A Radio and ultraviolet study of dual nuclei in galaxies

Abstract

This thesis is aimed at studying dual nuclei in galaxies using radio and UV observations. Dual nuclei are formed during galaxy mergers in our Universe. Since we know that almost all nearby galaxies host a super-massive black-hole (SMBH) in their centres, galaxy mergers should produce pairs of SMBHs embedded in the dual nuclei. The term dual nuclei includes active galactic nuclei (AGN) pairs, AGN+star-forming (SF) nuclei pairs or SF+SF pairs. When the SMBHs are accreting, they form dual or binary AGN and can give rise to double-peaked emission lines in the optical spectra of the merger remnant, although the double peaks could also be due to jet-ISM interaction or rotating disks. One of the best ways to confirm the presence of dual/binary AGN in double-peaked AGN (DPAGN) is by using high resolution radio observations. In this thesis we compiled a sample of 20 DPAGN from the literature and observed them at two or more frequencies using the Karl G. Jansky Very Large Array (VLA). One of the sample sources 2MASXJ1203 shows a prominent S-shaped morphology with highly symmetric radio jets that extend out to a radius of ∼ 1.5

00 (1.74 kpc) on either

side of the core which has a size of ∼ 0.1

00(116 pc). The radio jets have a helical structure resembling the precessing jets in the galaxy NGC 326 which has confirmed dual AGN. We fitted a simple model of precessing jets to 2MASXJ1203 and find that the precession timescale is around 105

years: this matches the source lifetime estimate via spectral aging. Also, the expected SMBH separation corresponding to this timescale is 0.02 pc. We concluded that the S-shaped radio jets are due to jet precession caused either by a binary/dual SMBH system, a single SMBH with a tilted accretion disk or a dual AGN system where a close pass of the secondary SMBH in the past has given rise to jet precession. For three other DPAGN sources in our sample, we have detected dual radio structures at separation of . 10 kpc. Using the spectral index maps and optical spectra of the sources, we have confirmed that one of them is a dual AGN (DAGN), while the other two can be dual AGN or AGN+star-forming nuclei

pairs. The fifth source has a clear core-jet structure in our observations. However, the Z-shaped morphology changes direction at larger scales as traced by low frequency 1.4GHz emission; this can be due to a dual/binary AGN. The sixth source has an extended radio morphology. The next 13 sources are single cores and one source is not detected at any frequency. We find that for our dual AGN detections, the DPAGN emission lines do not originate from the dual/binary AGN. Instead, they could be due to outflows or jets. Hence, we conclude that DPAGN identified in low resolution SDSS spectra are not good indicators of dual/binary AGN. On the other hand, closely interacting galaxies or merger remnants are good candidates for detecting dual/binary AGN. We have also done ultra-violet imaging observations of a sample of 15 dual nuclei galaxies to detect the UV emission from their nuclei as well as star formation from the surrounding regions. We have used the high resolution ultra-violet imaging telescope (UVIT) which is mounted on the Astrosat satellite for these observations. We have detected emission from most of the sources which are a mixed class of AGN-AGN, AGN-SF and SF-SF nuclei. For one of the sources MRK 212 we have done followup multi-wavelength studies to confirm the nature of the nuclei. A radio, optical and X-ray study of the merging galaxy MRK 212 is presented in this thesis. With radio and UV data, we have confirmed that MRK 212 is a dual AGN (at nuclear separations of ∼ 6kpc) system. The deep 15 ksec UVIT image has resolved the star-forming knots in one of the nuclei (source 2) and in the tidal arms. The radio and UV emission from the two sided structure in source 2 suggests that jet is inducing star-formation. The optical spectra could not reveal the nature of the AGN; we think that is because of dust obscuration or poor sensitivity of the data. The X-ray data shows that source one has a soft spectrum while source 2 has a hard spectrum. We conclude that MRK 212 is a good example of a dual AGN with clear signatures of jet induced star formation.