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1.
We present a 5–8 μm analysis of the Spitzer spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift   z < 0.15  , devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SBs) as the power source of the extreme infrared emission. Around ∼5 μm, an AGN is much brighter (by a factor of ≈30) than an SB of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in ∼70 per cent of our sample (50/71 sources). Through a simple analytical model, we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (∼23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.  相似文献   

2.
We present the rest-frame optical and infrared colours of a complete sample of  1114 z < 0.3  galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical  ( g − r )  colour and infrared  log( L 24/ L 3.6)  colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The  ( g − r )  colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the  log( L 24/ L 3.6)  colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red  ( g − r )  colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'.  相似文献   

3.
We study the N H distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20–40 keV band from INTEGRAL /Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed  ( N H≥ 1022 cm2)  sources is 43 per cent while the Compton thick AGN comprise 7 per cent of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported by Risaliti, Maiolino & Salvati using an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account, we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore, we show that the measured fraction of absorbed sources in our sample decreases from 80 to ∼20–30 per cent as a function of redshift with all Compton thick AGN having   z ≤ 0.015  . If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their N H distribution. We conclude that in the low-redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent.  相似文献   

4.
Optically very faint  ( R > 25.5)  sources detected by the Spitzer Space Telescope at 24 μm represent a very interesting population at redshift   z ∼ (1.5–3)  . They exhibit strong clustering properties, implying that they are hosted by very massive haloes, and their mid-infrared emission could be powered by either dust-enshrouded star formation and/or by an obscured active galactic nucleus (AGN). We report observations carried out with the Max Planck Millimetre Bolometer (MAMBO) array at the IRAM 30-m antenna on Pico Veleta of a candidate protocluster with five optically obscured sources selected from the 24-μm Spitzer sample of the First-Look Survey. Interestingly, these sources appear to lie on a high-density filament aligned with the two radio jets of an AGN. Four out of five of the observed sources were detected. We combine these measurements with optical, infrared and radio observations to probe the nature of the candidate protocluster members. Our preliminary conclusions can be summarized as follows: the spectral energy distributions (SEDs) of all sources include both AGN and starburst contributions; the AGN contribution to the bolometric luminosities ranges between 14 and 26 per cent of the total. Such a contribution is enough for the AGN to dominate the emission at 5.8, 8 and 24 μm, while the stellar component, inferred from SED fitting, prevails at 1.25 mm and at  λ < 4.5 μ  m. The present analysis suggests a coherent interplay at high z between extended radio activity and the development of filamentary large-scale structures.  相似文献   

5.
We describe improved modelling of the emission by dust in a toroidal-like structure heated by a central illuminating source within active galactic nuclei (AGNs). We have chosen a simple but realistic torus geometry, a flared disc, and a dust grain distribution function including a full range of grain sizes. The optical depth within the torus is computed in detail taking into account the different sublimation temperatures of the silicate and graphite grains, which solves previously reported inconsistencies in the silicate emission feature in type 1 AGNs. We exploit this model to study the spectral energy distributions (SEDs) of 58 extragalactic (both type 1 and type 2) sources using archival optical and infrared data. We find that both AGN and starburst contributions are often required to reproduce the observed SEDs, although in a few cases they are very well fitted by a pure AGN component. The AGN contribution to the far-infrared luminosity is found to be higher in type 1 sources, with all the type 2 requiring a substantial contribution from a circumnuclear starburst. Our results appear in agreement with the AGN unified scheme, because the distributions of key parameters of the torus models turn out to be compatible for type 1 and type 2 AGNs. Further support to the unification concept comes from comparison with medium-resolution infrared spectra of type 1 AGNs by the Spitzer observatory, showing evidence for a moderate silicate emission around 10 μm, which our code reproduces. From our analysis we infer accretion flows in the inner nucleus of local AGNs characterized by high equatorial optical depths  ( AV ≃ 100)  , moderate sizes  ( R max < 100 pc)  and very high covering factors (   f ≃ 80  per cent) on average.  相似文献   

6.
We present a result of cross-correlating the Infrared Astronomical Satellite Faint Source Catalogue with the spectroscopic catalogues of galaxies in the Fourth Data Release of the Sloan Digital Sky Survey, the Final Data Release of the 2dF Galaxy Redshift Survey (2dFGRS) and the Second Data Release of the 6dF Galaxy Survey. We have identified 324 ultraluminous infrared galaxies (ULIRGs) including 190 newly discovered ULIRGs, and two hyperluminous infrared galaxies. 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     , which is larger than that     of the sample of previously known ULIRGs.  相似文献   

