arXiv subjects

F. N. Owen

Publications and source records attributed to F. N. Owen.

At least 37 records · Page 2Linked to original sources

arXiv2002

Radio-selected Galaxies in Very Rich Clusters at z < 0.25: I. Multi-wavelength Observations and Data Reduction Techniques

Radio observations were used to detect the `active' galaxy population within rich clusters of galaxies in a non-biased manner that is not plagued by dust extinction or the K-correction. We present wide-field radio, optical (imaging and spectroscopy), and ROSAT All-Sky Survey (RASS) X-ray data for a sample of 30 very rich Abell (R > 2) cluster with z < 0.25. The VLA radio data samples the ultra-faint radio (L(1.4 GHz) > 2E22 W/Hz) galaxy population within these extremely rich clusters for galaxies with M_R < -21. This is the largest sample of low luminosity 20 cm radio galaxies within rich Abell clusters collected to date. The radio-selected galaxy sample represents the starburst (Star formation rate > 5 M_sun/yr) and active galactic nuclei (AGN) populations contained within each cluster. Archival and newly acquired redshifts were used to verify cluster membership for most (~95%) of the optical identifications. Thus we can identify all the starbursting galaxies within these clusters, regardless of the level of dust obscuration that would affect these galaxies being identified from their optical signature. Cluster sample selection, observations, and data reduction techniques for all wavelengths are discussed.

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arXiv2002

The Diversity of Extremely Red Objects

We present a multi-wavelength study of Extremely Red Objects (EROs) employing deep RIzJHK photometry of a 8.5'x8.5' region to identify 68 EROs with R-K>5.3 and K<20.5 (5-sigma). This is combined with an extremely deep 1.4-GHz radio map (sigma=3.5uJy), sensitive enough to detect an active galaxy with L_1.4>10^23 W/Hz at z>1 or a SFR of >25Mo/yr. We detect radio emission from 21 EROs at >12.6uJy and resolve a third of these with our 1.6'' FWHM beam. The SEDs of most of these radio EROs are consistent with dust-reddened starbursts at z~1. At z~1 the radio luminosities of these EROs indicate far-infrared luminosities of L_FIR>10^12 Lo, meaning half are ultraluminous infrared galaxies. We conclude that >16+/-5% of EROs with K<20.5 are luminous infrared galaxies at z>1. We also photometrically classify the EROs to investigate the mix of dusty/active and evolved/passive systems in the radio-undetected EROs. We suggest that at least 30%, and perhaps up to ~60%, of all EROs with R-K>5.3 and K<20.5 are dusty, star-forming systems at z>1. The SFD in this optically faint (R>26) population is rho^* (0.1-100Mo)=0.11+/-0.03 Mo/yr/Mpc^3, comparable to that in H-alpha emitting galaxies at z~1, and greater than that in UV-selected samples at these epochs. This support the claim of a strong increase in obscured star formation at high redshifts. Using the observed counts of the radio-detected EROs we model the break in the K-band number counts of all EROs at K~19-20 and propose that the passive ERO class dominates the total population in a narrow range around K~20, with dusty EROs dominating at fainter magnitudes. [Abridged]

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arXiv2002

GMOS Spectroscopy of SCUBA Galaxies Behind A851

We have identified counterparts to two submillimeter (submm) sources, SMM J09429+4659 and SMM J09431+4700, seen through the core of the z=0.41 cluster Abell 851. We employ deep 1.4-GHz observations and the far-infrared/radio correlation to refine the submm positions and then optical and near-infrared imaging to locate their counterparts. We identify an extremely red counterpart to SMM J09429+4659, while GMOS spectroscopy with Gemini-North shows that the R=23.8 radio source identified with SMM J09431+4700 is a hyperluminous infrared galaxy (L_FIR~1.5x10^13 L_sun) at z=3.35, the highest spectroscopic redshift so far for a galaxy discovered in the submm. The emission line properties of this galaxy are characteristic of a narrow-line Seyfert-1, although the lack of detected X-ray emission in a deep XMM-Newton observation suggests that the bulk of the luminosity of this galaxy is derived from massive star formation. We suggest that active nuclei, and the outflows they engender, may be an important part of the evolution of the brightest submm galaxies at high redshifts.

