arXiv subjects

Stephen Serjeant

Publications and source records attributed to Stephen Serjeant.

At least 109 records · Page 6Linked to original sources

arXiv2008

Timeline analysis and wavelet multiscale analysis of the AKARI All-Sky Survey at 90 micron

We present a careful analysis of the point source detection limit of the AKARI All-Sky Survey in the WIDE-S 90 $μ$m band near the North Ecliptic Pole (NEP). Timeline Analysis is used to detect IRAS sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90 $μ$m flux of a source. Combined with a robust noise measurement, the point source flux detection limit at S/N $>5$ for a single detector row is $1.1\pm0.1$ Jy which corresponds to a point source detection limit of the survey of $\sim$0.4 Jy. Wavelet transform offers a multiscale representation of the Time Series Data (TSD). We calculate the continuous wavelet transform of the TSD and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above $4σ$ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non-IRAS sources detected in Timeline Analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non-IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds.

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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

Optical Identification of 15 Micron Sources in the AKARI Performance Verification Field toward the North Ecliptic Pole

We present the results of optical identifications for 257 mid-infrared sources detected with a deep 15um survey over approximately 80 arcmin^2 area in the AKARI performance verification field near the North Ecliptic Pole. The 15um fluxes of the sources range from 1 mJy down to 40 uJy, approximately a half of which are below 100 uJy. Optical counterparts were searched for within a 2-3 arcsec radius in both the BVRi'z' catalog generated by using the deep Subaru/Suprime-cam field which covers one-third of the performance verification field, and the g'r'i'z' catalog based on observations made with MegaCam at CFHT. We found B-R and R-z' colours of sources with successful optical identifications are systematically redder than that of the entire optical sample in the same field. Moreover, approximately 40% of the 15um sources show colours R-L15>5, which cannot be explained by the spectral energy distribution (SED) of normal quiescent spiral galaxies, but are consistent with SEDs of redshifted (z>1) starburst or ultraluminous infrared galaxies. This result indicates that the fraction of the ultraluminous infrared galaxies in our faint 15um sample is much larger than that in our brighter 15um sources, which is consistent with the evolving mid-infrared luminosity function derived by recent studies based on the Spitzer 24um deep surveys. Based on an SED fitting technique, the nature of the faint 15um sources is further discussed for a selected number of sources with available K_s-band data.

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arXiv2007

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 um, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcmin, and the pixel pitch is matches the diffraction limit of the telescope. Derived point spread functions (PSFs) from observations of asteroids are similar to the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and its performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

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arXiv2007

AKARI infrared imaging of reflection nebulae IC4954 and IC4955

We present the observations of the reflection nebulae IC4954 and IC4955 region with the Infrared Camera (IRC) and the Far-Infrared Surveyor (FIS) on board the infrared astronomical satellite AKARI during its performance verification phase. We obtained 7 band images from 7 to 160um with higher spatial resolution and higher sensitivities than previous observations. The mid-infrared color of the S9W (9um) and L18W (18um) bands shows a systematic variation around the exciting sources. The spatial variation in the mid-infrared color suggests that the star-formation in IC4954/4955 is progressing from south-west to north-east. The FIS data also clearly resolve two nebulae for the first time in the far-infrared. The FIS 4-band data from 65um to 160um allow us to correctly estimate the total infrared luminosity from the region, which is about one sixth of the energy emitted from the existing stellar sources. Five candidates for young stellar objects have been detected as point sources for the first time in the 11um image. They are located in the red S9W to L18W color regions, suggesting that current star-formation has been triggered by previous star-formation activities. A wide area map of the size of about 1 x 1 (deg^2) around the IC4954/4955 region was created from the AKARI mid-infrared all-sky survey data. Together with the HI 21cm data, it suggests a large hollow structure of a degree scale, on whose edge the IC4954/4955 region has been created, indicating star formation over three generations in largely different spatial scales.

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arXiv2007

The Far-Infrared Properties of Spatially Resolved AKARI Observations

We present the spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification (PV) phase of the mission. We extracted reliable point sources matched with IRAS point source catalogue. By comparing IRAS and AKARI fluxes, we found that the flux measurements of some IRAS sources could have been over or underestimated and affected by the local background rather than the global background. We also found possible candidates for new AKARI sources and confirmed that AKARI observations resolved IRAS sources into multiple sources. All-Sky Survey observations are expected to verify the accuracies of IRAS flux measurements and to find new extragalactic point sources.