7.
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8.
The emission from individual X-ray sources in the Chandra Deep Fields and XMM – Newton Lockman Hole shows that almost half of the hard X-ray background above 6 keV is unresolved and implies the existence of a missing population of heavily obscured active galactic nuclei (AGN). We have stacked the 0.5–8 keV X-ray emission from optical sources in the Great Observatories Origins Deep Survey (GOODS; which covers the Chandra Deep Fields) to determine whether these galaxies, which are individually undetected in X-rays, are hosting the hypothesized missing AGN. In the 0.5–6 keV energy range, the stacked-source emission corresponds to the remaining 10–20 per cent of the total background – the fraction that has not been resolved by Chandra . The spectrum of the stacked emission is consistent with starburst activity or weak AGN emission. In the 6–8 keV band, we find that upper limits to the stacked X-ray intensity from the GOODS galaxies are consistent with the ∼40 per cent of the total background that remains unresolved, but further selection refinement is required to identify the X-ray sources and confirm their contribution.  相似文献   

9.
As part of a large spectroscopic survey of   z > 5  Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R -band dropout galaxies at   z ∼ 5  and above, only J104048.6−115550.2 at   z = 5.44  shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N  v is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope ( HST ) imaging. This, and the comparatively high Lyα/N  v flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux  ( I AB= 26.1)  is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.  相似文献   

10.
We present HST WFPC2 V -band imaging for 23 ultraluminous infrared galaxies (ULIRGs) taken from the QDOT redshift survey. The fraction of sources observed to be interacting is 87 per cent. Most of the merging systems show a number of compact 'knots', whose colour and brightness differ substantially from their immediate surroundings. Colour maps for nine of the objects show a non-uniform colour structure. Features include blue regions located towards the centres of merging systems which are likely to be areas of enhanced star formation, and compact red regions which are likely to be dust shrouded starbursts or active galactic nuclei. The host galaxies of the quasi-stellar objects (QSOs) in the sample were found to be either interacting systems or ellipticals. Our data show no evidence that ULIRGs are a simple transition stage between galaxy mergers and QSOs. We propose an alternative model for ULIRGs based on the morphologies in our sample and previous N -body simulations. Under this model ULIRGs as a class are much more diverse than a simple transition between galaxy merger and QSO. The evolution of IR power source and merger morphology in ULIRGs is driven solely by the local environment and the morphologies of the merger progenitors.  相似文献   

11.
We have cross-matched the 1.4-GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 deg2 (about 20 per cent of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio identifications – the largest and most homogeneous set of radio source spectra ever obtained. The 2dFGRS radio sources span the redshift range     to 0.438, and are a mixture of active galaxies (60 per cent) and star-forming galaxies (40 per cent). About 25 per cent of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We make a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star formation density of         .  相似文献   

12.
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   

13.
We present an X-ray spectroscopic study of the prototype far-infrared galaxy NGC 6240 from ASCA . The soft X-ray spectrum (below 2 keV) shows clear signatures of thermal emission well described by a multitemperature optically thin plasma, which probably originates in a powerful starburst. Strong hard X-ray emission is also detected with ASCA and its spectrum above 3 keV is extremely flat with a prominent iron K line complex, very similar to that seen in the Seyfert 2 galaxy NGC 1068 but about an order of magnitude more luminous ( L 3−10keV ≈ 1.4 × 1042 erg s−1). The hard X-ray spectrum indicates that only reflected X-rays of an active galactic nucleus (AGN) buried in a heavy obscuration ( N H > 2 × 1024 cm−2) are visible. This is evidence for an AGN in NGC 6240, emitting possibly at a quasar luminosity (∼ 1045 erg s−1), and suggests its significant contribution to the far-infrared luminosity.  相似文献   

14.
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   

15.
We identify eight   z > 1  radio sources undetected at 850 μm but robustly detected at 70 μm, confirming that they represent ultraluminous infrared galaxies (ULIRGs) with hotter dust temperatures  (〈 T d〉= 52 ± 10 K)  than submillimetre galaxies (SMGs) at similar luminosities and redshifts. These galaxies share many properties with SMGs: ultraviolet spectra consistent with starbursts, high stellar masses and radio luminosities. We can attribute their radio emission to star formation since high-resolution Multi-Element Radio Linked Interferometer Network (MERLIN) radio maps show extended emission regions (with characteristic radii of 2–3 kpc), which are unlikely to be generated by active galactic nucleus (AGN) activity. These observations provide the first direct confirmation of hot, dusty ULIRGs which are missed by current submillimetre surveys. They have significant implications for future observations from the Herschel Space Observatory and Submillimetre Common-User Bolometer Array 2 (SCUBA2), which will select high-redshift luminous galaxies with less selection biases.  相似文献   