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arXiv2001

Supermassive Black Hole Accretion History Inferred from a Large Sample of Chandra Hard X-ray Sources

We describe the optical, near-infrared, and radio properties of a sample of hard (2-7 keV) X-ray sources detected in a deep Chandra observation of the field surrounding the Abell 370 cluster. We combine these data with similar observations of the Chandra Deep Field-North and the Hawaii Survey Field SSA13 to obtain a sample of 69 hard X-ray sources (45 are spectroscopically identified) with extremely deep 20 cm observations. We find that about 4% of the >Lstar galaxy population is X-ray luminous at any time and hence that black hole accretion has a duration of about half a Gyr. We find that about 30% of the summed 2-7 keV flux from our total sample is from sources at z<1. We estimate the bolometric luminosities of accretion onto supermassive black holes and the mass inflow rates. The time history of the accretion rate density is evaluated, and its integrated value is reasonably consistent with the value inferred from the local black hole mass to bulge mass ratio.

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arXiv2001

GMRT Detection of HI 21 cm-line Absorption from the Peculiar Galaxy in Abell 2125

Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km/s. The estimated column density of atomic Hydrogen is 0.7e22(Ts/100K) per sq. cm. The HI absorption is redshifted by ~ 400 km/s compared to the [OIII] emission line from this system. We attribute this to an in-falling cold gas, or to an out-flowing ionised gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster potential.

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arXiv2000

Extended Extragalactic Radio Emission

Extended radio emission and its relation to parent galaxy properties is briefly reviewed. Our current understanding of the relation between absolute radio and optical luminosity, radio morphology and linear size is discussed. The impact of radio jets on dense cluster cores is discussed using M87 as an example. Finally, the relation of AGN's to star-bursting galaxies at high redshift is considered.

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arXiv2000

M87 at 90cm: A Different Picture

We report new radio imaging of the large scale radio structure of M87 with the VLA at 90 cm. These new images show the complex structure of the radio emission more clearly than previous attempts, some of which date back to the 1940's. The images suggest that the outward flow from the M87 nucleus extends well beyond the 2 kpc jet. Two ``bubbles'' of synchrotron emission appear to be inflated by this flow. A simple model of the emission, combined with our knowledge of the inner jet, suggests that the energy input into this region from the M87 nucleus exceeds the energy being radiated away as X-rays. This argues that the region within 40 kpc of the center of M87 is currently dominated by energy input from the M87 nucleus. The gas in the region is expanding, not flowing inward as is envisioned in the cooling flow model.

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arXiv1999

X-ray and Radio Interactions in the Cores of Cooling Flow Clusters

We present high resolution ROSAT x-ray and radio observations of three cooling flow clusters containing steep spectrum radio sources at their cores. All three systems exhibit strong signs of interaction between the radio plasma and the hot intracluster medium. Two clusters, A133 and A2626, show enhanced x-ray emission spatially coincident with the radio source whereas the third cluster, A2052, exhibits a large region of x-ray excess surrounding much of the radio source. Using 3-D numerical simulations, we show that a perturbed jet propagating through a cooling flow atmosphere can give rise to amorphous radio morphologies, particularly in the case where the jet was ``turned off'' and allowed to age passively. In addition, the simulated x-ray surface brightness produced both excesses and deficits as seen observationally.

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arXiv1999

Stormy Weather and Cluster Radio Galaxies

New adaptive mesh refinement N-body + hydrodynamics numerical simulations are used to illustrate the complex and changing cluster environments in which many radio galaxies live and evolve. Groups and clusters of galaxies form at the intersections of filaments where they continue to accrete gas and dark matter to the present day. The accretion process produces shocks, turbulence, and transonic bulk flows forming a kind of stormy weather within the intracluster medium (ICM). Radio sources embedded within the stormy ICM form distorted, complex morphologies as observed in recent VLA cluster surveys. We show that the bending of wide-angle tailed radio sources can be understood as the result of recent cluster-subcluster mergers. We use new MHD simulations to illustrate how cluster radio halos can be formed by the shocks and turbulence produced during cluster mergers. Finally, we discuss new observations of distant Abell clusters that reveal a class of weak radio sources, probably starbursts, likely produced during the formation of the clusters as they accrete material from the supercluster environment.

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arXiv1999

Rich Cluster and Non-Cluster Radio Galaxies & the (P,D) Diagram for a Large Number of FR I and FR II Sources

We present a comparison of the optical and radio properties of radio sources inside and outside the cores of rich clusters from combined samples of more than 380 radio sources. We also examine the nature of FR I and FR II host galaxies, and in particular, we illustrate the importance of selection effects in propagating the misconception that FR I's and FR II's are found in hosts of very different optical luminosity. Given the large sample size, we also discuss the power-size (P,D) distributions as a function of optical luminosity.

