A 610-MHz survey of the Lockman Hole with the Giant Metrewave Radio Telescope – I. Observations, data reduction and source catalogue for the central 5 deg2

We present observations of the Lockman Hole taken at 610 MHz with the Giant Metrewave Radio Telescope (GMRT). Twelve pointings were observed, covering a total area of ∼5 deg² with a resolution of  6 × 5 arcsec²  , position angle  +45°  . The majority of the pointings have a rms noise of ∼60 μJy beam⁻¹ before correction for the attenuation of the GMRT primary beam. Techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented along with a catalogue of 2845 sources detected above 6σ. Radio source counts are calculated at 610 MHz and combined with existing 1.4-GHz source counts, in order to show that pure luminosity evolution of the local radio luminosity functions for active galactic nuclei and starburst galaxies is sufficient to account for the two source counts simultaneously.     

Radio imaging of the Subaru/XMM–Newton Deep Field – I. The 100-μJy catalogue, optical identifications, and the nature of the faint radio source population

We describe deep radio imaging at 1.4 GHz of the 1.3-deg² Subaru/ XMM–Newton Deep Field (SXDF), made with the Very Large Array in B and C configurations. We present a radio map of the entire field, and a catalogue of 505 sources covering 0.8 deg² to a peak flux density limit of 100 μJy. Robust optical identifications are provided for 90 per cent of the sources, and suggested identifications are presented for all but 14 (of which seven are optically blank, and seven are close to bright contaminating objects). We show that the optical properties of the radio sources do not change with flux density, suggesting that active galactic nuclei (AGN) continue to contribute significantly at faint flux densities. We test this assertion by cross-correlating our radio catalogue with the X-ray source catalogue and conclude that radio-quiet AGN become a significant population at flux densities below 300 μJy, and may dominate the population responsible for the flattening of the radio source counts if a significant fraction of them are Compton-thick.     

Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 and 151 MHz

We present the results of optical spectroscopy of two flux-density-limited samples of radio sources selected at frequencies of 38 and 151 MHz in the same region around the North Ecliptic Cap, the 8C-NEC and 7C- iii samples respectively. Both samples are selected at flux density levels ≈20 times fainter than samples based on the 3C catalogue. They are amongst the first low-frequency selected samples with no spectral or angular size selection for which almost complete redshift information has been obtained, and contain many of the lowest-luminosity z >2 radio galaxies so far discovered. They will therefore provide a valuable resource for understanding the cosmic evolution of radio sources and their hosts and environments. The 151-MHz 7C- iii sample is selected to have S ₁₅₁≥0.5 Jy and is the more spectroscopically complete; out of 54 radio sources fairly reliable redshifts have been obtained for 44 objects. The 8C sample has a flux limit of S ₃₈≥1.3 Jy and contains 58 sources of which 46 have fairly reliable redshifts. We discuss possible biases in the observed redshift distribution, and some interesting individual objects, including a number of cases of probable gravitational lensing. Using the 8C-NEC and 7C- iii samples in conjunction, we form the first sample selected on low-frequency flux in the rest-frame of the source, rather than the usual selection on flux density in the observed frame. This allows us to remove the bias associated with an increasing rest-frame selection frequency with redshift. We investigate the difference this selection makes to correlations of radio source properties with redshift and luminosity by comparing the results from traditional flux-density selection with our new method. We show in particular that flux-density-based selection leads to an overestimate of the steepness of the correlation of radio source size with redshift.     

Observations of the Sunyaev–Zel'dovich effect in the z=0.78 cluster MS 1137.5+6625

We have observed the   z =0.78  cluster MS 1137.5+6625 with the Ryle Telescope (RT) at 15 GHz. After subtraction of contaminating radio sources in the field, we find a Sunyaev–Zel'dovich flux decrement of  -421±60 μJy  on the ≈0.65 k λ baseline of the RT, spatially coincident with the optical and X-ray positions for the cluster core.
For a spherical King-profile cluster model, the best fit to our flux measurement has a core radius   θ _C=20 arcsec  , consistent with previous X-ray observations, and a central temperature decrement  Δ T =650±92 μK  .
Using this model, we calculate that the cluster has a gas mass inside a     radius of  2.9×10¹³ M_⊙  for an  Ω _M =1  universe and  1.6×10¹³ M_⊙  for  Ω _M =0.3  ,  Ω_Λ=0.7  . We compare this model with existing measurements of the total mass of the cluster, based on gravitational lensing, and estimate a gas fraction for MS 1137.5+6625 of ≈8 per cent.     

