A spectral stacking analysis to search for faint outflow signatures inz∼ 6 quasars

Abstract

Aims. Outflows in quasars during the early epochs of galaxy evolution are an important part of the feedback mechanisms potentially affecting the evolution of the host galaxy. However, systematic observations of outflows are only now becoming possible with the advent of sensitive mm telescopes. In this study we use spectral stacking methods to search for faint high velocity outflow signal in a sample of [C ii] detected, z ∼ 6 quasars. Methods. We search for broad emission line signatures from high-velocity outflows for a sample of 26 z ∼ 6 quasars observed with the Atacama Large Millimeter Array (ALMA), with a detection of the [C ii] line. The observed emission lines of the sources are dominated by the host galaxy, and outflow emission is not detected for the individual sources. We use a spectral line stacking analysis developed for interferometric data to search for outflow emission. We stack both extracted spectra and the full spectral cubes. We also investigate the possibility that only a sub-set of our sample contributes to the stacked outflow emission. Results. We find only a tentative detection of a broad emission line component in the stacked spectra. When taking a region of about 2 around the source central position of the stacked cubes, the stacked line shows an excess emission due to a broad component of 1.1-1.5σ, but the significance drops to 0.4-0.7σ when stacking the extracted spectra from a smaller region. The broad component can be characterised by a line width of full width half-max FWHM > 700 km s −1 . Furthermore, we find a sub-sample of 12 sources the stack of which maximises the broad component emission.The stack of this sub-sample shows an excess emission due to a broad component of 1.2-2.5σ. The stacked line of these sources has a broad component of FWHM > 775 km s −1 . Conclusions. We find evidence suggesting the presence of outflows in a sub-sample of 12 out of 26 sources, and have demonstrated the importance of spectral stacking techniques in tracing faint signal in galaxy samples. However, deeper ALMA observations are necessary to confirm the presence of a broad component in the individual spectra.

Cool outflows in galaxies and their implications

“…The stacked cube also suggests that these quasar-driven outflows have extensions of several kpc. Stacking analyses no doubt miss a significant fraction of faint outflows due to orientation effects (Stanley et al 2019), so these results should be considered conservative lower limits.…”

Probing the Full CO Spectral Line Energy Distribution (SLED) in the Nuclear Region of a Quasar-starburst System at z = 6.003

“…The fluxes were calibrated using the standard flux calibrator P allas, while SDSS J2253+1608 was used as both the phase calibrator and the bandpass calibrator. The typical calibration uncertainty is < 5% in ALMA band 4, here we use 15% uncertainty that also includes the uncertainties of the old P allas flux model in early casa versions (Stanley et al 2019).…”

What do observations tell us about the highest-redshift supermassive black holes?

“…Bischetti et al (2019) studied 48 quasars at 4.5 < z < 7.1 and found broad [C ii] wings that are interpreted to trace outflowing gas with velocities v ∼ > 1,000 km s −1 , an extent of ∼ > 5 kpc, and a deduced a mass outflow rate of order ∼100 M yr −1 . On the other hand, Stanley et al (2019) found only marginal evidence (<3σ) for such outflow signatures in their analysis of 26 z ∼ 6 quasars (the outflow signatures may be more prominent in a subset of about half of these quasars).…”