arXiv subjects

A. Georgakakis

Publications and source records attributed to A. Georgakakis.

At least 127 records · Page 7Linked to original sources

arXiv2004

X-ray Bright Optically Inactive Galaxies (XBONG) in XMM-Newton/SDSS fields: more diluted than absorbed?

We explore the properties of X-ray Bright Optically Inactive Galaxies (XBONG) detected in the 0.5-8 keV spectral band in 20 public XMM fields overlapping with the SDSS. We constrain our sample to optically extended systems with log (f_X/f_o > -2) that have spectroscopic identifications available from the SDSS (r<19.2 mag). The resulting sample contains 12 objects with L_X (0.5-8keV)= 5 x 10^41 - 2x10^44 erg s-1 in the redshift range 0.06 < z < 0.45. The X-ray emission in four cases is extended suggesting the presence of hot gas associated with a cluster or group of galaxies. The X-ray spectral fits show that two additional sources are best fit with a thermal component emission (kT \~ 1 keV). Three sources aremost likely associated with AGN: their X-ray spectrum is described by a steep photon index ~ 1.9 typical of unobscured AGN while, they are very luminous in X-rays (L_X (0.5-8 keV) ~ 10^43 - 10^44 erg s-1.) Finally, three more sources could be associated with either normal galaxies or unobscured Low Luminosity AGN (L_X < 10^42 erg s-1). We find no evidence for significant X-ray absorbing columns in any of our XBONGs. The above suggest that XBONGs, selected in the total 0.5-8 keV band, comprise a mixed bag of objects primarily including normal elliptical galaxies and type-1 AGN whose optical nuclear spectrum is probably diluted by the strong stellar continuum. Nevertheless, as our sample is not statistically complete we cannot exclude the possibility that a fraction of optically fainter XBONG may be associated with heavily obscured AGN.

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arXiv2004

The Phoenix Deep Survey: the clustering and the environment of Extremely Red Objects

In this paper we explore the clustering properties and the environment of the Extremely Red Objects (EROs; I-K>4mag) detected in a ~180arcmin^2 deep (Ks~20mag) Ks-band survey of a region within the Phoenix Deep Survey, an on-going multiwavelength program aiming to investigate the nature and the evolution of faint radio sources. Using our complete sample of 289 EROs brighter than Ks=20mag we estimate a statistically significant (~3.7sigma) angular correlation function signal with amplitude Aw=8.7^{+2.1}_{-1.7}x10^{-3} consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range ro=12-17h^{-1}Mpc, implying that EROs trace regions of enhanced density. Using a novel method we further explore the association of EROs with galaxy overdensities by smoothing the K-band galaxy distribution using the matched filter algorithm of Postman et al. (1996) and then cross-correlating the resulting density maps with the ERO positions. Our analysis provides direct evidence that EROs are associated with overdensities at redshifts z>1. We also exploit the deep radio 1.4GHz data (limiting flux 60microJy) available to explore the association of EROs and faint radio sources and whether the two populations trace similar large scale structures. Cross-correlation of the two samples (after excluding 17EROs with radio counterparts) gives a 2sigma signal only for the sub-sample of high-z radio sources (z>0.6). Although the statistics are poor this suggests that it is the high-z radio sub-sample that traces similar structures with EROs.

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arXiv2004

XMM-{\em Newton} Observations of Optically Selected SDSS Clusters

We explore the X-ray properties of a subset of the optically selected SDSS cluster sample of Goto et al. (2002), by analysing seven public XMM-{\em Newton} pointings, with exposure times ranging from $\sim$ 4 to 46 ksec. There are in total 17 SDSS clusters out of which only eight are detected at X-ray wavelengths with $\rm f_{0.5-2keV}\magcir 1.2 \times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$. For the remaining 9 SDSS clusters we estimate their 3$σ$ luminosity upper limits (corresponding to $L_x\mincir 5\times 10^{42}$ ergs/sec in the 0.5-2 keV band). This relatively low luminosity suggests that if real structures, these galaxy aggregations correspond to poor groups of galaxies. Using the SDSS photometric catalogue we also derive the cluster optical $r$-band luminosities. The resulting scaling relations ($L_{\rm opt}-L_x$, $L_{\rm opt}-T_x$) are consistent with those of other recent studies.