Radio source stacking and the infrared/radio correlation at μJy flux densities

We investigate the infrared/radio correlation using the technique of source stacking, in order to probe the average properties of radio sources that are too faint to be detected individually. We compare the two methods used in the literature to stack sources and demonstrate that the creation of stacked images leads to a loss of information. We stack infrared sources in the Spitzer Extragalactic First Look Survey (xFLS) field, and the three northern Spitzer Wide-area Infrared Extragalactic survey (SWIRE) fields, using radio surveys created at 610 MHz and 1.4 GHz, and find a variation in the absolute strength of the correlation between the xFLS and SWIRE regions, but no evidence for significant evolution in the correlation over the 24-μm flux density range 150 μJy to 2 mJy. We carry out the first radio source stacking experiment using 70-μm-selected galaxies, and find no evidence for significant evolution over the 70-μm flux density range 10–100 mJy.     

The IPHAS-POSS-I proper motion survey of the Galactic plane

We present millimetre observations of a sample of 12 high-redshift ultraluminous infrared galaxies (ULIRGs) in the extended growth strip (EGS). These objects were initially selected on the basis of their observed mid-IR colours (  0.0 < [3.6]−[4.5] < 0.4  and  −0.7 < [3.6]−[8.0] < 0.5  ) to lie at high redshift  1.5 ≲ z ≲ 3  , and subsequent 20–38 μm mid-IR spectroscopy confirms that they lie in a narrow redshift window centred on   z ≈ 2  . We detect 9/12 of the objects in our sample at high significance  (>3σ)  with a mean 1200 μm flux of  〈 F _1200 μm〉= 1.6 ± 0.1  mJy. Our millimetre photometry, combined with existing far-IR photometry from the Far-IR Deep Extragalactic Legacy Survey (FIDELS) and accurate spectroscopic redshifts, places constraints both sides of the thermal dust peak. This allows us to estimate the dust properties, including the far-IR luminosity, dust temperature and dust mass. We find that our sample is similar to other high- z and intermediate- z ULIRGs, and local systems, but has a different dust selection function than submillimeter-selected galaxies. Finally, we use existing 20-cm radio continuum imaging to test the far-IR/radio correlation at high redshift. We find that our sample is consistent with the local relation, implying little evolution. Furthermore, this suggests that our sample selection method is efficient at identifying ultraluminous, starburst-dominated systems within a very narrow redshift range centred at   z ∼ 2  .     

The contribution of faint blue galaxies to the submillimetre counts and background

Observations in the submillimetre (submm) waveband have recently revealed a new population of luminous sources. These are proposed to lie at high redshift and to be optically faint because of their high intrinsic dust obscuration. The presence of dust has been previously invoked in optical galaxy count models which use the Bruzual & Charlot evolution models with an exponential τ =9 Gyr star formation rate (SFR) for spirals, and these fit the count data well from U to K . We now show that by using either a 1/ λ or Calzetti absorption law for the dust and re-distributing the evolved spiral galaxy ultraviolet (UV) radiation into the far-infrared (FIR), these models can account for all of the 'faint' ( 1 mJy) 850-μm galaxy counts, but fail to fit 'bright' ( 2 mJy) sources, indicating that another explanation for the submm counts may apply at brighter fluxes, e.g., quasi-stellar objects (QSOs) or ultraluminous infrared galaxies (ULIRGs). We find that the main contribution to the faint, submm number counts is in the redshift range 0.5< z <3, peaking at z ≈1.8. The above model, using either dust law, can also explain a significant proportion of the extragalactic background at 850 μm, as well as producing a reasonable fit to the bright 60-μm IRAS counts.     

A new search for distant radio galaxies in the Southern hemisphere – I. Sample definition and radio properties

This paper introduces a new program to find high-redshift radio galaxies in the Southern hemisphere through ultrasteep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices α⁸⁴³₄₀₈≤−1.0 ( S _ν∝ν^α) and flux densities S ₄₀₈≥ 200 mJy in a region of 0.35 sr, chosen by cross-correlating the revised 408 MHz Molonglo Reference Catalogue, the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region −40° < δ < −30°. We present Australia Telescope Compact Array (ATCA) high-resolution 1384 and 2368 MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408–2368 MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368 MHz. We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample.     

