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1.
Spectroscopic surveys of submillimetre(submm)-selected sources have uncovered optically bright galaxies at   z ≲ 1  close to the positions of several submm sources. Naive statistical analyses suggest that these galaxies are associated with the submm emission. However, in some cases, it is difficult to understand this association given the relatively modest redshifts and unprepossessing spectral characteristics of the galaxies. These are in stark contrast to those expected from the massive dust-enshrouded starbursts and AGN thought to power the bulk of the bright submm population. We present new observations of optically bright counterparts to two luminous submm sources, along with a compilation of previously proposed optically bright counterparts with   z ≲ 1  . We suggest that the majority of these associations between bright galaxies and submm sources may be as a result of the action of the foreground galaxies as gravitational lenses on the much fainter and more distant submm sources. We discuss the implications of this conclusion for our understanding of the SCUBA population.  相似文献   

2.
We present confusion-limited submillimetre (submm) observations with the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope of the   z = 2.83  Lyman-break galaxy (LBG), Westphal–MM8, reaching an 850 μm sensitivity even greater than that achieved in the SCUBA map of the Hubble Deep Field region. The detection of MM8  ( S 850 μm= 1.98 ± 0.48 mJy)  , along with the literature submm detections of lensed LBGs, suggests that the LBG population may contribute significantly to the source counts of submm-selected galaxies in the 1–2 mJy regime. Additionally, submm-luminous LBGs are a viable progenitor population for the recently discovered evolved galaxies at   z ∼ 2–2.5  . These observations represent an important baseline for SCUBA2 observations which will regularly map large regions of the sky to this depth.  相似文献   

3.
We present an analysis of the X-ray point source populations in 182 Chandra images of galaxy clusters at   z > 0.1  with exposure time >10 ks, as well as 44 non-cluster fields. The analysis of the number and flux of these sources, using a detailed pipeline to predict the distribution of non-cluster sources in each field, reveals an excess of X-ray point sources associated with the galaxy clusters. A sample of 148 galaxy clusters at  0.1 < z < 0.9  , with no other nearby clusters, shows an excess of 230 cluster sources in total, an average of ∼1.5 sources per cluster. The lack of optical data for these clusters limits the physical interpretation of this result, as we cannot calculate the fraction of cluster galaxies hosting X-ray sources. However, the fluxes of the excess sources indicate that over half of them are very likely to be active galactic nuclei (AGN), and the radial distribution shows that they are quite evenly distributed over the central 1 Mpc of the cluster, with almost no sources found beyond this radius. We also use this pipeline to successfully reproduce the results of previous studies, particularly the higher density of sources in the central 0.5 Mpc of a few cluster fields, but show that these conclusions are not generally valid for this larger sample of clusters. We conclude that some of these differences may be due to the sample properties, such as the size and redshift of the clusters studied, or a lack of publications for cluster fields with no excess sources. This paper also presents the basic X-ray properties of the galaxy clusters, and in subsequent papers in this series the dependence of the AGN population on these cluster properties will be evaluated.
In addition the properties of over 9500 X-ray point sources in the fields of galaxy clusters are tabulated in a separate catalogue available online or at http://www.sc.eso.org~rgilmour .  相似文献   

4.
We present William Herschel Telescope spectropolarimetry observations of a complete RA-limited sample of nine low-redshift  (0.05< z <0.2)  3CR radio sources in order to investigate the nature of the ultraviolet (UV) excess in nearby powerful radio galaxies. Of the nine galaxies studied in detail from this sample, we find that four show a measurable UV excess following nebular continuum subtraction, but none of the sources shows significant polarization in the UV. One of the radio galaxies with a UV excess – 3C 184.1 – shows evidence for broad permitted lines and hence direct active galactic nucleus (AGN) light. In the remaining three galaxies we argue that the most likely contributor to the UV excess is a young stellar component. For these three galaxies we find that the best-fitting model for the optical/UV continuum consists of a combination of an old stellar population  (10–15 Gyr  old elliptical galaxy) plus a reddened young stellar population  (0.05–2 Gyr)  . The reddened young stellar component typically accounts for half of the total flux at 4780 Å, following nebular continuum subtraction, and   E ( B - V )  values of between 0.2 and 0.7 mag are required. However, for the majority of sources in our sample (six out of nine), continuum modelling provides no evidence for a significant young stellar component in the nuclear regions of the host galaxies. Our results are discussed in the context of far-infrared evidence for star formation activity.  相似文献   

