arXiv subjects

A. Georgakakis

Publications and source records attributed to A. Georgakakis.

137 records · Page 8Linked to original sources

arXiv2003

The XMM-Newton/2dF survey I: X-ray properties of normal galaxies

This paper explores the X-ray properties of `normal' galaxies using a shallow XMM-Newton survey covering an area of ~1.5deg2. The X-ray survey overlaps with the 2dF Galaxy Redshift Survey. Compared with previous studies this has the advantage of high quality spectra and spectral classifications to bj=19.4. Moreover, sources with optical spectra revealing powerful AGNs can easily be discarded from the normal galaxy sample used here. In particular, we present stacking analysis results for 200 galaxies from the 2dFGRS at =0.1. We detect a strong signal for the whole sample (~6sigma) in the soft 0.5-2keV band corresponding to a flux of ~7*10^-16cgs and a luminosity of ~2*10^40cgs. A statistically significant signal is also detected for both the early and late galaxy sub-samples with X-ray luminosities of ~3*10^40 and ~5*10^39cgs respectively. In contrast, no signal is detected in the hard 2-8keV band for any of the above samples. The mean L_X/L_B ratio of the spiral galaxy sample is consistent with both local (<100Mpc) and distant (z~1) samples suggesting little or no evolution of the X-ray emission mechanisms relative to the optical. The 0.5-2keV XRB contribution of the spiral galaxy sub-sample at z~0.1 is estimated to be 0.4% in broad agreement with the XRB fractions estimated in previous studies.Assuming that star-forming galaxies evolve with redshift as (1+z)^{k} the present data combined with previous studies suggest k<3. The k values are constrained by the relatively low fraction of the soft X-ray background that remains unresolved by deep surveys (6-26%). The mean X-ray emissivity of spiral galaxies at z~0.1 is also estimated and is found to be consistent within the uncertainties with that of local HII galaxy samples.

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arXiv2003

XMM-Newton observations of an absorbed z=0.67 QSO: no dusty torus?

We present XMM-Newton observations of AXJ0341.4-4453, a mildly reddened A_V<7 QSO at a redshift of z=0.672. The XMM-Newton spectrum shows a large obscuring column N_H~10^23 cm^{-2} corresponding to A_V~70, in agreement with previous results based on the lower limit of the ASCA hardness ratio. The X-ray spectrum is represented by a 'scattering' model with Gamma~2.0 with the scattered power-law normalization being a few per cent of the hard component. No FeK line is detected with a 90 per cent upper limit on its equivalent width of ~360eV. The large discrepancy between the column density observed in X-rays and that inferred from the Balmer decrement can be explained by dust sublimation near the nucleus. Then, the X-ray and the optical obscuration come from two different regions: the X-ray close to the accretion disk while the optical at much larger >0.25pc scales.

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arXiv2002

The Phoenix Deep Survey: The 1.4 GHz microJansky catalogue

The initial Phoenix Deep Survey (PDS) observations with the Australia Telescope Compact Array have been supplemented by additional 1.4 GHz observations over the past few years. Here we present details of the construction of a new mosaic image covering an area of 4.56 square degrees, an investigation of the reliability of the source measurements, and the 1.4 GHz source counts for the compiled radio catalogue. The mosaic achieves a 1-sigma rms noise of 12 microJy at its most sensitive, and a homogeneous radio-selected catalogue of over 2000 sources reaching flux densities as faint as 60 microJy has been compiled. The source parameter measurements are found to be consistent with the expected uncertainties from the image noise levels and the Gaussian source fitting procedure. A radio-selected sample avoids the complications of obscuration associated with optically-selected samples, and by utilising complementary PDS observations including multicolour optical, near-infrared and spectroscopic data, this radio catalogue will be used in a detailed investigation of the evolution in star-formation spanning the redshift range 0 < z < 1. The homogeneity of the catalogue ensures a consistent picture of galaxy evolution can be developed over the full cosmologically significant redshift range of interest. The 1.4 GHz mosaic image and the source catalogue are available on the web at http://www.atnf.csiro.au/~ahopkins/phoenix/ or from the authors by request.