A far-infrared view of the Lockman hole from ISO 95-μm observations – II. Optical identifications and insights into the nature of the far-infrared sources

We present the optical identifications of a 95-μm ISOPHOT sample in the Lockman hole over an area of approximately half a deg². The Rodighiero et al. catalogue includes 36 sources, making up a complete flux-limited sample for   S _95 μm≥ 100 mJy  . Reliable sources were detected, with decreasing but well-controlled completeness, down to   S _95 μm≃ 20 mJy  . We have combined mid-infrared (IR) and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15-μm associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed spectral energy distributions (SEDs). Four of these 95-μm sources have been classified as faint IR (FIR) galaxies  ( L _FIR < 1. e 11 L_⊙)  , six as luminous IR galaxies (LIRGs) and only one as an ultraluminous IR galaxy (ULIRG). We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 μm. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at   z < 0.2  ), their luminosities and star formation rates (SFR; median value  ∼ 10 M_⊙ yr⁻¹  ), the sources unveiled by ISOPHOT at 95 μm seem to correspond to the low redshift  ( z < 0.3)  FIRBACK 175-μm population, composed of dusty, star-forming galaxies with moderate SFRs. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes.     

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

We present deep multifrequency observations using the Giant Metrewave Radio Telescope (GMRT) at 153, 244, 610 and 1260 MHz of a field centred on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as haloes and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey (WENSS), NRAO (National Radio Astronomy Observatories) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters Survey (FIRST). Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index,  α∼ 0.8 [S(ν) ∝ν^−α  ], which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with  α≥ 1.3  . We examine their optical fields and discuss the nature of some of these sources.     

Gravitational lensing of extended high-redshift sources by dark matter haloes

High-redshift galaxies and quasi-stellar objects (QSOs) are most likely to be strongly lensed by intervening haloes between the source and the observer. In addition, a large fraction of lensed sources is expected to be seen in the submillimetre region, as a result of the enhanced magnification bias on the steep intrinsic number counts. We extend in three directions Blain's earlier study of this effect.
First, we use a modification of the Press–Schechter mass function and detailed lens models to compute the magnification probability distribution. We compare the magnification cross-sections of populations of singular isothermal spheres and Navarro, Frenk & White (NFW) haloes and find that they are very similar, in contrast to the image-splitting statistics which were recently investigated in other studies. The distinction between the two types of density profile is therefore irrelevant for our purposes.
Secondly, we discuss quantitatively the maximum magnification, μ _max, that can be achieved for extended sources (galaxies) with realistic luminosity profiles, taking into account the possible ellipticity of the lensing potential. We find that μ _max plausibly falls into the range     for sources of     effective radius at redshifts within     .
Thirdly, we apply our model for the lensing magnification to a class of sources following the luminosity evolution typical for a unified scheme of QSO formation. As a result of the peculiar steepness of their intrinsic number counts, we find that the lensed source counts at a fiducial wave length of 850 μm can exceed the unlensed counts by several orders of magnitude at flux densities ≳100 mJy, even with a conservative choice of the maximum magnification.     

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

We present a careful analysis of the point-source detection limit of the AKARI All-Sky Survey in the WIDE-S 90-μm band near the North Ecliptic Pole (NEP). Timeline analysis is used to detect IRAS ( Infrared Astronomy Satellite ) sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90-μm flux of a source. Combined with a robust noise measurement, the point-source flux detection limit at signal-to-noise ratio  (S/N) > 5  for a single detector row is  1.1 ± 0.1 Jy  which corresponds to a point-source detection limit of the survey of ∼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.     

A SCUBA scan-map of the Hubble Deep Field: measuring the bright end of the submillimetre source counts

Using the 850-μm SCUBA camera on the JCMT and a scanning technique different from other sub-mm surveys, we have obtained a 125 arcmin² map centred on the Hubble Deep Field . The 1 σ sensitivity to point sources is roughly 3 mJy and thus our map probes the brighter end of the sub-mm source counts. We find six sources with a flux greater than about 12 mJy (>4 σ ) and, after a careful accounting of incompleteness and flux bias, estimate the integrated density of bright sources     (68 per cent confidence bounds).     