5.
We present the results of fitting deep off-nuclear optical spectra of radio-quiet quasars, radio-loud quasars and radio galaxies at z ≃0.2 with evolutionary synthesis models of galaxy evolution. Our aim was to determine the age of the dynamically dominant stellar populations in the host galaxies of these three classes of powerful active galactic nuclei (AGN). Some of our spectra display residual nuclear contamination at the shortest wavelengths, but the detailed quality of the fits longward of the 4000-Å break provides unequivocal proof, if further proof were needed, that quasars lie in massive galaxies with (at least at z ≃0.2) evolved stellar populations. By fitting a two-component model we have separated the very blue (starburst and/or AGN contamination) from the redder underlying spectral energy distribution, and find that the hosts of all three classes of AGN are dominated by old stars of age 8–14 Gyr. If the blue component is attributed to young stars, we find that, at most, 1 per cent of the visible baryonic mass of these galaxies is involved in star formation activity at the epoch of observation, at least over the region sampled by our spectroscopic observations. These results strongly support the conclusion reached by McLure et al. that the host galaxies of luminous quasars are massive ellipticals which have formed by the epoch of peak quasar activity at z ≃2.5.  相似文献   

6.
We have used deep ground-based imaging in the near-infrared (near-IR) to search for counterparts to the luminous submillimetre (submm) sources in the catalogue of Smail et al. For the majority of the submm sources the near-IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10 per cent of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep Hubble Space Telescope ( HST ) and ground-based R -band images used by Smail et al. We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2 σ limits of ( I − K )≳6.8 and ( I − K )≳6.0, respectively. Both sources thus class as extremely red objects (EROs). Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. This connection provides important insights into the nature of the enigmatic ERO population and faint submm galaxies in general. From the estimated surface density of these submm-bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide useful insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank-field submm surveys will be extremely difficult owing to their faintness ( K ∼20.5, I ≳26.5). Finally, we discuss the radio and submm properties of the two submm-bright EROs discovered here and suggest that both galaxies lie at z ≳2.  相似文献   

7.
We present XMM data for the supercluster A901/2, at   z ∼ 0.17  , which is combined with deep imaging and 17-band photometric redshifts (from the COMBO-17 survey), two degree field (2dF) spectra and Spitzer 24 μm data, to identify active galactic nuclei (AGN) in the supercluster. The 90 ksec XMM image contains 139 point sources, of which 11 are identified as supercluster AGN with   L X(0.5−7.5 keV) > 1.7 × 1041 erg cm−2 s−1  . The host galaxies have   M R < −20  and only two of eight sources with spectra could have been identified as AGN by the detected optical emission lines. Using a large sample of 795 supercluster galaxies, we define control samples of massive galaxies with no detected AGN. The local environments of the AGN and control samples differ at ≳98 per cent significance. The AGN host galaxies lie predominantly in areas of moderate projected galaxy density and with more local blue galaxies than the control sample, with the exception of one very bright type I AGN very near the centre of a cluster. These environments are similar to, but not limited to, cluster outskirts and blue groups. Despite the large number of potential host galaxies, no AGN are found in regions with the highest galaxy density (excluding some cluster cores where emission from the intra-cluster medium obscures moderate luminosity AGN). AGN are also absent from the areas with lowest galaxy density. We conclude that the prevalence of cluster AGN is linked to their environment.  相似文献   

