Faint Flux Research Articles

The 1.4-GHz and Hα luminosity functions and star formation rates from faint radio galaxies

A sample of over 1000 objects selected from a 1.4-GHz survey made by the Australia Telescope Compact Array (ATCA) is used to study the properties of the faint radio source population. The sample, covering an area of ≈3 deg2, is 50 per cent complete to 0.2 mJy. Over 50 per cent of the radio sources are found to have optical counterparts brighter than R≈21.5. Spectroscopic observations of 249 optically identified radio sources have been made, using the 2-degree Field (2dF) facility at the Anglo-Australian Telescope (AAT). Redshifts and equivalent widths of several spectral features (e.g. Hα and [O ii] λ3727) sensitive to star formation have been measured. On the basis of the photometric and spectroscopic data, the optically identified radio sources are classified as (i) absorption-line galaxies, (ii) star-forming galaxies and (iii) Seyfert-like galaxies. The spectroscopic sample is corrected for incompleteness and used to estimate the 1.4-GHz and Hα luminosity functions (LFs) and luminosity density distributions. The 1.4-GHz LF of the star-forming population has a much steeper faint-end slope (1.85) than the ellipticals (1.35). This implies an increasing preponderance of star-forming galaxies among the optically identified (i.e. z≲1) radio sources at fainter flux densities. The Hα LF of the faint radio population agrees with published Hα LFs derived from local samples selected by Hα emission. This suggests that the star-forming faint radio population is coincident with the Hα-selected population. The 1.4-GHz and Hα luminosity densities have been used to estimate the star formation rates (SFRs). The two SFRs agree, both giving a SFR density of 0.032 M⊙ yr−1 Mpc−3 in the range z≲1. Radio selection appears to be as effective as Hα selection in finding the galaxies that dominate star formation at a given epoch. Although the sample contains many galaxies lying beyond z≈0.3, it does not reveal a significant rise in the global star formation rate with increasing redshift. This result suggests that the optical counterparts of galaxies undergoing vigorous star formation at redshifts beyond z≈0.3 are generally fainter than R≈21.

X-ray spectra of the RIXOS source sample

We present results of an extensive study of the X-ray spectral properties of sources detected in the RIXOS survey, which is a large, nearly complete sample of objects detected serendipitously in ROSAT PSPC fields down to a flux limit of 3×10−14 erg cm−2 s−1 (0.5–2 keV). We show that for X-ray surveys containing sources with low count rate, such as RIXOS, spectral slopes estimated using simple hardness ratios in the ROSAT band can be biased. Instead, we analyse three-colour X-ray data using statistical techniques appropriate to the Poisson regime which remove the effects of this bias. We also show that the use of three-colour data enables some discrimination between thermal and non-thermal spectra. We have then applied this technique to the RIXOS survey to study the spectral properties of the sample. For the AGN we find an average energy index of 1.05±0.05, with no evidence for spectral evolution with redshift. Individual AGN are shown to have a range of properties, including soft X-ray excesses and intrinsic absorption. Narrow-emission-line galaxies (NELGs) also seem to fit to a power-law spectrum, which may indicate a non-thermal origin for their X-ray emission. We infer that most of the clusters in the sample have a bremsstrahlung temperature <3 keV, although some show evidence for a cooling flow. The stars deviate strongly from a power-law model but fit to a thermal model. Finally, we have analysed the whole RIXOS sample (extending the flux cut-off to the sensitivity threshold of each individual observation) containing 1762 sources to study the relationship between spectral slope and flux. We find that the mean spectral slope of the sources hardens at lower fluxes, in agreement with results from other samples. However, a study of the individual sources demonstrates that the majority have relatively soft spectra even at faint flux levels, and the hardening of the mean is caused by the appearance of a population of very hard sources at the lowest fluxes. This has implications for the nature of the soft X-ray background.

Fitting the spectrum of the X-ray background: the effects of high-metallicity absorption

Recent work by Risaliti, Maiolino & Salvati suggests that more than half of all Seyfert 2 galaxies in the local Universe are Compton-thick (NH>1024 cm−2). This has implications for AGN synthesis models for the X-ray background, the flexibility of which for the inclusion of large numbers of high-z type 2 sources we examine here. We highlight the importance of Compton down-scattering in determining the individual source spectra and the fit to the X-ray background spectrum, and demonstrate how parameter space ‘opens up’ considerably if a super-solar iron abundance is assumed for the absorbing material. This is illustrated with a model which satisfies the present constraints, but which predicts substantial numbers of type 2 sources at the faint flux levels soon to be probed for the first time by the Chandra and XMM missions. We demonstrate also how a strong negative K-correction facilitates the detection of sources with 10∼24NH1025 cm−2 out to the highest redshifts at which they could plausibly exist.

