QUASARS

Twin galaxies in the early Universe | OPTICON

Serendipitous discovery of a physical binary quasar at redshift z=1.76

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arXiv pdf
ADS link

Abstract

Binary quasars are extremely rare objects, used to investigate clustering on very small scales at different redshifts. The cases where the two quasar components are gravitationally bound, known as physical binary quasars, can also exhibit enhanced astrophysical activity and therefore are of particular scientific interest. Here we present the serendipitous discovery of a physical pair of quasars with an angular separation of Δθ = (8.76 ± 0.11) arcsec. The redshifts of the two quasars are consistent within the errors and measured as z = (1.76 ± 0.01). Under the motivated assumption that the pair does not arise from a single gravitationally lensed quasar, the resulting projected physical separation was estimated as (76 ± 1) kpc. For both targets we detected Si IV, C IV, C III], and Mg II emission lines. However, the two quasars show significantly different optical colours, one being among the most reddened quasars at z > 1.5 and the other with colours consistent with typical quasar colours at the same redshift. Therefore it is ruled out that the sources are a lensed system. This is our second serendipitous discovery of a pair of two quasars with different colours, having a separation ≲ 10 arcsec, which extends the very limited catalogue of known quasar pairs. We ultimately argue that the number of binary quasars may have been significantly underestimated in previous photometric surveys, due to the bias arising from paired quasars with very different colours.

Figures

Below is shown a 1 arcmin × 1 arcmin field around the two sources (marked A and B) as imaged in the r-band by SDSS DR12. North is up and East is to the left. We have over-plotted information on the proper motion from Gaia DR2 (Gaia Collaboration et al. 2018) with red arrows and red error ellipses showing the 2-σ uncertainty on the proper motion. The quasars both have proper motions consistent with 0, whereas the two other objects are moving significantly and are hence unrelated. We also plot a schematic view of the slit during the GTC observation (dotted lines), which was centred on source A and aligned with the parallactic angle, oriented at 115o East of North (EoN) at the time of the observation. The position angle between the two objects is 124o EoN, and this was the slit angle used during the NOT observation (full drawn lines).

Optical field of view centred on the double quasar.

Here are shown the spectra from the Nordic Optical Telescope, obtained using a slit properly aligned with both quasars. The top panel illustrates the 2-dimensional spectrum with the traces of both quasars separated by 8.76 arcsec. The two bottom panels show the 1-dimensional spectra covering the region from C III] to Mg II (marked with dashed, red lines). The spectra are not corrected for telluric absorption. The red dots show the g, r and i-band photometry from SDSS.

Optical spectrum of the A and B components.