8.
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.  相似文献   

9.
We use morphological information of X-ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at   z ≈ 1  that is not likely to be associated with major mergers. Deep X-ray observations are combined with high-resolution optical data from the Hubble Space Telescope in the All-wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X-ray sources in the redshift interval  0.5 < z < 1.3  . The sample is split into discs, early-type bulge-dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X-ray luminosity function and luminosity density of AGN at   z ≈ 1  are then calculated as a function of morphological type. We find that disc-dominated hosts contribute  30 ± 9  per cent to the total AGN space density and  23 ± 6  per cent to the luminosity density at   z ≈ 1  . We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN     compared to predictions for the stochastic fuelling of massive black holes in disc galaxies.  相似文献   

10.
We present detailed observations of MRC 0116+111, revealing a luminous, miniradio halo of ∼240-kpc diameter located at the centre of a cluster of galaxies at redshift   z = 0.131  . Our optical and multiwavelength Giant Metrewave Radio Telescope and Very Large Array radio observations reveal a highly unusual radio source: showing a pair of giant (∼100-kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 – the dominant central radio galaxy in the Virgo cluster. However, in MRC 0116+111 we do not detect any ongoing active galactic nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located ∼100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intracluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration. We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intracluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio jet inflated plasma bubbles in the hot cluster atmosphere.  相似文献   

11.
We explore the role of active galactic nuclei (AGN) in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1 Gyr) the termination of their star formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the All-wavelength Extended Groth strip International Survey (AEGIS). Stacking the X-ray photons at the positions of galaxies  (0.4 < z < 0.9)  not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-infrared (IR) properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that black hole (BH) accretion outlives the termination of the star formation. This is also supported by our finding that post-starburst galaxies at z ≈ 0.8 and AGN are associated, in agreement with recent results at low z . A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after quenching their star formation.  相似文献   

12.
We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H  i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O  iii ] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H  i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.  相似文献   

13.
We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.  相似文献   

14.
We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disc and massive H  i gas content. Radio continuum observations of the galaxy were carried out at 320, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disc at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with active galactic nucleus (AGN) activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the Hα flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centres can host significant AGN activity. We also mapped the H  i gas disc and velocity field in PGC 045080. The H  i disc extends well beyond the optical disc and appears warped. In the H  i intensity maps, the disc appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disc but is fairly uniform in the other half. We derived the H  i rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of ∼190 km s−1.  相似文献   

15.
We present further spectroscopic observations for a sample of galaxies selected in the vacuum ultraviolet (UV) at 2000 Å from the FOCA balloon-borne imaging camera of Milliard et al. This work represents an extension of the initial study by Treyer et al. Our enlarged catalogue contains 433 sources (≃3 times as many as in our earlier study) across two FOCA fields. 273 of these are galaxies, nearly all with redshifts z ≃0–0.4. Nebular emission-line measurements are available for 216 galaxies, allowing us to address issues of excitation, reddening and metallicity. The UV and H α luminosity functions strengthen our earlier assertions that the local volume-averaged star formation rate is higher than indicated from earlier surveys. Moreover, internally within our sample, we do not find a steep rise in the UV luminosity density with redshift over 0< z <0.4. Our data are more consistent with a modest evolutionary trend, as suggested by recent redshift survey results. Investigating the emission-line properties, we find no evidence for a significant number of AGN in our sample; most UV-selected sources to z ≃0.4 are intense star-forming galaxies. We find that the UV flux indicates a consistently higher mean star formation rate than that implied by the H α luminosity for typical constant or declining star formation histories. Following Glazebrook et al., we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. We develop a simple algorithm which explores the scatter in the UV flux–H α relation in the context of various burst scenarios. Whilst we can explain most of our observations in this way, there remains a small population with extreme UV–optical colours which cannot be understood.  相似文献   

16.
We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range  0.01 < z < 0.16  . The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of Hα equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour–magnitude space or colour–concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced Hα emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a 'transition' population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys.  相似文献   