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arXiv2004

The XMM-{\em Newton}/2dF Survey - VI: Clustering and Bias of the Soft X-ray Point Sources

We study the clustering properties of X-ray sources detected in the wide area ($\sim 2 $deg$^{2}$) bright, contiguous XMM-{\em Newton}/2dF survey. We detect 432 objects to a flux limit of $5\times 10^{-15}$ ergcm$^{-2}$s$^{-1}$ in the soft 0.5-2 keV band. Performing the standard angular correlation function analysis, a $\sim 3σ$ correlation signal between 0 and 150 arcsec is detected: $w(θ<150^{''})\simeq 0.114\pm 0.037$. If the angular correlation function is modeled as a power law, $w(θ)=(θ_{\circ}/θ)^{γ-1}$, then for its nominal slope of $γ=1.8$ we estimate, after correcting for the integral constraint and the amplification bias, that $θ_{\circ}\simeq 10.4 \pm 1.9$ arcsec. Using a clustering evolution model which is constant in comoving coordinates ($ε=-1.2$), a luminosity dependent density evolution model for the X-ray luminosity function and the concordance cosmological model ($Ω_{\rm m}=1-Ω_Λ=0.3$) we obtain, by inverting Limber's integral equation, a spatial correlation length of $r_{\circ}\sim 16$ $h^{-1}$ Mpc. This value is larger than that of previous {\sc ROSAT} surveys as well as of the optical two-degree quasar redshift survey. Only in models where the clustering remains constant in physical coordinates ($ε=-3$), do we obtain an $r_{\circ}$ value ($\sim 7.5$ $h^{-1}$ Mpc) which is consistent with the above surveys.

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arXiv2004

The Phoenix Deep Survey: the radio properties of the hard X-ray selected sample

The radio properties of hard (2-8keV) X-ray selected sources are explored by combining a single 50ks XMM-Newton pointing with the ultra-deep and homogeneous Phoenix radio (1.4GHz) survey (Hopkins et al. 2003). A total of 43 sources are detected above the X-ray flux limit f_X(2-8keV)=7.7e-15cgs with 14 of them exhibiting radio emission above ~40muJy (3sigma). The X-ray/radio matched population lies in the borderline between radio loud and quiet AGNs and comprises sources with both soft and hard X-ray spectral properties suggesting both obscured and unobscured systems. The spectroscopically identified sub-sample (total of 6 X-ray/radio matches) comprises narrow emission line AGNs (4) with hard X-ray spectral properties and broad line sources (2) with soft X-ray spectra. We find evidence that the fraction of X-ray/radio matches increases from ~20% for sources with rest-frame column density N_H<1e22cm^-2 to \~50% for more absorbed systems. Poor statistics however, limit the significance of the above result to the ~2sigma level. Also, the X-ray/radio matched sources have flatter coadded X-ray spectrum (Gamma=1.78^{+0.05}_{-0.03}) compared to sources without radio emission (Gamma=2.00^{+0.03}_{-0.04}). A possible explanation for the higher fraction of absorbed sources with radio emission at the muJy level is the presence of circum-nuclear starburst activity that both feeds and obscures the central engine. For a small sub-sample of z~0.4 radio emitting AGNs with N_H>1e22cm^-2 their combined spectrum exhibits a soft X-ray component that may be associated with star-formation activity, although other possibilities cannot be excluded. We also find that radio emitting AGNs make up about 13-20% of the hard-band X-ray background depending on the adopted normalisation.

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arXiv2004

The XMM-Newton/2dF survey VII. Is there any X-ray absorption in optically selected QSOs?