Can the faint submillimetre galaxies be explained in the Λ cold dark matter model?

We present predictions for the abundance of submillimetre galaxies (SMGs) and Lyman-break galaxies (LBGs) in the Λ cold dark matter cosmology. A key feature of our model is the self-consistent calculation of the absorption and emission of radiation by dust. The new model successfully matches the LBG luminosity function, as well as reproducing the properties of the local galaxy population in the optical and infrared. The model can also explain the observed galaxy number counts at 850 μm, but only if we assume a top-heavy initial mass function for the stars formed in bursts. The predicted redshift distribution of SMGs depends relatively little on their flux over the range 1–10 mJy, with a median value of   z ≈ 2.0  at a flux of 5 mJy, in good agreement with the recent measurement by Chapman et al. The counts of SMGs are predicted to be dominated by ongoing starbursts. However, in the model these bursts are responsible for making only a few per cent of the stellar mass locked up in massive ellipticals at the present day.     

Clarifying the nature of the brightest submillimetre sources: interferometric imaging of LH 850.02

We present high-resolution interferometric imaging of LH 850.02, the brightest 850- and 1200-μm submillimetre (submm) galaxy in the Lockman Hole. Our observations were made at 890 μm with the Submillimetre Array (SMA). Our high-resolution submm imaging detects LH 850.02 at  ≳6σ  as a single compact (size ≲1 arcsec or ≲8 kpc) point source and yields its absolute position to ∼0.2-arcsec accuracy. LH 850.02 has two alternative radio counterparts within the Submillimetre Common User Bolometer Array (SCUBA) beam (LH 850.02N and LH 850.02S), both of which are statistically very unlikely to be so close to the SCUBA source position by chance. However, the precise astrometry from the SMA shows that the submm emission arises entirely from LH 850.02N, and is not associated with LH 850.02S (by far the brighter of the two alternative identifications at 24 μm). Fits to the optical–infrared (IR) multicolour photometry of LH 850.02N and LH 850.02S indicate that both lie at   z ≈ 3.3  , and are therefore likely to be physically associated. At these redshifts, the 24-μm-to-submm flux density ratios suggest that LH 850.02N has an Arp 220-type starburst-dominated far-IR spectral energy distribution (SED), while LH 850.02S is more similar to Mrk 231, with less dust enshrouded star formation activity, but a significant contribution at 24 μm (rest frame 5–6 μm) from an active nucleus. This complex mix of star formation and active galactic nucleus (AGN) activity in multicomponent sources may be common in the high-redshift ultraluminous galaxy population, and highlights the need for precise astrometry from high-resolution interferometric imaging for a more complete understanding.     

The European Large Area ISO Survey: optical identifications of 15-μm and 1.4-GHz sources in N1 and N2

We present the multiwavelength properties and catalogue of the 15 μm and 1.4 GHz radio sources detected in the European Large Area ISO Survey ( ELAIS ) areas N1 and N2. Using the optical data from the Wide Field Survey we use a likelihood ratio method to search for the counterparts of the 1056 and 691 sources detected at 15 μm and 1.4 GHz, respectively, down to flux limits of   S ₁₅= 0.5 mJy  and   S _1.4 GHz= 0.135 mJy  . We find that ∼92 per cent of the 15 μm ELAIS sources have an optical counterpart down to   r '= 24  . All mid-infrared (IR) sources with fluxes   S ₁₅≥ 3 mJy  have an optical counterpart. The magnitude distribution of the sources shows a well-defined peak at relatively bright magnitudes   r '∼ 18  . The mid-IR-to-optical and radio-to-optical flux diagrams are presented and discussed in terms of actual galaxy models. About 15 per cent of the sources are bright galactic stars; of the extragalactic objects ∼65 per cent are compatible with being normal or starburst galaxies and ∼25 per cent active galactic nuclei (AGNs). Objects with mid-IR-to-optical fluxes larger than 100 are found, comprising ∼20 per cent of the sample. We suggest that that these sources are highly obscured luminous and ultraluminous starburst galaxies and AGNs.     

