An ALMA Band 7 survey of SDSS/Herschel quasars in Stripe 82: I. The properties of the 870 micron counterparts

TL;DR

This study uses ALMA Band 7 imaging at 0.8″ resolution to resolve the 870 μm counterparts of 152 FIR-bright SDSS quasars in Stripe 82, addressing the origin of their FIR emission and the role of multiplicity and environment in concurrent SMBH growth and star formation. It finds that ~60% of fields host a single submm counterpart, with multiplicities increasing with redshift and showing no strong dependence on balnicity, implying mergers are important but not the sole driver. Serendipitous CO(6-5) and CO(7-6) detections reveal moderate gas excitation, with no evidence for widespread high-J CO emission or dominant XDR influence. The results suggest many FIR-bright quasars reside in over-dense environments with companion sources contributing to FIR flux, which can inflate extreme SFR inferences; a follow-up SED reanalysis will reassess L_IR and SFRs to clarify the true star-forming activity.

Abstract

(Abridged) Quasar studies with Herschel/SPIRE often report host luminosities ranging from 10^{12} to 10^{14} Lsun, suggestive of star formation rates (SFRs) of up to several thousand Msun/yr. Due to the limited spatial resolution of SPIRE, it is uncertain whether the far-infrared (FIR) emission originates from the quasar itself, nearby sources, or unrelated sources within the SPIRE beam. High-resolution observations at wavelengths close to the SPIRE coverage are needed to pinpoint the true source of the FIR emission. In this work we unambiguously identify the ALMA Band 7 counterparts of a statistical sample of 152 FIR-bright SDSS quasars and estimate the multiplicity rates among these systems. Based on the multiplicities, we assess the importance of mergers as triggers for concomitant accretion onto supermassive black holes (SMBHs) and extreme star formation. In ~60% of cases, the submm emission originates from a single counterpart within the SPIRE beam, centred on the optical coordinates of the quasar. The multiplicity rate increases by a factor of ~2.5 between redshifts 1 and 2.5. The incidence of multiplicities is consistent among broad absorption line (BAL) quasars and non-BAL quasars. The multiplicities observed in a fraction of the sample indicate that, while mergers enhance gas inflow efficiency, there must be viable alternatives for driving synchronous SMBH growth and intense star formation in isolated systems. We report the serendipitous detection of two CO(6-5) and three CO(7-6) transitions out of the eight such transitions expected based on the spectral setup and the redshifts of the objects in the sample. Higher transitions are not detected, indicating that the quasars are not exciting sufficiently the gas in their hosts. Finally, we also detect a potential emission of H2O, HCN (10-9) or a combination of both in the spectrum of a quasar at redshift 1.67.

Figures (8)

• Figure 1: Properties of the sample of 152 quasars. From left to right and top to bottom: spectroscopic redshift; Herschel/SPIRE 250 $\mu$m flux (S$_{250}$); quasar bolometric luminosity (L$_{bol}$) and Eddington ratio ($\lambda_{\rm Edd}$) from the SDSS DR16 catalogue.
• Figure 2: Left: Histogram of the distance to the phase centre for the primary (open histogram) and secondary (grey histogram) counterparts. Right: Distribution of the Band 7 (870 ${\mu}$m) peak fluxes of the 209 ALMA sources in the 152 quasar fields (black histogram). Primary counterparts, i.e. counterparts extracted at the phase centre (i.e. at the optical coordinates of the quasars) are shown in a shaded histogram.
• Figure 3: Example ALMA Band 7 images. From top to bottom and left to right: J010524.39-002527.1: Single 870 ${\mu}$m counterpart centred on the SDSS coordinates; J021417.64-010524.8: ALMA 870 ${\mu}$m counterpart centred at the location of the quasar, with secondary counterpart; J015017.71+002902.4: ALMA 870 ${\mu}$m counterpart centred at the location of the quasar, with two secondary, brighter counterparts; J005921.54+004350.0: Single 870 ${\mu}$m counterpart, not associated with the SDSS quasar; J014555.58-003125.8: Three 870 ${\mu}$m counterpart, none of which associated with the quasar; and J020327.40-001625.9: no detection. The ALMA beam is shown as a black ellipse at the bottom left corner of each image.
• Figure 4: Contribution of the brightest (black filled circles), second and third brightest (green open circles and red open triangles, respectively) counterparts to the total 870 ${\mu}$m flux on each of the ALMA Band 7 maps, as a function of the 250 ${\mu}$m originally associated to each SDSS quasar.
• Figure 5: The Band 7 spectra of the two quasars with unambiguous CO(6-5) transition detections. Left and middle columns: The filled blue histograms show the extracted spectra within the extraction aperture smoothed to a spectral resolution of $\sim$111 km s$^{-1}$, while the solid black histogram shows the raw-resolution spectra of the peak flux within the aperture (units of Jy/beam). The greyed out regions are the per-channel $\pm1\sigma$, while the dotted lines are the per-channel $\pm3\sigma$ level. The magenta line shows the continuum that was subtracted from the spectrum to assess the significance of absorption detections (there were none). The red dashed lines indicate the expected location of the CO transition shifted to the optical (SDSS) redshift of each object. The orange line shows the single-Gaussian fit to the line. Right column: ALMA pipeline continuum fitting output from the spectral window where the lines were identified. The binned spectrum in red; the region in which the continuum has been extracted is shown in cyan; the level of the continuum is shown in black. The x-axis at the bottom indicates the channels, the one at the top the frequency (in GHz). These plots have been extracted directly from the logs of the pipeline runs ("weblogs").