We explore the X-ray properties of optically selected QSOs spectroscopically identified in the course of the 2dF QSO survey (2QZ). Our main goal is to expand to higher redshifts previous findings suggesting the presence of a fraction of X-ray obscured sources among the low redshift optically selected broad line AGN population. The X-ray data are from the wide field (~2.5 sq. deg), shallow $[f(0.5 - 8 keV) ~10^-14 egs s^-1 cm^-2 XMM-Newton/2dF survey. A total of 96 2QZ QSOs overlap with the area covered by our X-ray survey. 66 of them have X-ray counterparts while 30 remain undetected in our X-ray survey. The 66 X-ray detected QSOs have a mean photon index of ~2 suggesting little or no X-ray obscuration for most of these sources. Individual X-ray spectral fittings reveal only 1 source (intrinsic Lx(0.5-8 keV) ~ 10^44 erg s^-1 at z=0.82) that is likely to be obscured (NH~10^23 cm^-2) at the 90% confidence level. Additionally, there are 9 2QZ sources that show evidence for moderate absorption (mean observed NH of ~10^21 cm^-2). For the 30 QSOs that remain undetected in our X-ray survey we use stacking analysis to estimate a mean hardness ratio of -0.59 +/- 0.11 suggesting that the bulk of this population has NH consistent with the Galactic value. However, we cannot exclude the possibility that some of these sources have enhanced photoelectric absorption that is not revealed in the mean stacked spectrum. We estimate a lower limit to the fraction of optically selected QSO with X-ray absorption of about 10% (10 out of 96).

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arXiv2004

The XMM-Newton/2dF survey - V. the radio properties of the X-ray population

In this paper we cross-correlate the FIRST 1.4GHz radio survey with a wide field (~ 1.6deg^2) shallow [f(0.5-8keV) ~ 1e-14erg/s/cm^2] XMM-Newton survey. We find 12 X-ray/radio matches representing 4% of the X-ray selected sample. Most of them are found to be associated with AGNs (total of 9) on the basis of the observed optical spectra (3), radio morphology (2) or X-ray/optical properties (4), while one radio source is identified with an X-ray selected cluster. We also find two sources associated with low redshift galaxies with narrow emission line optical spectra, X-ray luminosity L(0.5-8keV) ~ 1e41erg/s, radio luminosity density L(1.4GHz) ~ 5e22W/Hz and logf_X/f_{opt} ~ -2 suggesting `normal' star-forming galaxies. We argue that radio surveys combined with X-ray sample could provide a powerful tool for identifying X-ray selected `normal' galaxies powered by stellar processes. Finally, radio loud and quiet systems in the present sample have mean X-ray spectral properties consistent with Gamma ~ 1.9.

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arXiv2004

The Clustering of XMM-Newton Hard X-ray Sources

This paper presents the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field ($\approx \rm 2 deg^{2}$) shallow [$f_X(\rm 2-8 keV)\approx 10^{-14}\rm erg cm^{-2} s^{-1}$] and contiguous XMM-{\it Newton} survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect a $\sim 4σ$ correlation signal out to 300 arcsec with $w(θ< 300^{''})\simeq 0.13 \pm 0.03$. Modeling the two point correlation function as a power law of the form $w(θ)=(θ_{\circ}/θ)^{γ-1}$ we find: $θ_{\circ}=48.9^{+15.8}_{-24.5}$ arcsec and $γ=2.2\pm {0.30}$. Fixing the correlation function slope to $γ=1.8$ we obtain $θ_{\circ}=22.2^{+9.4}_{-8.6}$ arcsec. Using Limber's intergral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from $r_{\circ}\sim 9$ to 19 $h^{-1}$ Mpc (for $γ=1.8$ and the {\em concordance} cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics. The relatively large correlation length is comparable to that of extremely red objects and luminous radio sources.

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arXiv2004

The Clustering of XMM-Newton Hard X-ray Sources

We present the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg cm^{-2} s^{-1}] and contiguous XMM-Newton survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect a ~ 4σcorrelation signal out to 300 arcsec with w(theta < 300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain theta_o=22.2^{+9.4}_{-8.6} arcsec. Using Limber's integral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from r_o ~ 9 to 19 h^{-1} Mpc (for gamma=1.8 and the concordance cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics.