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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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arXiv2007

First Constraints on Source Counts at 350 Microns

We have imaged a $\sim$6 arcminute$^2$ region in the Boötes Deep Field using the 350 $μ$m-optimised second generation Submillimeter High Angular Resolution Camera (SHARC II), achieving a peak 1$σ$ sensitivity of $\sim$5 mJy. We detect three sources above 3$σ$, and determine a spurious source detection rate of 1.09 in our maps. In the absence of $5σ$ detections, we rely on deep 24 $μ$m and 20 cm imaging to deduce which sources are most likely to be genuine, giving two real sources. From this we derive an integral source count of 0.84$^{+1.39}_{-0.61}$ sources arcmin$^{-2}$ at $S>13$ mJy, which is consistent with 350 $μ$m source count models that have an IR-luminous galaxy population evolving with redshift. We use these constraints to consider the future for ground-based short-submillimetre surveys.

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arXiv2006

Ultraluminous and Hyperluminous Infrared Galaxies in the SDSS, 2dFGRS and 6dFGS

We present a result of cross-correlating the Infrared Astronomical Satellite Faint Source Catalogue (IRAS FSC) with the spectroscopic catalogues of galaxies in the Fourth Data Release of Sloan Digital Sky Survey (SDSS), the Final Data Release of 2dF Galaxy Redshift Survey (2dFGRS) and the Second Data Release of 6dF Galaxy Survey (6dFGS). We have identified 324 ultraluminous infrared galaxies (ULIRGs) including 190 newly discovered ULIRGs, and 2 hyperluminous infrared galaxies (HLIRGs). Adding these new ULIRGs, we increase the number of known ULIRGs by about 30 per cent. The reliability of the cross-correlation is estimated using the likelihood ratio method. The incompleteness of our sample introduced by the identification procedure in this study is estimated to be about 5 per cent. Our sample covers the redshift range of z=0.037$-$0.517 with a median redshift of $\bar{z}$=0.223, which is larger than that ($\bar{z}$=0.184) of the sample of previously known ULIRGs.

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arXiv2006

Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F)

AKARI (formerly ASTRO-F) is an infrared space telescope designed for an all-sky survey at 10-180 (mu)m, and deep pointed surveys of selected areas at 2-180 (mu)m. The deep pointed surveys with AKARI will significantly advance our understanding of galaxy evolution, the structure formation of the Universe, the nature of the buried AGNs, and the cosmic infrared background. Here we describe the important characteristics of the AKARI mission: the orbit, and the attitude control system, and investigate the optimum survey area based on the updated pre-flight sensitivities of AKARI, taking into account the cirrus confusion noise as well as the surface density of bright stars. The North Ecliptic Pole (NEP) is concluded to be the best area for 2-26 (mu)m deep surveys, while the low-cirrus noise regions around the South Ecliptic Pole (SEP) are worth considering for 50-180 (mu)m pointed surveys to high sensitivities limited by the galaxy confusion noise. Current observational plans of these pointed surveys are described in detail. Comparing these surveys with the deep surveys with the Spitzer Space Telescope, the AKARI deep surveys are particularly unique in respect of their continuous wavelength coverage over the 2-26 (mu)m range in broad-band deep imaging, and their slitless spectroscopy mode over the same wavelength range.

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arXiv2004

The local sub-mm luminosity functions and predictions from Spitzer to Herschel

We present new determinations of the local sub-mm luminosity functions, solving the ``sub-mm Olbers' Paradox.'' We also present predictions of source counts and luminosity functions in current and future far-infrared to sub-mm surveys. Using the sub-mm colour temperature relations from the SCUBA Local Universe Galaxy Survey, and the discovery of excess 450 micron excess emission in these galaxies, we interpolate and extrapolate the IRAS detections to make predictions of the SEDs of all 15411 PSC-z galaxies from 50-1300 microns. Despite the long extrapolations we find excellent agreement with (a) the 90 micron luminosity function of Serjeant et al. (2001), (b) the 850 micron luminosity function of Dunne et al. (2000), (c) the mm-wave photometry of Andreani & Franceschini (1996); (d) the asymptotic differential and integral source count predictions at 50-1300 microns by Rowan-Robinson (2001). We find the local 850 micron sub-mm luminosity density converges to (7.3+/-0.2)x10^{19} h_{65} W/Hz/Mpc^3. Remarkably, the local spectral luminosity density and the extragalactic background light together strongly constrain the cosmic star formation history for a wide class of evolutionary assumptions. We find that the extragalactic background light, the 850 micron 8mJy source counts, and the Omega_* constraints all independently point to a decline in the comoving star formation rate at z>1. In order to reconcile this with direct determinations, we suggest either there is a top-heavy initial mass function at high redshifts, and/or there is stronger evolution in the more luminous far-infrared galaxies than seen in the population as a whole.

