A MERLIN Survey of Weak, Compact VLA FIRST Sources.

Mike Garrett (JIVE, The Netherlands)

Simon Garrington (NRAL, UK)

We have observed with MERLIN at 6cm 127 weak (> 10mJy/beam) unresolved (< 5 arcsec) VLA "FIRST" (Becker et al. 1995 Ap.J. 450, 559) sources all of which lie within 1.5 degees of a strong VLBI calibrator J1159+291.

Each source was observed in phase-reference (cycle time of 9 minutes) and snapshot mode (5 x 3 mins) resulting in an r.m.s. noise of 0.3mJy and a resolution of around 60 mas. About 50% of the sources were detected by MERLIN and have compact structure (< 60 mas) with peak flux densities > 2mJy/beam. Of these sources 60% were unresolved by MERLIN, 10% were partially resolved and the remainder are doubles or "complex". 30% of the sources are identified with optical counterparts brighter than 20th magnitude.The absolute MERLIN positions are accurate to a few mas. The preliminary images are presented here.

More recently (27-28 May 1997) we observed a subset of 40 of the sources detected by MERLIN (chosen by proximity to the calibrator) with EVN+HN+MERLIN at 18cm. Each source was observed for 6 x 3.4 mins over 12 hrs. The MERLIN data have already been analysed and images at 0.3 arcsec resolution show that the majority have little intermediate structure (0.5 -- 3.0 arcsec) and are probably flat or peak spectrum compact sources.

MAIN GOALS of this project:

define a sample of weak sources suitable for follow-up VLBI observations, selected purely on the basis that they are unresolved by the VLA, detected by MERLIN and lie within 1.5 degrees of a suitable phase-reference calibrator

to determine the VLBI radio structure of such a sample

classify the sources on the basis of their radio structure, spectra and optical ids

Advantages of this technique

There are many advantages to targeting sources nearby bright VLBI calibrators. The most obvious benefit is that :

the painful process of identifying phase calibrators is avoided
the solutions from the phase-reference can be very accurate (the calibrator is bright and nearby by definition)
the observing efficiency is much higher since the sources are grouped together in the same part of the sky.

MERLIN Images and Sample on-line

PostScript versions of the MERLIN 6cm maps and MERLIN 18cm maps are available.

Tables including all the sources in the original sample, and another with only those detected by MERLIN.

Some of the more interesting maps are available as colour gif files.