SDSS low z quasar companion galaxies

TL;DR

This paper tackles whether low-redshift QSO activity is driven by nearby galactic companions. Using SDSS DR16 data, the authors select QSOs with $0.1<z<0.35$ and $M(r)<-21.3$ and search for spectroscopic companions within $PD<700$ kpc and $\Delta V<1000$ km s$^{-1}$, yielding 651 companions in 447 QSO fields after stringent quality cuts. They find that most QSOs have a single companion, with only a few residing in richer groups, and that the contamination from chance alignments is $<5\%$, but the average number of companions per QSO is about $2.2$ when accounting for spectroscopic coverage. Emission-line diagnostics show no significant enhancement of star formation in companion galaxies compared to non-associated field galaxies, suggesting that close companions and recent star formation are not strongly linked to QSO nuclear activity in this sample. Overall, environment appears to play a limited role in triggering activity for these nearby QSOs, though a minority inhabit richer galactic environments.

Abstract

We investigate the relationship between quasars and close companion galaxies using photometry and spectroscopy from the large data set of the SDSS DR16 survey. From the SDSS-QSO catalog of quasars, we selected objects with 0.1 < z < 0.35 and absolute magnitude M(r) <-21.3. For all these targets, we searched for candidate companion galaxies that have a projected distance from the target < 700 kpc and a radial velocity difference from the QSO DeltaV < 1000 km/sec. We find that in 447 QSO at least one companion galaxy is associated. A total of 691 associated galaxies are found in these QSO fields. In the majority of them there is just one associated galaxy but in few cases, many companions are also discovered and in two cases the QSO is found in rich galaxy environments. The possible contamination due to a chance projection of the companion galaxies is found less than 5%. Based on the available data we expect on average to find \sim2 associated companion galaxies for each QSO. A small fraction (13%) of the companion galaxies exhibits [OII] emission lines as signature of recent star formation. However a similar fraction (16%) of unassociated galaxies in the same QSO fields show [OII] emission. This study suggests that there is no significant link between the presence of these close companion galaxies or the signature of recent star formation and the QSO nuclear activity.

Figures (9)

• Figure 1: Absolute magnitude distribution of 447 (blue) QSO (this work) compared with the distribution of a sample of 416 QSO (red) from SDSS Stripe 82 survey Falomo_2014.
• Figure 2: Top-Left panel Color image of the field from SDSS DR16 (Field 2.5$\times$2.5 arcmin; North up East left). Bottom-Left panel Contour plot of the field. Right panel The optical spectrum of the QSO (top) and that of the companion galaxy (bottom). The main spectral features in the QSO and the companion galaxy are marked.
• Figure 3: Absolute magnitude (M$_r$) distribution of 651 companion galaxies in 447 QSO fields (see text). The average absolute magnitude is $<M_r>$ = --21.8 $\pm 0.9$
• Figure 4: Definition of the two subsamples of companion galaxies (see text). Red and black point represent galaxies that are associated and non associated to the QSO, respectively. The two rectangles define the sub-samples (see also text). Subsample A (magenta rectangle): 0.1$<z<$0.35 and PD $<$300 kpc. Subsample B (blue rectangle): 0.25$<z<$0.35 and PD $<$600 kpc.
• Figure 5: The expected number of associated companion galaxies for the 3112 QSO fields as a function of the galaxy magnitude. This is derived from the observed fraction of companion galaxies with spectroscopic observations and the magnitude distribution of all galaxies in the defined QSO fields (see text). On average we found that in a QSO we expect to have 2.2 associated galaxies.