Summary of the ‘Sub-microJansky Radio Sky’ Workshop

AbstractThe Square Kilometre Array Radio Telescope is the next generation radio telescope. An international project is currently under way to design and build an instrument having an effective collecting area two orders of magnitude greater than that of any existing telescope. A number of separate studies are presently investigating how to design the Square Kilometre Array to best carry out the kinds of observations desired by the astronomical community. We present a summary of one of these studies, a workshop called The ‘Sub-microJansky Radio Sky’ held at the ATNF, Sydney, on 17 June 1998. This workshop addressed the nature of the radio sky at the very faint flux densities likely to be attainable by the Square Kilometre Array. In particular, each speaker investigated a separate population of radio sources and how the expected appearance of that population at such faint flux densities would dictate how to refine some of the design constraints for the Square Kilometre Array.

High-redshift galaxies in the Hubble Deep Field- II. Colours and number counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U300 ≈ 26 mag is not caused by incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of ≈ 0.5 mag is observed at faint fluxes in the colour-magnitude diagram, (U300 - V606)eff versus V606. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels (in the AB system) of AB ≈ 27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7 per cent of the sources in U300, 30 per cent in B450 and 35 per cent in V606 are Lyman-break ‘dropouts’, i.e. candidate star-forming galaxies at z > 2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have ⟨z⟩ ∼ 0.6. To AB ≈ 29 mag, the integrated light from resolved galaxies in the l-band is 2.1+0.4+0.3, and its spectrum is well described by a broken power law (lν ∝ λ2 from 2000 to 8000 Å and lν ∝ λ from 8000 to 22 000 Å). We discuss the predictions for the counts, colours and luminosity densities from standard low-q0 pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.

A Catalog of 203 Galaxy Clusters Serendipitously Detected in theROSATPSPC Pointed Observations

We present a catalog of 203 clusters of galaxies serendipitously detected in 647 ROSAT PSPC high Galactic latitude pointings covering 158 deg2. This is one of the largest X-ray-selected cluster samples, comparable in size only to the ROSAT All-Sky Survey sample of nearby clusters (Ebeling et al. 1997). We detect clusters in the inner 175 of the ROSAT PSPC field of view using the spatial extent of their X-ray emission. Fluxes of detected clusters range from 1.6 × 10-14 to 8 × 10-12 ergs s-1 cm-2 in the 0.5-2 keV energy band. X-ray luminosities range from 1042 ergs s-1, corresponding to very poor groups, to ~5 × 1044 ergs s-1, corresponding to rich clusters. The cluster redshifts range from z = 0.015 to z > 0.5. The catalog lists X-ray fluxes, core radii, and spectroscopic redshifts for 73 clusters and photometric redshifts for the remainder. Our detection method, optimized for finding extended sources in the presence of source confusion, is described in detail. Selection effects necessary for a statistical analysis of the cluster sample are comprehensively studied by Monte Carlo simulations. We have optically confirmed 203 of 223 X-ray sources as clusters of galaxies. Of the remaining 20 sources, 19 are likely false detections arising from blends of unresolved point X-ray sources. Optical identifications of the remaining object are hampered by a bright nearby star. Above a flux of 2 × 10-13 ergs s-1 cm-2, 98% of extended X-ray sources are optically confirmed clusters. The number of false detections and their flux distribution are in perfect agreement with simulations. The log N-log S relation for clusters derived from our catalog shows excellent agreement with counts of bright clusters derived from the Einstein Extended Medium Sensitivity Survey and ROSAT All-Sky Survey. At fainter fluxes, our log N-log S relation agrees with the smaller area WARPS survey. Our cluster counts appear to be systematically higher than those from a 50 deg2 survey by Rosati et al. In particular, at a flux of 2 × 10-13 ergs s-1 cm-2, we find a surface density of clusters of 0.57 ± 0.07 deg-2, which is a factor of 1.3 more than was found by Rosati et al. This difference is marginally significant at the ~2 σ level. The large area of our survey makes it possible to study the evolution of the X-ray luminosity function in the high luminosity range inaccessible with other, smaller area ROSAT surveys.