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arXiv1999

An X-ray and Optical Investigation of the Environments Around Nearby Radio Galaxies

Investigations of the cluster environment of radio sources have not shown a correlation between radio power and degree of clustering. However, it has been demonstrated that extended X-ray luminosity and galaxy clustering do exhibit a positive correlation. This study investigates a complete sample of 25 nearby (z less than 0.06) radio galaxies which are not cataloged members of Abell clusters. The environment of these radio galaxies is studied in both the X-ray and the optical by means of the ROSAT All-Sky Survey (RASS), ROSAT pointed observations, and the Palomar optical Digitized Sky Survey (DSS). X-ray luminosities and extents are determined from the RASS, and the DSS is used to quantify the degree of clustering via the spatial two-point correlation coefficient, Bgg. Of the 25 sources, 20 are greater than sigma detections in the X-ray and 11 possessed Bgg's significantly in excess of that expected for an isolated galaxy. Adding the criterion that the X-ray emission be resolved, 10 of the radio galaxies do appear to reside in poor clusters with extended X-ray emission suggestive of the presence of an intracluster medium. Eight of these galaxies also possess high spatial correlation coefficients. Taken together, these data suggest that the radio galaxies reside in a low richness extension of the Abell clusters. The unresolved X-ray emission from the other galaxies is most likely associated with AGN phenomena. Furthermore, although the sample size is small, it appears that the environments of FR I and FR II sources differ. FR I's tend to be more frequently associated with extended X-ray emission (10 of 18), whereas FR II's are typically point sources or non-detections in the X-ray (none of the 7 sources exhibit extended X-ray emission).

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arXiv1999

Radio Constraints on the Identifications and Redshifts of Submm Galaxies

We present deep radio maps from the Very Large Array (VLA) for 16 sources detected in a sub-millimeter (submm) survey of the distant Universe. Our deep VLA 1.4-GHz maps allow us to identify radio counterparts or place stringent limits (<20uJy in the source plane) on the radio flux of the submm sources. We compare the spectral indices of our sources between 850um and 1.4GHz to empirical and theoretical models for distant starburst galaxies and active galactic nuclei (AGN) as a function of redshift. In this way we can derive redshift limits for the submm sources, even in the absence of an optical or near-infrared counterpart. We conclude that the submm population brighter than \~1mJy has a median redshift of at least ~2, more probably ~2.5-3, with almost all galaxies at z>>1. This estimate is a strong lower limit as both misidentification of the radio counterparts and non-thermal emission from an AGN will bias our redshift estimates to lower values. The high median redshift means that the submm population, if predominately powered by starbursts, contributes a substantial fraction of the total star formation density at high redshifts. A comparison of the spectral index limits with spectroscopic redshifts for proposed optical counterparts to individual submm galaxies suggests that half of the submm sources remain unidentified and thus their counterparts must be fainter than I>24.

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arXiv1999

The Discovery of ERO Counterparts to Faint Submm Galaxies

We use deep ground-based imaging in the near-IR to search for counterparts to the luminous submm sources in the catalog of Smail et al (1998). 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% of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep HST and ground-based R-band images used in Smail et al (1998). We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2-sigma limits of (I-K)>6.8 and (I-K)>6.0 respectively. Both sources thus class as 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. (1998). From the 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 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.

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arXiv1999

Hubble Space Telescope Near-infrared and Optical Imaging of Faint Radio Sources in the Distant Cluster Cl0939+4713

We present deep Hubble Space Telescope NICMOS near-infrared and WFPC2 optical imaging of a small region in the core of the distant rich cluster Cl0939+4713 (z=0.41). We compare the optical and near-infrared morphologies of cluster members and find apparent small-scale optical structures within the galaxies which are absent in the near-infrared. We conclude that strong dust obscuration is a common feature in the late-type galaxies in distant clusters. We then concentrate on a sample of ten faint radio galaxies lying within our NICMOS field and selected from a very deep 1.4-GHz VLA map of the cluster with a 1sigma flux limit of 9uJy. Using published data we focus on the spectral properties of the eight radio-selected cluster members and show that these comprise a large fraction of the post-starburst population in the cluster. The simplest interpretation of the radio emission from these galaxies is that they are currently forming massive stars, contradicting their classification as post-starburst systems based on the optical spectra. We suggest that this star formation is hidden from view in the optical by the same obscuring dust which is apparent in our comparison on the optical and near-infrared morphologies of these galaxies. We caution that even in the restframe optical the effects of dust cannot be ignored when comparing samples of distant galaxies to low-redshift systems, particularly if dust is as prevelant in distant galaxies as appears to be the case in our study.

