luminosity, LIR, in SMGs, is dominated by star formation and not AGN activity. We find that high redshift SMGs lie on the relation between LIR and LPAH6.2 (or LPAH7.7 or LPAH11.3) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the star formation rate in SMGs. SMGs are consistent with being a short lived cool phase in a massive merger. Although there appears to be an AGN present in many SMGs, it does not appear to have become strong enough to heat the dust and dominate the mid- or far-infrared emission.

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arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) - IV: Radio-mm-FIR photometric redshifts

We present the redshift distribution of the SHADES galaxy population based on the rest-frame radio-mm-FIR colours of 120 robustly detected 850um sources in the Lockman Hole East (LH) and Subaru XMM-Newton Deep Field (SXDF). The redshift distribution derived from the full SED information is shown to be narrower than that determined from the radio-submm spectral index, as more photometric bands contribute to a higher redshift accuracy. The redshift distribution of sources derived from at least two photometric bands peaks at z ~ 2.4 and has a near-Gaussian distribution, with 50 per cent (interquartile range) of sources at z=1.8-3.1. We find a statistically-significant difference between the measured redshift distributions in the two fields; the SXDF peaking at a slightly lower redshift (median z ~ 2.2) than the LH (median z ~ 2.7), which we attribute to the noise-properties of the radio observations. We demonstrate however that there could also be field-to-field variations that are consistent with the measured differences in the redshift distributions, and hence, that the incomplete area observed by SHADES with SCUBA, despite being the largest sub-mm survey to date, may still be too small to fully characterize the bright sub-mm galaxy population. Finally we present a brief comparison with the predicted, or assumed, redshift distributions of sub-mm galaxy formation and evolution models, and we derive the contribution of these SHADES sources and the general sub-mm galaxy population to the star formation-rate density at different epochs.

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arXiv2006

The Infrared Properties of Submillimeter Galaxies: Clues From Ultra-Deep 70 Micron Imaging

We present 70 micron properties of submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey (GOODS) North field. Out of thirty submillimeter galaxies (S_850 > 2 mJy) in the central GOODS-N region, we find two with secure 70 micron detections. These are the first 70 micron detections of SMGs. One of the matched SMGs is at z ~ 0.5 and has S_70/S_850 and S_70/S_24 ratios consistent with a cool galaxy. The second SMG (z = 1.2) has infrared-submm colors which indicate it is more actively forming stars. We examine the average 70 micron properties of the SMGs by performing a stacking analysis, which also allows us to estimate that S_850 > 2 mJy SMGs contribute 9 +- 3% of the 70 micron background light. The S_850/S_70 colors of the SMG population as a whole is best fit by cool galaxies, and because of the redshifting effects these constraints are mainly on the lower z sub-sample. We fit Spectral Energy Distributions (SEDs) to the far-infrared data points of the two detected SMGs and the average low redshift SMG (z_{median}= 1.4). We find that the average low-z SMG has a cooler dust temperature than local ultraluminous infrared galaxies (ULIRGs) of similar luminosity and an SED which is best fit by scaled up versions of normal spiral galaxies. The average low-z SMG is found to have a typical dust temperature T = 21 -- 33 K and infrared luminosity L_{8-1000 micron} = 8.0 \times 10^11 L_sun. We estimate the AGN contribution to the total infrared luminosity of low-z SMGs is less than 23%.

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arXiv2006

Infrared spectral energy distributions of submillimetre galaxies

Submillimetre (sub-mm) galaxies have very high infrared (IR) luminosities and are thousands of times more numerous at z~2 than local ultra-luminous IR galaxies. They therefore represent a key phase in galaxy evolution which can be missed in optical surveys. Determining their contribution to the global star formation rate requires dissecting their IR emission into contributions from starbursts (SB) and active galactic nuclei (AGN). There are several examples of AGN systems which masquerade as SBs in either the IR or X-ray, and SBs can often look like AGN in some wavebands. A combination of SB and AGN emission is not unreasonable, given models of merger-driven evolution. To assess in detail what powers the intense IR luminosity of sub-mm galaxies it is important to obtain a complete multi-wavelength picture. Mid-IR spectroscopy is a particularly good probe of where the intense IR luminosity is coming from. We present the first results from a program to obtain Spitzer IRS spectroscopy of a sample of high redshift galaxies in the GOODS-N field, a large fraction of which are sub-mm galaxies. This field is already home to the deepest X-ray, optical, IR and radio data. We piece together the sub-mm data with the Spitzer photometry and IRS spectra to provide a well sampled IR spectral energy distribution (SED) of sub-mm galaxies and determine the contribution to the bolometric luminosity from the AGN and SB components.