16.
We present near-infrared observations of a sample of mainly interacting ultraluminous infrared galaxies, comprising H - and K -band spectra. Our main aims are to investigate the power source of these extremely luminous objects and the various excitation mechanisms of the strong molecular hydrogen emission often seen in such objects. Broadened emission lines were only detected in one object, IRAS 23498, consistent with the previous results for this galaxy. The [Si  vi ] emission line was detected in IRAS 17179 and 20210, both classified as Sy2s. Two of the samples were unclassified, IRAS 00150 and 23420, which exhibit neither [Si  vi ] emission nor broadened H  i emission. However this does not rule out the presence of an obscured AGN. Analysis of the molecular hydrogen emission showed that the major excitation mechanism for most objects was thermal. Modelling of the more luminous objects indicates that for IRAS 20210 10 per cent, and for both IRAS 23365 and IRAS 23420 30 per cent, of the 1–0 S(1) line emission has a non-thermal origin.  相似文献   

17.
We show that the far-IR properties of distant Luminous and UltraLuminous InfraRed Galaxies (LIRGs and ULIRGs, respectively) are on average divergent from analogous sources in the local Universe. Our analysis is based on Spitzer Multiband Imaging Photometer (MIPS) and Infrared Array Camera (IRAC) data of   L IR > 1010 L⊙, 70 μm  selected objects in the  0.1 < z < 2  redshift range and supported by a comparison with the IRAS Bright Galaxy Sample. The majority of the objects in our sample are described by spectral energy distributions (SEDs) which peak at longer wavelengths than local sources of equivalent total infrared luminosity. This shift in SED peak wavelength implies a noticeable change in the dust and/or star-forming properties from   z ∼ 0  to the early Universe, tending towards lower dust temperatures, indicative of strong evolution in the cold dust, 'cirrus', component. We show that these objects are potentially the missing link between the well-studied local IR-luminous galaxies, Spitzer IR populations and SCUBA sources – the   z < 1  counterparts of the cold   z > 1  SubMillimetre Galaxies (SMGs) discovered in blank-field submillimetre surveys. The Herschel Space Observatory is well placed to fully characterize the nature of these objects, as its coverage extends over a major part of the far-IR/sub-mm SED for a wide redshift range.  相似文献   

18.
We investigate the environments and clustering properties of starburst galaxies selected from the 2dF Galaxy Redshift Survey (2dFGRS) in order to determine which, if any, environmental factors play a role in triggering a starburst. We quantify the local environments, clustering properties and luminosity functions of our starburst galaxies and compare to random control samples. The starburst galaxies are also classified morphologically in terms of their broad Hubble type and evidence of tidal merger/interaction signatures. We find the starburst galaxies to be much less clustered on large (5–15 Mpc) scales compared to the overall 2dFGRS galaxy population. In terms of their environments, we find just over half of the starburst galaxies to reside in low to intermediate luminosity groups, and a further ∼30 per cent residing in the outskirts and infall regions of rich clusters. Their luminosity functions also differ significantly from that of the overall 2dFGRS galaxy population, with the sense of the difference being critically dependent on the way their star formation rates are measured. In terms of pin-pointing what might trigger the starburst, it would appear that factors relating to their local environment are most germane. Specifically, we find clear evidence that the presence of a near neighbour of comparable luminosity/mass within 20 kpc is likely to be important in triggering a starburst. We also find that a significant fraction (20–30 per cent) of the galaxies in our starburst samples have morphologies indicative of either an ongoing or a recent tidal interaction and/or merger. These findings notwithstanding, there remain a significant portion of starburst galaxies where such local environmental influences are not in any obvious way playing a triggering role, leading us to conclude that starbursts can also be internally driven.  相似文献   

19.
We present the K -band Hubble diagrams ( K – z relations) of submillimetre-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 radio galaxies. Like radio galaxies, the HLIRG K – z relation at   z ≲ 3  is consistent with a passively evolving ∼3 L * instantaneous starburst starting from a redshift of   z ∼ 10  . In contrast, many submillimetre-selected galaxies are ≳2 mag 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 submillimetre-selected galaxies due to the 60-μm selection of some, so the location on the K – z relation may be related to the presence of the most massive active galactic nucleus. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.  相似文献   

20.
We present Hubble Space Telescope Wide Field Planetary Camera 2 I -band imaging for a sample of nine hyperluminous infrared galaxies (HLIRGs) spanning a redshift range     . Three of the sample have morphologies showing evidence for interactions and six are quasi-stellar objects (QSOs). Host galaxies in the QSOs are detected reliably out to     . The detected QSO host galaxies have an elliptical morphology with scalelengths spanning     and absolute k -corrected magnitudes spanning     There is no clear correlation between the infrared (IR) power source and the optical morphology. None of the sources in the sample, including F15307+3252, shows any evidence for gravitational lensing. We infer that the IR luminosities are thus real. Based on these results, and previous studies of HLIRGs, we conclude that this class of object is broadly consistent with being a simple extrapolation of the ULIRG population to higher luminosities; ULIRGs being mainly violently interacting systems powered by starbursts and/or active galactic nuclei. Only a small number of sources, the infrared luminosities of which exceed 1013 L⊙, are intrinsically less luminous objects that have been boosted by gravitational lensing.  相似文献   

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