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arXiv2004

The XMM-Newton/2dF survey IV. The X-ray spectral properties of the hard sources

We present an analysis of the X-ray spectral properties of 61 hard X-ray selected (2-8 keV) sources from the bright ($f_{2-8 keV}>10^{-14}$ \funits) {\it XMM-Newton}/2dF survey. This comprises of 9 {\it XMM-Newton} pointings in the North Galactic Pole region ($\sim 1.6 \rm deg^2$) and overlaps with the SDSS, 2QZ and 2dFGRS surveys. Our sources contribute about 50 per cent of the 2-10 keV X-ray background down to the flux limit of $10^{-14}$ cgs. The hardness ratio distribution of the sample suggests a deficit of heavily absorbed sources. A spectral fit to the co-added total source spectrum yields a steep photon index, $Γ=1.83^{+0.04}_{-0.05}$. All but 8 sources have optical counterparts down to the SDSS photometric limit of $r\approx22.5$. Spectroscopic identifications exist for 34 sources. The vast majority are associated with Broad-Line (BL) AGN (24 sources) while only 7 present narrow or no emission lines. Five sources are probably associated with Galactic stars. Finally, for another 17 probable AGN we present photometric redshifts. The combined spectrum of the 24 spectroscopically identified BL AGN is steep ($Γ=2.02^{+0.04}_{-0.05}$), while that of the 7 AGN, which do not present broad lines is flatter with $Γ=1.64^{+0.11}_{-0.11}$. The spectrum of the 8 optically unidentified sources is flat with $Γ\approx 1.1$. Spectral fits to the individual BL AGN reveal large absorption (rest-frame column density $\rm>10^{22} cm^{-2}$) in only two cases. The individual spectra of the NL AGN present significant evidence for even a moderate absorption ($\rm 3\times 10^{21} cm^{-2}$) in only one case.

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arXiv2004

The XMM/2dF survey III: Comparison between optical and X-ray cluster detection methods

We directly compare X-ray and optical techniques of cluster detection by combining SDSS photometric data with a wide-field ($\sim 1.8$ deg$^{2}$) XMM-{\em Newton} survey in the North Galactic Pole region. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey. The most probable cause of the small number of optical cluster candidates detected in our X-ray survey is that they are relatively poor clusters, fainter than the X-ray flux limit (for extended sources) of our survey $f_{x}(0.3-2 {\rm keV}) \simeq 2 \times 10^{-14} erg cm^{-2} s^{-1}$.

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arXiv2003

Radio Properties of EROs in the Phoenix Deep Survey

Insensitive to dust obscuration, radio wavelengths are ideal to study star-forming galaxies free of dust induced biases. Using data from the Phoenix Deep Survey, we have identified a sample of star-forming extremely red objects (EROs). Stacking of the radio images of the radio-undetected star-forming EROs revealed a significant radio detection. Using the expected median redshift, we estimate an average star-formation rate of 61 M_sun/yr for these galaxies.

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arXiv2003

The Phoenix Deep Survey: Evolution of Star Forming Galaxies

The Phoenix Deep Survey (PDS) is a multiwavelength survey based on deep 1.4 GHz radio observations used to identify a large sample of star forming galaxies to z=1. Photometric redshifts are estimated for the optical counterparts to the radio-detected galaxies, and their uncertainties quantified by comparison with spectroscopic redshift measurements. The photometric redshift estimates and associated best-fitting spectral energy distributions are used in a stacking analysis exploring the mean radio properties of U-band selected galaxies. Average flux densities of a few microJy are measured.

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arXiv2003

The XMM-Newton/2dF survey - II. The nature of X-ray faint optically bright X-ray sources

In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide area (2.5deg^2) shallow (f(0.5-8keV)~10e-14cgs) XMM-Newton survey. We find a total of 26 sources (5% of the total X-ray selected population) with log f_X/f_{opt}<-0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (total of 8) and broad line AGNs (total of 8).We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low redshift galaxies with narrow emission line optical spectra, X-ray luminosities L_X(0.5-8keV)~10e41cgs and logf_X/f_opt ~ -2 suggesting `normal' star-forming galaxies. Despite the small number statistics the sky density of `normal' X-ray selected star-forming galaxies at the flux limit of the present sample is low consistent with previous ROSAT HRI deep surveys. Also, the number density estimated here is in good agreement with both the logN-logS of `normal' galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.