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arXiv2006

The Hubble Deep Field North SCUBA Super-map IV - Characterizing submillimetre galaxies using deep Spitzer imaging

We present SEDs, Spitzer colours, and IR luminosities for 850 micron selected galaxies in the GOODS-N field. Using the deep Spitzer Legacy images and new data and reductions of the VLA-HDF radio data, we find statistically secure counterparts for 60 per cent (21/35) of our submm sample, and identify tentative counterparts for another 12 objects. This is the largest sample of submm galaxies with statistically secure counterparts detected in the radio and with Spitzer. We find that in most cases the 850 micron emission is dominated by a single 24 micron source. A composite rest-frame SED shows that the submm sources peak at longer wavelengths than those of local ULIRGs of the same luminosity and therefore appear to be cooler. The SEDs of submm galaxies are also different from those of their high redshift neighbours, the near-IR selected BzK galaxies, whose mid-IR to radio SEDs are more like those of local ULIRGs. Using 24 micron. 850 micron and 1.4 GHz observations, we fit templates that span the mid-IR through radio to derived the integrated IR luminosity of the submm galaxies. By themselves, 24 micron and radio fluxes are able to predict LIR reasonably well because they are relatively insensitive to temperature. However, the submm flux by itself consistently overpredicts LIR when using spectral templates which obey the local ULIRG temperature-luminosity relation. The shorter Spitzer wavelengths sample the stellar bump at the redshifts of the submm sources, and we find that the Spitzer photometry alone provides a model independent estimate of the redshift. Using X-ray and mid-IR data, only 5 per cent of our secure counterparts show strong evidence for an active galactic nucleus dominating the IR luminosity.

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arXiv2006

Using Spitzer to probe the nature of submillimetre galaxies in GOODS-N

How does the submm galaxy population detected by SCUBA fit into galaxy evolution? How do these rare starbursting systems, which contribute significantly to high redshift star-formation, relate to other galaxy populations? Deep radio observations have been most useful for studying these systems, but still leave a significant fraction of the population unidentified. Now with the deep IRAC and MIPS images from the GOODS Spitzer Legacy program and a re-analysis of the deep radio data, we are able to identify counterparts for a large fraction of SCUBA galaxies in GOODS-N. All of these counterparts are detected by Spitzer. Given the vast multi-wavelength data in this field, we can study the spectral energy distributions (SEDs) of these systems and determine what is fueling their intense infrared luminosities. A rest-frame composite optical-to-radio SED for all spectroscopically identified submm sources shows that the average SCUBA galaxy is consistent with models of ultraluminous starburst galaxies, although cooler than those observed locally. Because of this, the submm flux alone consistently overestimates IR when using spectral templates which obey the local ULIRG temperature-luminosity relation. The wide range of 24/850 micron flux ratios as a function of redshift indicates the presence of strong mid-IR features, to be confirmed with deep IRS spectroscopy. The IRAC colours of the submm systems provide useful redshift constraints, since, at these redshifts, IRAC samples the stellar bump. The Spitzer photometry of this large sample of submm galaxies has allowed us to put constraints on many of the outstanding issues in submm astronomy.

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arXiv2006

On the cross-correlation of sub-mm sources and optically-selected galaxies

Bright sub-mm galaxies are expected to arise in massive highly-biased haloes, and hence exhibit strong clustering. We argue that a valuable tool for measuring these clustering properties is the cross-correlation of sub-mm galaxies with faint optically-selected sources. We analyze populations of SCUBA-detected and optical galaxies in the GOODS-N survey area. Using optical/IR photometric-redshift information, we search for correlations induced by two separate effects: (1) cosmic magnification of background sub-mm sources by foreground dark matter haloes traced by optical galaxies at lower redshifts; and (2) galaxy clustering due to sub-mm and optical sources tracing the same population of haloes where their redshift distributions overlap. Regarding cosmic magnification, we find no detectable correlation. Our null result is consistent with a theoretical model for the cosmic magnification, and we show that a dramatic increase in the number of sub-mm sources will be required to measure the effect reliably. Regarding clustering, we find evidence at the 3.5-sigma level for a cross-correlation between sub-mm and optical galaxies analyzed in identical photometric redshift slices. The data hint that the sub-mm sources have an enhanced bias parameter compared to the optically-selected population (with a significance of 2-sigma). The next generation of deep sub-mm surveys can potentially perform an accurate measurement of each of these cross-correlations, adding a new set of diagnostics for understanding the development of massive structure in the Universe.

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arXiv2005

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.

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arXiv2004

Optical and near-infrared properties of submillimetre galaxies in the GOODS-North field

We investigate the photometric redshift distribution, colours and morphologies of a large sample of submillimetre sources in the GOODS-North region. With the depth achieved by the deep ACS images, optical counterparts have been found for all the radio-detected sub-mm sources, and we have used several techniques to identify counterparts to the radio-undetected sources. The sub-mm counterparts have a median photometric redshift, from optical and near-IR photometry, of 1.9. Our sample contains a much higher fraction of EROs than other sub-mm surveys, and larger angular sizes than the average galaxy at all redshifts. The sub-mm galaxy counterparts are consistent with massive star-forming galaxies at high redshift.

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