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arXiv2004

Dust and gas obscuration in ELAIS Deep X-ray Survey reddened quasars

Hard X-ray surveys have uncovered a large population of heavily obscured AGN. They also reveal a population of quasars with moderate obscuration at both visible and X-ray wavelengths. We use Chandra selected samples of quasars from the ELAIS Deep X-ray Survey (EDXS) and CDF-N to investigate the obscuration towards the nuclei of moderately obscured AGN. We find an inverse correlation between the optical to X-ray flux ratio and the X-ray hardness ratio which can be interpreted as due to obscuration at visible and X-ray wavelengths. We present detailed optical and near-infrared data for a sample of optically-faint (R>23) quasars from the EDXS. These are used to constrain the amount of rest-frame UV/optical reddening towards these quasars. It is found that optically-faint quasars are mostly faint due to obscuration, not because they are intrinsically weak. After correcting for reddening, the optical magnitudes of most of these quasars are similar to the brighter quasars at these X-ray fluxes. Combining with gas column densities inferred from the X-ray observations we consider the gas-to-dust ratios of the obscuring matter. We find that the quasars generally have higher gas-to-dust absorption than that seen in the Milky Way - similar to what has been found for nearby Seyfert galaxies. We consider the possible existence of a large population of X-ray sources which have optical properties of Type 1 (unobscured) quasars, but X-ray properties of Type 2 (obscured) quasars. We show that the observed distribution of optical-to-X-ray flux ratios of quasars at z>1 is skewed to low values compared to the intrinsic distribution...

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arXiv2003

The K-band Hubble diagram of sub-mm galaxies and hyperluminous galaxies

We present the K-band Hubble diagrams (K-z relations) of sub-mm-selected galaxies and hyperluminous galaxies (HLIRGs). We report the discovery of a remarkably tight K-z relation of HLIRGs, indistinguishable from that of the most luminous radiogalaxies. Like radiogalaxies, the HLIRG K-z relation at z<~3 is consistent with a passively evolving ~3L* instantaneous starburst starting from a redshift of z~10. In contrast, many sub-mm selected galaxies are >~2 magnitudes fainter, and the population has a much larger dispersion. We argue that dust obscuration and/or a larger mass range may be responsible for this scatter. The galaxies so far proved to be hyperluminous may have been biased towards higher AGN bolometric contributions than sub-mm-selected galaxies due to the 60 micron selection of some, so the location on the K-z relation may be related to the presence of the most massive AGN. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.

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arXiv2003

The local sub-mm luminosity functions and predictions from ASTRO-F/SIRTF to Herschel

We present new determinations of the local sub-mm luminosity functions. We find the local sub-mm luminosity density converging to 7.3+/-0.2 x 10^19 W/Hz/Mpc^3 /h_65 at 850um solving the ``sub-mm Olbers' Paradox.'' Using the sub-mm colour temperature relations from the SCUBA Local Universe Galaxy Survey, and the discovery of excess 450um excess emission in these galaxies, we interpolate and extrapolate the IRAS detections to make predictions of the SEDs of all 15411 PSC-z galaxies from 50-3000um. Despite the long extrapolations we find excellent agreement with (a) the 90um luminosity function of Serjeant et al. (2001), (b) the 850um luminosity function of Dunne et al. (2000), (c) the mm-wave photometry of Andreani & Franceschini (1996); (d) the asymptotic differential and integral source count predictions at 50-3000um by Rowan-Robinson (2001). Remarkably, the local luminosity density and the extragalactic background light together strongly constrain the cosmic star formation history for a wide class of evolutionary assumptions. We find that the extragalactic background light, the 850um 8mJy source counts, and the Omega_* constraints all independently point to a decline in the comoving star formation rate at z>1.

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arXiv2001

The local star formation rate and radio luminosity density

We present a new determination of the local volume-averaged star formation rate from the 1.4 GHz luminosity function of star forming galaxies. Our sample, taken from the B<=12 Revised Shapley-Ames catalogue (231 normal spiral galaxies over effective area 7.1 sr) has ~100% complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fit Schechter function has a faint-end slope of -1.27+/-0.07 in agreement with the local Halpha luminosity function, characteristic luminosity L*=(2.6+/-0.7)*10^{22} W/Hz and density phi* = (4.8 +/-1.1)*10^{-4} / Mpc^3. The inferred local radio luminosity density of (1.73+/-0.37+/-0.03)*10^{19} W/Hz/Mpc^3 (Poisson noise, large scale structure fluctuations) implies a volume averaged star formation rate ~2 x larger than the Gallego et al. Halpha estimate, i.e. rho(1.4 GHz} = (2.10+/-0.45+/-0.04) *10^{-2}$ Msun/yr/Mpc^3 for a Salpeter initial mass function from 0.1-125 Msun and Hubble constant of 50 km/s/Mpc. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical-UV SFRs are easily underestimated, particularly at high redshift.

