EDP Sciences
Free access
Volume 437, Number 3, July III 2005
Page(s) 1135 - 1146
Section Celestial mechanics and astrometry
DOI http://dx.doi.org/10.1051/0004-6361:20042426

A&A 437, 1135-1146 (2005)
DOI: 10.1051/0004-6361:20042426

Near-infrared astrometry and photometry of Southern ICRF quasars

J. I. B. Camargo, G. Daigne, C. Ducourant and P. Charlot

Observatoire Aquitain des Sciences de l'Univers, CNRS-UMR 5804, BP 89, 33270 Floirac, France
    e-mail: camargo@obs.u-bordeaux1.fr

(Received 23 November 2004 / Accepted 10 March 2005)

We present high quality astrometry, along with J, H, and $K_{\rm s}$ photometry, for 30 southern ICRF quasars observed with the ESO NTT/SOFI at La Silla. Our main purpose is to document the optical counterpart of ICRF quasars, by reporting their position in a stellar reference frame and their near-infrared photometry. Our investigation lies within the scope of extending the (radio) extragalactic reference frame into the optical range. An internal precision generally better than 10 mas was obtained in positioning the near-infrared counterpart of these quasars within their stellar surroundings. The actual astrometric accuracy is estimated from the departure between the observed and the radio (VLBI) positions of quasars, and is about 35 mas, mainly due to the statistical uncertainties of the stellar reference catalogue (UCAC2). The positional accuracy was improved to 25 mas for a subset of 14 quasars observed with extended fields of view, as obtained from mosaics of overlapping images. The 3-band photometry was achieved with a precision most often better than 0.04 mag, based on calibration with the 2MASS magnitudes. A spectral index $\alpha$ ( $F_{\nu}\propto\nu^{\alpha}$) was derived from the observed quasar fluxes in the near-infrared. With a mean value $\langle\alpha\rangle=-1$, this spectral index does not show any significant correlation with the radio spectral index.

Key words: astrometry -- infrared: general -- methods: numerical -- reference systems

SIMBAD Objects
Tables at the CDS

© ESO 2005