Optical Identification of the Hardest X‐Ray Source in theASCALarge Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band, the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H = 6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured AGN. The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

Extragalactic source counts and contributions to the anisotropies of the cosmic microwave background: predictions for the Planck Surveyor mission

We present predictions for the counts of extragalactic sources, the contributions to fluctuations and their spatial power spectrum in each channel foreseen for the Planck Surveyor (formerly COBRAS/SAMBA) mission. The contribution to fluctuations due to clustering of both radio and far--IR sources is found to be generally small in comparison with the Poisson term; however the relative importance of the clustering contribution increases and may eventually become dominant if sources are identified and subtracted down to faint flux limits. The central Planck frequency bands are expected to be ``clean'': at high galactic latitude (|b|>20), where the reduced galactic noise does not prevent the detection of the extragalactic signal, only a tiny fraction of pixels is found to be contaminated by discrete extragalactic sources. Moreover, removal of contaminating signals is eased by the substantial difference between their power spectrum and that of primordial fluctuations.

The origin of the cosmic soft X-ray background: optical identification of an extremely deep ROSAT survey

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was obtained with the ROSAT position-sensitive proportional counter (PSPC) and reaches a flux limit of 1.6 x 10(-15) erg cm(-2) s(-1) (0.5-2.0 keV). Above a flux limit of 2 x 10(-15) erg cm(-2) s(-1) we define a complete sample of 70 sources, of which 59 are identified. For a further five sources we have tentative identifications and for a further four the X-ray error boxes are blank to R = 23 mag. At brighter fluxes (greater than or equal to 10(-14) erg cm(-2) s(-1)) we confirm the results of previous less deep X-ray surveys, with 84 per cent of our sources being QSOs. However, at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition, at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally >0.3. Most of these groups are poor systems of low X-ray Luminosity and the number that we find is consistent with a zero evolutionary scenario, unlike the situation for high-luminosity clusters at the same redshift. To a flux limit of 2 x 10(-15) erg cm(-2) s(-1), QSOs contribute > 31 per cent of the cosmic soft X-ray background (XRB), groups/clusters contribute similar to 10 per cent and NELGs contribute similar to 8 per cent. However, the QSO differential source count slope below 10(-14) erg cm(-2) s(-1) is similar to-1.4, severely sub-Euclidean, as is the (poorly defined) group/cluster slope, whereas the differential NELG slope is close to Euclidean (similar to-2.4). If the NELG source counts continue to rise at that slope, all of the remaining cosmic soft XRB will be explained by a flux limit of similar to 1-2 x 10(-16) erg cm(-2) s(-1), with NELGs contributing about one quarter of the XRB. The average NELG X-ray spectrum is harder than that of the QSOs, and similar to that of the remaining unresolved cosmic XRB, suggesting that NELGs will also be substantial contributors to the XRB at higher energies. The observed NELGs lie in the redshift range 0.1-0.6 and have M-R = -20 to -23, approximately 3 mag more luminous than typical field galaxies. They have predominantly blue colours, and some are definitely spirals, but the presence of some ellipticals cannot yet be ruled out. Many are in interacting or disturbed systems. The NELGs have optical spectra similar to those of the majority of the field galaxy population at a similar redshift and may simply be the more luminous members of the emission-line field galaxy population. Based on optical line ratios and X-ray/optical flux ratios, the NELGs, both as a sample and within individual galaxies, appear to be a mixture of starburst galaxies and true active galactic nuclei (AGN).

High-redshift galaxies in the Hubble Deep Field - II. Colours and number counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U=26 mag is not due to incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of about 0.5 magnitude is observed at faint fluxes in the colour-magnitude diagram, (U-V)_{eff} vs. V. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels of AB=27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7% of the sources in U, 30% in B and 35% in V are Lyman-break ``dropouts'', i.e. candidate star-forming galaxies at z>2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have =0.6. To AB=29 mag, the integrated light from resolved galaxies in the I-band is 2.1\pm 0.4 \times 10^{-20} erg/cm^2/s/Hz/sr, and its spectrum is well described by a broken power-law. We discuss the predictions for the counts, colours, and luminosity densities from standard low-q_0 pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.

Limits on Cosmological Models from Radio‐selected Gravitational Lenses

We are conducting a redshift survey of 177 flat-spectrum radio sources in 3 samples covering the 5GHz flux ranges 50-100, 100-200 and 200-250 mJy. So far, we have measured 124 redshifts with completenesses of 80%, 68% and 58% for the bright, intermediate, and faint flux ranges. Using the newly determined redshift distribution we can derive cosmological limits from the statistics of the 6 gravitational lenses in the JVAS sample of 2500 flat-spectrum radio sources brighter than 200 mJy at 5GHz. For flat cosmological models with a cosmological constant, the limit using only radio data is Omega_0 > 0.27 at 2-sigma (0.47 0.38 at 2-sigma (0.64 0.26 at 2-sigma in flat cosmologies. The increasing fraction of radio galaxies as compared to quasars at fainter radio fluxes (rising from ~10% at 1 Jy to ~50% at 0.1 Jy) explains why lensed optical emission is common for radio lenses and partly explains the red color of radio-selected lenses.