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arXiv2000

HST imaging survey of sub-mJy star-forming galaxies I: morphologies at z ~ 0.2

We present the first results of our HST WFPC2 F814W snapshot imaging survey, targeting virtually all sub-mJy decimetric radio-selected star-forming galaxies. The radio selection at ~1 GHz is free from extinction effects and the radio luminosities are largely unaffected by AGN contamination, making these galaxies ideal tracers of the cosmic star formation history. A sub-sample of 4 targets is presented here, selected at 1.4 GHz from the spectroscopically homogenous and complete samples of Benn et al. (1993) and Hopkins et al. (1999). The redshifts are confined to a narrow range around z~0.2, to avoid differential evolution, with a radio luminosity close to L* where the galaxies dominate the comoving volume-averaged star formation rate. We find clearly disturbed morphologies resembling those of ultraluminous infrared galaxies, indicating that galaxy interactions may be the dominant mechanism for triggering star formation at these epochs. The morphologies are also clearly different from coeval quasars and radiogalaxies, as found in star-forming galaxies selected at other wavelengths. This may prove challenging for models which propose direct causal links between AGN evolution and the cosmic star formation history at these epochs. The asymmetries are typically much larger than seen in the CFRS at similar redshifts, optical luminosities and Halpha-derived star formation rates, indicating the possible existence of an obscuration-related morphological bias in such samples.

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arXiv1998

The radio luminosity density of star-burst galaxies and co-moving star formation rate

We present a new determination of the co-moving star formation density at redshifts z~<0.35 from the 1.4 GHz luminosity function of sub-mJy star-burst galaxies. Our sample, taken from Benn et al. (1993), is insensitive to dust obscuration. The shape of the Luminosity function of this sample is indistinguishable from a number of reasonable a prior models of the luminosity function. Using these shapes we calculate the modest corrections (typically ~< 20 per cent to the observed 1.4 GHz luminosity density. We find that the cosmic variance in our estimate of this luminosity density is large. We find a luminosity density in broad agreement with that from the RSA sample by Condon et al. (1987). We infer a co-moving star formation rate surprisingly similar to coeval estimates from the Canada-France Redshift Survey, in both ultraviolet and Halpha, although the later may also be affected by Cosmic variance. We conclude that the intermediate 0.05<z<0.3 star-formation rate is not yet well determined partially due to uncertain extinction corrections, but also partially due to cosmic variance. We suggest that deep moderate area radio surveys will improve this situation considerably.

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arXiv1998

IRAS F10214+4724: the inner 100pc

We use new near-infrared spectroscopy and our published optical spectroscopy of the gravitationally-lensed Seyfert-2 galaxy F10214+4724 to study both the links between the starburst and AGN in this object and the properties of the inner narrow-line clouds. The UV spectrum is consistent with a compact, moderately- reddened starburst providing about half the UV light. Spectroscopy of the Halpha /[NII] line blend has enabled us to distinguish emission from the narrow-line region of the Seyfert-2 and a moderately-reddened emission line region which we argue is associated with the starburst. Estimates of the star formation rate from the UV continuum flux and the Halpha flux are broadly consistent. We can explain the unusual emission line properties of F10214+4724 in terms of conventional models for nearby Seyfert-2 galaxies if lensing is preferentially magnifying the side of the inner narrow-line region between the AGN and the observer, and the other side is both less magnified and partially obscured by the torus. The hydrogen densities of clouds in this region are high enough to make the Balmer lines optically thick and to suppress forbidden emission lines with low critical densities. We have deduced the column density of both ionised and neutral gas in the narrow-line clouds, and the density of the ionised gas. Using these we have been able to estimate the mass of the inner narrow-line clouds to be ~ 1 solar mass, and show that the gas:dust ratio NH/E(B-V) in these clouds must be ~1.3x10^{27}m^{-2}mag^{-1}, significantly higher than in the Milky Way. The cloud properties are consistent with the those of the warm absorbers seen in the X-ray spectra of Seyfert-1 galaxies. Our results favour models in which narrow-line clouds start close to the nucleus and flow out.

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