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arXiv2001

Cold gas in elliptical galaxies

We explore the evolution of the cold gas (molecular and neutral hydrogen) of elliptical galaxies and merger remnants ordered into a time sequence on the basis of spectroscopic age estimates. We find that the fraction of cold gas in early merger remnants decreases significantly for ~1-2 Gyr, but subsequent evolution toward evolved elliptical systems sees very little change. This trend can be attributed to an initial gas depletion by strong star-formation which subsequently declines to quiescent rates. This explanation is consistent with the merger picture for the formation of elliptical galaxies. We also explore the relation between HI-to-H2 mass ratio and spectroscopic galaxy age, but find no evidence for a statistically significant trend. This suggests little net HI to H2 conversion for the systems in the present sample.

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arXiv2000

The globular cluster system of the young elliptical NGC 6702

We study the globular cluster (GC) system of the dust-lane elliptical galaxy NGC 6702, using B, V and I-band photometric observations carried out at the Keck telescope. This galaxy has a spectroscopic age of ~2Gyrs suggesting recent star-formation. We find strong evidence for a bimodal GC colour distribution, with the blue peak having a colour similar to that of the Galactic halo GCs. Assuming that the blue GCs are indeed old and metal-poor, we estimate an age of 2-5Gyrs and supersolar metallicity for the red GC subpopulation. Despite the large uncertainties, this is in reasonable agreement with the spectroscopic galaxy age. Additionally, we estimate a specific frequency of S_{N}=2.3\pm1.1 for NGC 6702. We predict that passive evolution of NGC 6702 will further increase its specific frequency to S_{N}~2.7 within 10Gyrs, in closer agreement to that of typical present-day ellipticals. We also discuss evidence that the merger/accretion event that took place a few Gyrs ago involved a high gas fraction.

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arXiv2000

Microjansky radio sources in DC0107-46 (Abell 2877)

The cluster DC0107-46 (Abell 2877) lies within the Phoenix Deep Survey, made at 1.4 GHz with the Australia Telescope Compact Array. Of 89 known optical cluster members, 70 lie within the radio survey area. Of these 70 galaxies, 15 (21%) are detected, with luminosities as faint as 10^20 W/Hz. Spectroscopic observations are available for 14/15 of the radio-detected cluster galaxies. Six galaxies show only absorption features and are typical low-luminosity AGN radio sources. One galaxy hosts a Seyfert 2 nucleus, two are star-forming galaxies, and the remaining five may be star-forming galaxies, AGNs, or both.

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arXiv1999

The Phoenix radio survey: the angular correlation function

Using the Phoenix radio survey, a homogeneous survey selected at 1.4GHz and covering an area of ~3deg^2, we analyse the clustering of the sub-mJy radio population using angular correlation function analysis. Extensive simulations are carried out to investigate the significance of the estimated angular correlation amplitudes. Our analysis show that for the S_1.4>0.5mJy sub-samples the radio source distribution is anisotropic at the 2$σ$ significance level. Additionally, we estimate upper limits for the angular correlation amplitudes that, despite the large uncertainties, are in good agreement with the amplitude estimates for sources brighter than 1mJy, detected in the FIRST radio survey (Cress {et al.} 1997). Adopting a radio luminosity function and assuming an evolving spatial correlation function of the form ξ(r)=(r/r_{o})^{-γ} (1+z)^{-(3+ε)}, with the evolution parametrised by ε, we find an upper limit for the angular correlation length r_{o}~9h^-1Mpc for S_1.4>0.5mJy and γ=2.1. This agrees well with the value r_{o}~6-8h^-1Mpc estimated from the FIRST radio survey for sources brighter than 1mJy. Additionally, we quantify the characteristics, in terms of areal coverage and limiting flux density, of future deep radio surveys to yield a significant correlation amplitude detection and to explore possible changes of the correlation amplitude with flux density.

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arXiv1999

The Phoenix survey: the pairing fraction of faint radio sources

The significance of tidal interactions in the evolution of the faint radio population (sub-mJy) is studied using a deep and homogeneous radio survey (1.4 GHz), covering an area of 3.14 deg$^2$ and complete to a flux density of 0.4 mJy. Optical photometric and spectroscopic data are also available for this sample. A statistical approach is employed to identify candidate physical associations between radio sources and optically selected `field' galaxies. We find an excess of close pairs around optically identified faint radio sources, albeit at a low significance level, implying that the pairing fraction of the sub-mJy radio sources is similar to that of `field' galaxies (at the same magnitude limit) but higher than that of local galaxies.