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arXiv1999

Cluster Mergers as Triggers of Star Formation and Radio Emission: A Comparative Study of the Rich Clusters A2125 and A2645

We report a detailed optical study of the clusters Abell 2125 and 2645. Both clusters have z=0.25 and richness class 4, yet contrast strongly in blue fraction and radio galaxy population. We find 27 spectroscopically confirmed radio galaxies in the blue cluster, A2125 and only four in A2645. The excess radio population in A2125 occurs entirely at L(20cm) < 10^23 W/Hz, where one expects star-formation to be responsible for the radio emission. Most of the radio galaxies have optical properties consistent with types later than E/S0, but emission lines weaker than would be expected for the SFR's implied by the radio emission. Thus we suspect dust obscuration is important. The cluster- cluster merger in A2125 seems likely to play a part in these phenomena.

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arXiv1998

Cluster-subcluster Mergers and the Formation of Narrow-angle Tailed Radio Sources

We have examined the ROSAT PSPC X-ray properties of a sample of 15 Abell clusters containing 23 narrow-angle tailed (NAT) radio galaxies. We find that clusters with NATs show a significantly higher level of substructure than a similar sample of radio-quiet clusters, indicating that NAT radio sources are preferentially located in dynamically complex systems. Also, the velocity distribution of the NAT galaxies is similar to that of other cluster members; these velocities are inadequate for producing the ram pressure necessary to bend the radio jets. We therefore propose a new model for NAT formation, in which NATs are associated with dynamically complex clusters undergoing merger events. The U-shaped NAT morphology is produced in part by the merger-induced bulk motion of the ICM bending the jets.

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arXiv1998

Constraints on UV Absorption in the Intracluster Medium of Abell 1030

We present results from an extensive HST spectroscopic search for UV absorption lines in the spectrum of the quasar B2~1028+313, which is associated with the central dominant galaxy in the cluster Abell~1030 ($z=0.178$). This is one of the brightest known UV continuum sources located in a cluster, and therefore provides an ideal opportunity to obtain stringent constraints on the column densities of any cool absorbing gas that may be associated with the intracluster medium (ICM). Our HST spectra were obtained with the FOS and GHRS, and provide continuous coverage at rest-frame wavelengths from $\sim 975$ to 4060~Å, thereby allowing the investigation of many different elements and ionization levels. We utilize a new technique that involves simultaneous fitting of large numbers of different transitions for each species, thereby yielding more robust constraints on column densities than can be obtained from a single transition. This method yields upper limits of $\lesssim 10^{11} - 10^{13}$ cm$^{-2}$ on the column densities of a wide range of molecular, atomic and ionized species that may be associated with the ICM. We also discuss a possible \Lya and C IV absorption system associated with the quasar. We discuss the implications of the upper limits on cool intracluster gas in the context of the physical properties of the ICM and its relationship to the quasar.

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arXiv1998

X-ray Properties of B2 1028+313: A Quasar at the Center of the Abell Cluster A1030

X-ray observations with the ROSAT HRI and with ASCA are presented for the nearby radio quasar B2 1028+313, which is located in the cD galaxy at the center of the Abell cluster A1030. We also analyze archival ROSAT PSPC observations. We find that the X-ray emission is dominated by the quasar. The flux varied by a factor of about two between the ROSAT HRI and ASCA observations, which were about one year apart. The X-ray spectrum of the quasar is fit by a single power-law, except at low energies where there is a soft excess. Although the shape of the soft excess is not strongly constrained, it can be fit by a blackbody with a temperature of about 30 eV. There was evidence for extended X-ray emission, which contributed about 25% of the total flux. However, this emission does not appear to be normal X-ray emission from intracluster gas or a central cooling flow. The extended X-ray emission appears to be quite soft; if its spectrum is modeled as thermal emission, the temperature is ~0.2 keV, rather than the 5-10 keV expected for ICM emission. The radial surface distribution of the emission was not fit by either the beta model which usually describes ICM emission, or by a cooling flow model. The ASCA and ROSAT spectra showed no convincing evidence for a thermal component with a cluster-like temperature, either in the overall spectral shape or in emission lines. In addition, the ROSAT PSPC image showed that the extended X-ray emission was highly elongated to the NNW and SSE, in the same direction as the extended radio emission from the quasar. We suggest that the extended emission is inverse Compton emission from the extended radio lobes.

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