Exposing New Components of the X-Ray Background with Multi-Wavelength Sky Surveys

The evolving class of objects responsible for the majority of the cosmic X-ray background (XRB) remains at large nearly three and a half decades after the discovery of the XRB. Surveys of sources selected on the basis of their X-ray properties alone provide an unbiased picture of the X-ray sky, but to date they have not been ideal for the discovery of rare types of X-ray sources at faint fluxes: large-area X-ray surveys have been restricted to bright sources, while deep X-ray surveys have been limited to very small patches of sky. X-ray selection coupled with another selection criterion, e.g., a radio or infrared detection, complements “pure” X-ray surveys by (1) permitting the exploration of large areas of sky to faint flux limits for types of extragalactic X-ray sources not well represented in other surveys, and (2) assisting the location of the optical counterparts of these X-ray sources. Using this approach, I have searched for new components of the XRB among the faintest X-ray sources detected by the Einstein Observatory.

Faint galaxies and the X‐ray background

AbstractWe summarise our recent work on the faint galaxy contribution to the cosmic X‐ray background (XRB). At bright X‐ray fluxes (in the ROSAT pass band), broad line QSOs dominate the X‐ray source population, but at fainter fluxes there is evidence for a significant contribution from emission‐line galaxies. Here we present statistical evidence that these galaxies can account for a large fraction of the XRB. We also demonstrate that these galaxies have significantly harder X‐ray spectra than QSOs in the ROSAT band. Finally we present preliminary findings from infra‐red spectroscopy on the nature of this X‐ray emitting galaxy population. We conclude that a hybrid explanation consisting of obscured/Type 2 AGN surrounded by starburst activity can explain the properties of these galaxies and perhaps the origin of the entire XRB.

X-RAY OBSERVATIONS OF ACTIVE GALACTIC NUCLEI IN THE COSMOLOGICAL CONTEXT

We review recent discoveries of obscured Active Galactic Nuclei at significant redshifts with X-ray observations. Most, if not all, the X-ray luminous galaxies found at faint X-ray fluxes could in fact be obscured AGN, where the obscuring material is also forming stars copiously. Given that AGN of various types appear to dominate the source populations at all X-ray fluxes, we also discuss their clustering properties. Current results imply that the population of X-ray selected AGN is not strongly biased, and therefore X-ray observations can be used to trace the distribution of matter out to redshifts ∼ 2.

The UK ROSAT Deep Survey

AbstractWe present a summary of the identification content of the UK ROSAT Deep Survey. The survey is based on a 115ksec PSPC observation and comprises a sample of 70 sources to a flux limit of 2 × 10−15 ergs cm−2 s−1 (0.5–2 keV). At bright fluxes the survey is dominated by broad line QSOs, at intermediate fluxes it contains a number of groups and clusters of galaxies, but at faint fluxes the survey contains many galaxies with narrow optical emission lines (NELGs). The average X‐ray spectrum of the NELGs is harder than that of the QSOs and similar to that of the X‐ray background. The NELGs have optical spectra similar to the majority of the field galaxy population in the same redshift range and may simply be the more luminous members of the emission line field galaxy population. Based on optical line ratios and X‐ray/optical ratios, the NELGs, both as a sample and within individual galaxies, appear to be a mixture of starburst galaxies and true AGN.

Calibration of the Debris Project ISOPHOT Data

The Astronomical Observing Templates (AOTs) for photometry with the ISOPHOT photometer were designed to include measurements of a black-body radiator (FCS) with known output power. However, the short-period drifts of the C-100 detectors at the beginning of a measurement are on the same order of magnitude as the response to both faint source fluxes and the corresponding FCS power. Thus other techniques for calibrating ISOPHOT data on faint sources are sought.