A limit on the number density of bright z≈ 7 galaxies

We present a survey of bright optical dropout sources in two deep, multiwavelength surveys comprising 11 widely separated fields, aimed at constraining the galaxy luminosity function at   z ≈ 7  for sources at  5–10  L * ( z = 6)  . Our combined survey area is 225 arcmin² to a depth of   J _AB= 24.2  (3σ) and 135 arcmin² to   J = 25.3  (4σ). We find that infrared data longwards of 2 μm are essential for classifying optical dropout sources, and in particular for identifying cool Galactic star contaminants. Our limits on the number density of high-redshift sources are consistent with current estimates of the Lyman break galaxy luminosity function at   z = 6  .     

A deep Giant Metre-wave Radio Telescope 610-MHz survey of the 1HXMM–Newton/Chandra survey field

We present the results of a deep 610-MHz survey of the 1 ^H XMM–Newton / Chandra survey area with the Giant Metre-wave Radio Telescope. The resulting maps have a resolution of ∼7 arcsec and an rms noise limit of 60 μJy. To a 5σ detection limit of 300 μJy, we detect 223 sources within a survey area of 64 arcmin in diameter. We compute the 610-MHz source counts and compare them to those measured at other radio wavelengths. The well-known flattening of the Euclidean-normalized 1.4-GHz source counts below ∼2 mJy, usually explained by a population of starburst galaxies undergoing luminosity evolution, is seen at 610 MHz. The 610-MHz source counts can be modelled by the same populations that explain the 1.4-GHz source counts, assuming a spectral index of −0.7 for the starburst galaxies and the steep spectrum active galactic nucleus (AGN) population. We find a similar dependence of luminosity evolution on redshift for the starburst galaxies at 610 MHz as is found at 1.4 GHz (i.e.  ' Q '= 2.45^+0.3_−0.4  ).     

Restrictions to the galaxy evolutionary models from the Hawaiian Deep Fields SSA13 and SSA22

Quantitative structural analysis of the galaxies present in the Hawaiian Deep Fields SSA13 and SSA22 is reported. The structural parameters of the galaxies have been obtained automatically by fitting a two-component model (Sérsic r ^{1/ n} bulge and exponential disc) to the surface brightness of the galaxies. The galaxies were classified on the basis of the bulge-to-total luminosity ratio  ( B / T )  . The magnitude selection criteria and the reliability of our method have been checked by using Monte Carlo simulations. A complete sample of objects up to redshift 0.8 has been achieved. Spheroidal objects (E/S0) represent ≈33 per cent and spirals ≈41 per cent of the total number of galaxies, while mergers and unclassified objects represent ≈26 per cent. We have computed the comoving space density of the different kinds of object. In an Einstein–de Sitter universe, a decrease in the comoving density of E/S0 galaxies is observed as redshift increases (≈30 per cent less at   z =0.8)  , while for spiral galaxies a relatively quiet evolution is reported. The framework of hierarchical clustering evolution models of galaxies seems to be the most appropriate to explain our results.     

Probing the submillimetre number counts at f850 μm < 2 mJy

We have conducted a submillimetre mapping survey of faint, gravitationally lensed sources, where we have targeted 12 galaxy clusters and additionally the New Technology Telescope (NTT) Deep Field. The total area surveyed is 71.5 arcmin² in the image plane; correcting for gravitational lensing, the total area surveyed is 40 arcmin² in the source plane for a typical source redshift z ≈ 2.5. In the deepest maps, an image plane depth of 1σ rms ∼0.8 mJy is reached. This survey is the largest survey to date to reach such depths. In total 59 sources were detected, including three multiply imaged sources. The gravitational lensing makes it possible to detect sources with flux density below the blank field confusion limit. The lensing-corrected fluxes range from 0.11 to 19 mJy. After correcting for multiplicity, there are 10 sources with fluxes <2 mJy of which seven have submJy fluxes, doubling the number of such sources known. Number counts are determined below the confusion limit. At 1 mJy, the integrated number count is  ∼10⁴ deg⁻²  , and at 0.5 mJy it is  ∼2 × 10⁴ deg⁻²  . Based on the number counts, at a source plan flux limit of 0.1 mJy, essentially all of the 850-μm background emission has been resolved. The dominant contribution (>50 per cent) to the integrated background arises from sources with fluxes S ₈₅₀ between 0.4 and 2.5 mJy, while the bright sources S ₈₅₀ > 6 mJy contribute only 10 per cent.