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Keywords:

  • techniques: interferometric;
  • galaxies: active;
  • galaxies: ISM;
  • galaxies: starburst;
  • radio continuum: galaxies

ABSTRACT

We have used the European very long baseline interferometry (VLBI) Network (EVN) to observe a sample of Lyman break analogues (LBAs), nearby (z < 0.3) galaxies with properties similar to the more distant Lyman break galaxies (LBGs). The study of LBGs may help define the feedback relationship between black holes (BHs) and their host galaxies. Previous Very Large Array (VLA) observations have shown that the kpc-scale radio emission from LBAs is dominated by starbursts. The main targets of this VLBI experiment were selected because they possessed emission-line properties between starbursts and Type 2 (obscured) active galactic nuclei (AGN). Eight targets (three star-forming LBAs, four composite LBAs and one Type 1 AGN) were observed at 5 GHz, four of which (one star-forming LBA and three composite LBAs) were also observed at 1.7 GHz. One star-forming LBA and one composite LBA were detected above 5σ at 1.7 GHz (only), while the AGN was detected at 5 GHz. In both LBAs, the radio luminosity (LR) exceeds that expected from supernovae (remnants) based on a comparison with Arp 220, Arp 229A and Mrk 273, by factors of inline image. The composite LBA exhibits a compact core emitting around 10 per cent of the VLA flux density. The high Tb of 3.5 × 107 K and excess core LR with respect to the LR/LX relation of radio-quiet AGN indicate that this LBA possesses an obscured AGN (MBH∼ 105 − 7 M). In three other composite LBAs detected previously in the X-ray, no radio sources were detected, indicating either variability or the presence of an obscured AGN below our radio sensitivity. While weak AGN may coexist with the starbursts as shown in at least one of the LBAs, their contribution to the total radio flux is fairly minimal. Our results show that the detection of such weak AGN presents a challenge at radio, X-ray and optical emission-line wavelengths at z∼ 0.2, indicating the great difficulties that need to be overcome in order to study similar processes at high redshift when these types of galaxies were common.