Redshift 6.4 host galaxies of 10^8 solar mass black holes: low star formation rate and dynamical mass

TL;DR

This study uses ALMA observations to analyze two z=6.4 quasars, revealing low star formation rates, dynamical masses, and potential galaxy rotation, indicating stellar mass growth lags black hole accretion at this epoch.

Contribution

First detailed ALMA analysis of z=6.4 quasar host galaxies showing low star formation and dynamical mass, suggesting delayed stellar growth relative to black hole accretion.

Findings

One quasar shows detectable continuum emission, the other does not.

Star formation rate in one host is below typical values at similar luminosities.

Dynamical mass inferred from [CII] line is lower than expected from local black hole-velocity dispersion relations.

Abstract

We present ALMA observations of rest-frame far-infrared continuum and [CII] line emission in two z=6.4 quasars with black hole masses of ~10^8 M_sun. CFHQS J0210-0456 is detected in the continuum with a 1.2 mm flux of 120+/-35 microJy, whereas CFHQS J2329-0301 is undetected at a similar noise level. J2329-0301 has a star formation rate limit of <40 M_sun/yr, considerably below the typical value at all redshifts for this bolometric luminosity. By comparison with hydro simulations, we speculate that this quasar is observed at a relatively rare phase where quasar feedback has effectively shut down star formation in the host galaxy. [CII] emission is also detected only in J0210-0456. The ratio of [CII] to far-infrared luminosity is similar to that of low redshift galaxies of comparable luminosity, suggesting the previous finding of an offset in the relationships between this ratio and far-infrared luminosity at low- and high-redshift may be partially due to a selection effect from the limited sensitivity of previous observations. The [CII] line of J0210-0456 is relatively narrow (FWHM=189+/-18 km/s), indicating a dynamical mass substantially lower than expected from the local black hole - velocity dispersion correlation. The [CII] line is marginally resolved at 0.7" resolution with the blue and red wings spatially offset by 0.5" (3 kpc) and a smooth velocity gradient of 100 km/s across a scale of 6 kpc, possibly due to rotation of a galaxy-wide disk. These observations are consistent with the idea that stellar mass growth lags black hole accretion for quasars at this epoch with respect to more recent times.