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arXiv2003

The XMM-2df Cluster Survey

We present the results from a shallow (2-10 ksec) XMM/2dF survey. Our survey covers 18 XMM fields ($\sim 5 {\rm deg}^2$) previously spectroscopically followed up with the Anglo-Australian telescope 2-degree field facility. About half of the fields are also covered by the Sloan Digital Sky Survey (SDSS). We are searching for extended sources using the XMM SAS maximum likelihood algorithm in the 0.3-2 keV band and we have detected 14 candidate clusters down to a flux of $\sim2\times10^{-14} cgs$. Our preliminary results show that: i) the redshift distribution peaks at relatively high redshifts ($\sim0.4$) as expected from the Rosati et al. $Φ(L)$, ii) some of our X-ray clusters appear to have optical counterparts.

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arXiv2003

QSO Photometric Redshift estimation for the XMM-Newton/2dF Survey

The technique of estimating redshifts using photometric rather than spectroscopic observations has recently received great attention due to its simplicity and the accuracy of the results obtained. In this work, we estimate photometric redshifts for an X-ray selected QSO sample. This is the first time this technique is applied on such a sample. We first calculate the accuracy of the results obtained by comparing photometric to spectroscopic redshifts for a sub-sample of our QSO sample: for the majority (~67%) of the objects in this sub-sample, photometric redshift estimates are correct within Dz<0.3. We then derive the photometric redshift distribution for the whole QSO sample. In the future, we expect to use the photometric redshift distribution in order to derive the distributions of properties such as the Hardness Ratio and hence the hydrogen column density, the luminosity function etc. As an example, we estimate here the dependence of the Hardness Ratio of the QSO sample on photometric redshift.

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arXiv2003

Extremely red galaxies in the Phoenix Deep Survey

The Phoenix Deep Survey (PDS) is a multiwavelength survey based on deep 1.4 GHz radio observations used to identify a large sample of star forming galaxies to z=1. Here we present an exploration of the evolutionary constraints on the star-forming population imposed by the 1.4 GHz source counts, followed by an analysis of the average properties of extremely red galaxies in the PDS, by using the "stacking" technique.

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arXiv2003

The Phoenix Deep Survey: X-ray properties of faint radio sources

In this paper we use a 50ks XMM-Newton pointing overlapping with the Phoenix Deep Survey, a homogeneous radio survey reaching muJy sensitivities, to explore the X-ray properties and the evolution of star-forming galaxies. UV, optical and NIR photometry is available and is used to estimate photometric redshifts and spectral types for radio sources brighter than R=21.5mag (total of 82). Sources with R<21.5mag and spiral galaxy SEDs (34) are grouped into two redshift bins with a median of z=0.240 and 0.455 respectively. Stacking analysis for both the 0.5-2 and 2-8keV bands is performed on these subsamples. A high confidence level signal (>3.5sigma) is detected in the 0.5-2keV band corresponding to a mean flux of ~3e-16cgs for both subsamples. This flux translates to mean luminosities of ~5e40 and 1.5e41cgs for the z=0.240 and 0.455 subsamples respectively. Only a marginally significant signal (2.6sigma) is detected in the 2-8keV band for the z=0.455 subsample. We argue that the stacked signal above is dominated by star-formation. The mean L_X/L_B ratio and the mean L_X of the two subsamples are found to be higher than optically selected spirals and similar to starbursts. We also find that the mean L_X and L_1.4 of the faint radio sources studied here are consistent with the L_X-L_1.4 correlation of local star-forming galaxies. Moreover, the X-ray emissivity of sub-mJy sources to z~0.3 is found to be elevated compared to local HII galaxies. The observed increase is consistent with L_X evolution of the form (1+z)^3. Assuming that our sample is indeed dominated by starbursts this is direct evidence for evolution of such systems at X-ray wavelengths. Using an empirical L_X to SFR conversion we estimate a global SFR density at z~0.3 of \~0.029M_o/yr/Mpc in agreement with previous studies.

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