17.
We present a photometric far-ultraviolet (FUV) to K s-band study of the field around quasar SDSS J092712.65+294344.0. The SDSS spectrum of this object shows various emission lines with two distinct redshifts, at   z = 0.699  and 0.712. Because of this peculiar spectroscopic feature, this source has been proposed as a candidate recoiling or binary black hole. A third alternative model involves two galaxies moving in the centre of a rich galaxy cluster. Here, we present a study addressing the possible presence of such a rich cluster of galaxies in the SDSS J092712.65+294344.0 field. We observed the  3.6 × 2.6  arcmin2 field in the K s band and matched the near-infrared data with the FUV and near-ultraviolet images in the Galaxy Evolution Explorer archive and the ugriz observations in the SDSS. From various colour–colour diagrams, we were able to classify the nature of 32 sources, only 6–11 of which have colours consistent with galaxies at   z ≈ 0.7  . We compare these numbers with the surface density of galaxies, stars and quasars and the expectations for typical galaxy clusters both at low and high redshift. Our study shows that the galaxy cluster scenario is in clear disagreement with the new observations.  相似文献   

18.
We show that the luminosity functions of the distant rich clusters Abell 665 ( z =0.182) and Abell 963 ( z =0.206) are flat or gradually rising down to MR =−14, with α≈−1.2±0.4 [here α is the logarithmic slope of the luminosity function: φ( L )∝ L α at the faint end]. We do not confirm the steep luminosity functions (α≤−1.8) that have been recently proposed for these two clusters.
Several technical points are discussed in detail. In particular, we compute the corrections to the background contamination caused by gravitational lensing from the cluster dark matter, and show that the corrections are small unless we wish to determine variations in the luminosity function on small scales.
Recent observations have also shown that the field galaxy luminosity function at z ≈0.2 is also shallow between MB =−19 and MB =−13. Abell 665 and 963 are two of the richest clusters known at that redshift. We therefore propose that the galaxy luminosity function might be universal in this magnitude range at z =0.2.
The dwarf galaxies that we see in Abell 665 have a colour distribution that is strongly peaked at B − R =1.9. We compute K -corrections based on the spectral energy distributions of local galaxies, and show that these are probably dwarf spheroidal galaxies. This might suggest that the dwarf spheroidal population observed in Virgo already existed at z =0.2.  相似文献   

19.
We present a comparison between the published optical, infrared (IR) and CO spectroscopic redshifts of 15 (sub)mm galaxies and their photometric redshifts as derived from long-wavelength (radio–mm–far-IR) photometric data. The redshift accuracy measured for 12 submillimetre (submm) galaxies with at least one robustly determined colour in the radio–mm–far-IR regime is  δ z ≈ 0.30 (rms)  . Despite the wide range of spectral energy distributions in the local galaxies that are used in an unbiased manner as templates, this analysis demonstrates that photometric redshifts can be efficiently derived for submm galaxies with a precision of  δ z < 0.5  using only the rest-frame far-IR to radio wavelength data.  相似文献   

20.
The spatial clustering amplitude ( B gq) is determined for a sample of 44 powerful active galactic nuclei (AGN) at z ≃0.2. No significant difference is detected in the richness of the cluster environments of the radio-loud and radio-quiet subsamples, both of which typically inhabit environments as rich as Abell class ≃0. Comparison with radio luminosity-matched samples from Hill & Lilly and Wold et al. suggests that there is no epoch-dependent change in environment richness out to at least z ≥0.5 for either radio galaxies or radio quasars. Comparison with the APM cluster survey shows that, contrary to current folklore, powerful AGN do not avoid rich clusters, but rather display a spread in cluster environment, which is perfectly consistent with being drawn at random from the massive elliptical population. Finally, we argue that virtually all Abell class ≃0 clusters contained an active galaxy during the epoch of peak quasar activity at z ∼2.5.  相似文献   

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