A cosmic collision caught in the act – beginning of a galaxy merger

Galaxies are the largest directly observable objects in the universe and learning about their formation is key to understanding how the Universe evolves. They probably form in the process of merging and close binary quasars may represent a very early stage of this process. A merger will likely produce a supermassive binary black hole. Less then 20 AGN (Active Galactic Nuclei) pairs are currently known. Most of them are high redshift objects with separation 3-10 arcsec, what corresponds to distances of ~10-80 kpc. The observed 3-10 arcsec binary quasars are probably bound to each other and represent galaxy pairs undergoing the loop following the first close encounter. However, still open question is if the host galaxies of the quasars interact in the process of merging. If so, we should observe morphological and kinematical distortions in these systems and their character should depends on the evolutionary stage of the pair. Strong evidence for the interaction of the host galaxies of the binary AGN system SDSS J1254+0846 (separation 3.8 arcseconds) has been provided through the observed distortion of the optical light from one of the host galaxies, showing obvious tidal tails (Green et al.2010). Here we report that radio-loud/radio-quiet binary quasar system associated with the radio source 1641+320 (FIRST J164311.3+315618) shows disturbed radio morphology possibly indicating that the two quasars are in the process of merging. Publication describing this discovery will appear soon in Astrophysical Journal (http://arxiv.org/abs/1105.3242).

The radio-loud/radio-quiet binary quasar associated with the radio source 1641+320 has been classified by Brotherton et al., 1999 based on the optical observations. It is a small separation, 2.3’’ (15.2 kpc), quasar pair with redshift z=0.586 and one of the four known quasar pairs with greatly discrepant optical and radio flux ratios. On this basis the gravitational lens hypothesis has been ruled out. The radio-loud component of this pair is characterized by strong, narrow emission line spectrum and X-ray emission. In the radio-quiet one the starburst has been triggered. The radio-loud component of the binary quasar, 1641+320, is a Compact Steep Spectrum (CSS) radio source. The radio 1.66 GHz and 5 GHz MERLIN observations of the 1641+320 were undertaken by us in 2007 and 2009. The 1.66 GHz MERLIN radio image of 1641+320 shows four components of the source (Fig.1a). A component indicated as C, which position is well correlated with the position of the optical counterpart is a radio core. Components E, W1 and W2 are probably radio jest/lobes. W2 is also the only one component showing polarization and only at the frequency of 1.66 GHz. We suggests that W2 component could be the most current jet direction and particle ejection after the whole structure has been disturbed in the merging process. The optical image of 1641+320 taken by the Hubble Space Telescope show distortions of the optical structure correlated with the radio one.

1641+320 (FIRST J1643+3156) is an unusual and statistically very rare, low redshift binary quasar. The complex radio morphology of 1641+320 indicates on the intermittent activity with a possible rapid change of the jet direction and/or restart of the jet due to the interaction with the companion (Fig.1a,b.c). The host galaxy of the radio-loud quasar is also highly disturbed (Fig.1c) and we suggest that the first pericentric passage took place in this pair igniting and/or changing the radio activity and morphology in the radio-loud AGN.

Authors: dr Magdalena Kunert-Bajraszewska, Centrum Astronomii UMK, ul. Gagarina 11, 87-100 Toruń
dr Agnieszka Janiuk, Centrum Fizyki Teoretycznej PAN, ul. Lotników 32/46, 02-668 Warszawa

Figure 1. a) the 1.66 GHz MERLIN radio image of 1641+320; b) the 5 GHz MERLIN radio image of 1641+320; c) optical image of 1641+320 from Hubble Space Telescope with radio contours at 5 GHz. A cross indicates the position of an optical object found using the most actual version of SDSS. Vectors represent the polarized flux density.