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arXiv1999

The 1.4 GHz and Halpha Luminosity Functions and Star Formation Rates from Faint Radio Galaxies

Using a recently completed survey of faint (sub-mJy) radio sources, selected at 1.4 GHz, a dust-free estimate of the local star formation rate (SFR) is carried out. The sample is 50% complete to 0.2 mJy, with over 50% of the radio sources having optical counterparts brighter than R = 21.5. Spectroscopic observations of 249 optically identified radio sources have been made, using the 2-degree Field (2dF) facility at the Anglo-Australian Telescope (AAT). Redshifts and equivalent widths of several spectral features (e.g., Hαand [OII]3727) sensitive to star formation have been measured and used to identify the star-forming and absorption-line systems. The spectroscopic sample is corrected for incompleteness and used to estimate the 1.4 GHz and Hαluminosity functions (LFs) and luminosity density distributions. The 1.4 GHz LF of the star-forming population has a much steeper faint-end slope (1.85) than that for the ellipticals (1.35). This implies an increasing preponderance of star-forming galaxies among the optically identified (i.e., z < 1) radio sources at fainter flux densities. The HαLF of the faint radio population agrees with published HαLFs derived from local samples selected by Hαemission. This suggests that the star-forming faint radio population is coincident with the Hαselected galaxies. The 1.4 GHz and Hαluminosity densities have been used to estimate the SFRs. The two estimates agree, both giving a SFR density of $0.032 M_\odot yr^{-1} Mpc^{-3}$ in the range z < 1.

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arXiv1999

The Phoenix survey: optical and near-infrared observations of faint radio sources

Using a deep Australia Telescope Compact Array (ATCA) radio survey covering an area of ~3deg^{2} to a 4σsensitivity of \ge 100 \muJy at 1.4GHz, we study the nature of faint radio galaxies. About 50% of the detected radio sources are identified with an optical counterpart revealed by CCD photometry to m_{R}=22.5 mag. Near-infrared (K-band) data are also available for a selected sample of the radio sources, while spectroscopic observations have been carried out for about 40% of the optically identified sample. These provide redshifts and information on the stellar content. Emission-line ratios imply that most of the emission line sources are star-forming galaxies, with a small contribution (\approx 10%) from Sy1/Sy2 type objects. We also find a significant number of absorption line systems, likely to be ellipticals. These dominate at high flux densities (> 1 mJy) but are also found at sub-mJy levels. Using the Balmer decrement we find a visual extinction A_{V}=1.0 for the star-forming faint radio sources. This moderate reddening is consistent with the V-R and R-K colours of the optically identified sources. For emission line galaxies, there is a correlation between the radio power and the Hαluminosity, in agreement with the result of Benn et al. (1993). This suggests that the radio emission of starburst radio galaxies is a good indicator of star-formation activity.

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arXiv1996

The nature of the faint galaxies in the Hubble Deep Field

We present a study of the galaxies found in the Hubble Deep Field. A high proportion of HDF galaxies are undergoing a strong episode of star formation, as evidenced by their very blue colours. A wide range of morphological types is found, with a high proportion of peculiar and merger morphologies. Fitting the multiband spectra with redshifted SEDs of galaxy types E to HII, we predict the spectral types and redshifts of galaxies detected in the HDF. We find a median redshift of 1.6, with $68\%$ having $z > 1$ and $31\%$ with $z >2$. The I-band absolute magnitude distributions as a function of galaxy types show a plausible trend of decreasing luminosity towards later types. The derived I-band luminosity function agrees well with that from the Canada-France survey (Lilly et al 1996) for $z < 1$ and shows strong luminosity evolution at $M_I < -21$ for $1 < z < 3$, comparable to the rate seen in quasars and starburst galaxies. We have predicted infrared and submillimetre fluxes assuming most of the galaxies are undergoing a strong starburst. Several planned space-borne and ground-based deep surveys are capable of detecting interesting numbers of HDF galaxies.

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