The Angular Two‐Point Correlation Function for the FIRST Radio Survey

The FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey now covers 1550 deg2 of sky, where 07h16 α 17h40 and 28°.3 δ 42°. This yields a catalog of 138,665 sources above the survey threshold of 1 mJy, about one-third of which are in double-lobed and multicomponent sources. We have used these data to obtain the first high-significance measurement of the two-point angular correlation for a deep radio sample. We find that the correlation function between 002 and 2° is well fitted by a power law of the form Aθγ, where A ≈ 3 × 10–3 and γ ≈ -1.1. On small scales (θ < 02), double and multicomponent sources are shown to have a larger clustering amplitude than that of the whole sample. Sources with flux densities below 2 mJy are found to have a shallower slope than that obtained for the whole sample, consistent with there being a significant contribution from starbursting galaxies at these faint fluxes. The cross-correlation of radio sources and Abell clusters is determined. A preliminary approach to inferring spatial information is outlined.

X-ray QSO evolution from a very deep ROSAT survey

In the deepest optically identified X-ray survey yet performed, we have identified 32 X-ray selected QSOs to a flux limit of 2x10^{-15} erg cm^{-2} s^{-1} (0.5-2 keV). The survey, performed with the ROSAT PSPC, has 89% spectroscopic completeness. The QSO log(N)-log(S) relation is found to have a break to a flat slope at faint fluxes. The surface density of QSOs at the survey limit is 230+/-40 per square degree, the largest so far of any QSO survey. We have used this survey to measure the QSO X-ray luminosity function at low luminosities (Lx<10^{44.5} erg s^{-1}) and high redshifts (1<z<2.5). The highest redshift QSO in the survey has z=3.4. Combined with the QSOs from the Einstein EMSS at bright fluxes, we find pure luminosity evolution of the form Lx proportional to (1+z)^{3.0(+0.2,-0.3)} is an adequate description of the evolution of the X-ray luminosity function at low redshifts. A redshift cutoff in the evolution is required at z=1.4 ^{+0.4}_{-0.17} (for qo=0.5). We discuss the form of this evolution, its dependence on the model assumed and the errors on the derived parameters. We show that most previous X-ray surveys, including the EMSS, are consistent with a power law luminosity evolution index of 3.0. The contribution of QSOs to the 1-2 keV cosmic X-ray background is found to be between 31% and 51%.

ROSAT PSPC spectra of X-ray-selected narrow-emission-line galaxies

We analyse the ROSAT PSPC spectrum of 19 X-ray selected Narrow Emission Line Galaxies (NELGs) discovered during the optical identification of sources in the ROSAT UK Deep Survey. Their properties are compared to those of broad line Active Galactic Nuclei (AGN) in the same sample. Counts in three spectral bands have been extracted for all the sources, and have been fitted with a power-law model assuming the Galactic value for NH. The average slope of NELGs is \alpha=0.45+-0.09, whilst for the AGN it is \alpha=0.96+-0.03. The power-law model is a good fit for ~90% of NELGs and ~75% of AGN. Recent work shows that the fractional surface density of NELGs increases with respect to AGN at faint fluxes. Thus they are expected to be an important component of the residual soft (<2 keV) X--ray background. The slope of the X--ray background (\alpha~0.4, 1-10 keV) is harder than that of AGN (\alpha~1) but our results show that it is consistent with the summed spectrum of the NELGs in the deep survey (\alpha~0.4). This may finally reconcile the spectrum of the background with the properties of the sources that constitute it.

A deep ROSAT survey — XII. The X-ray spectra of faint ROSAT sources

Optical spectroscopy has enabled us to identify the optical counterparts to over 200 faint X-ray sources to a flux limit of S_(0.5-2keV)=4x10^-15 erg s^-1 cm^-2 on 5 deep ROSAT fields. Here we present a spectral analysis of all the X-ray sources to investigate claims that the average source spectra harden at faint X-ray flux. From a hardness ratio analysis we confirm that the average spectra from 0.5-2 keV harden from an equivalent photon index of $\Gamma=2.2$ at S_(0.5-2keV)=1x10^-13 erg s^-1 cm^-2 to $\Gamma=1.7$ below 1x10^-14 erg s^-1 cm^-2. These spectral changes are due to the emergence of an unidentified source population rather than the class of X-ray QSOs already identified. The 128 QSOs detected so far show no evidence for spectral hardening over this energy range and retain a mean photon index of $\Gamma=2.2$. Recent work suggests that many of the remaining unidentified sources are X-ray luminous galaxies. Taking a subset identified as the most likely galaxy candidates we find that these show significantly harder spectra than QSOs. The emission line galaxies in particular show spectra more consistent with the residual X-ray background, with $\Gamma=1.51 \pm 0.1$ from 0.1-2 keV. Individually the galaxies appear to be a mixture of absorbed and unabsorbed X-ray sources. Combined with recent cross-correlation results and work on the source number count distribution, these results suggest that we may be uncovering the missing hard component of the cosmic X-ray background.