Near-infrared astrometry and photometry of Southern ICRF quasars

Abstract

Abstract.We present high quality astrometry, along with J, H, and K 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 α (F ν ∝ ν α ) was derived from the observed quasar fluxes in the near-infrared. With a mean value α = −1, this spectral index does not show any significant correlation with the radio spectral index.

The influence of radio-extended structures on offsets between the optical and VLBI positions of sources in the ICRF2

“…In this context, the determination of accurate optical positions of ICRF sources have been determined (see, for instance; Assafin et al 2003;Camargo et al 2005;Assafin et al 2007;Aslan et al 2010) and provided numbers about the alignment between the optical and radio frames. This work focuses on statistically significant optical to radio positional differences and aims at investigating them.…”