Molecular gas in z~6 quasar host galaxies

Abstract

We investigate the molecular gas content of z ∼ 6 quasar host galaxies using the Institut de Radioastronomie Millimétrique / Northern Extended Millimeter Array. We target the 3 mm dust continuum, and the line emission from CO(6-5), CO(7-6), [C i] 2−1 in 10 infrared-luminous quasars that have been previously studied in their 1 mm dust continuum and [C ii] line emission. We detect CO(7-6) at various degrees of significance in all the targeted sources, thus doubling the number of such detections in z ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperature T dust ∼ 47 K and an optical depth τ ν = 0.2 at the [C ii] frequency. Our study provides us with four independent ways to estimate the molecular gas mass, M H2 , in the targeted quasars. This allows us to set constraints on various parameters used in the derivation of molecular gas mass estimates, such as the mass per luminosity ratios α CO and α [CII] , the gas-to-dust mass ratio δ g/d , and the carbon abundance [C]/H 2 . Leveraging either on the dust, CO, [C i], or [C ii] emission yields mass estimates of the entire sample in the range M H2 ∼ 10 10 to 10 11 M . We compare the observed luminosities of dust, [C ii], [C i], and CO(7-6) with predictions from photo-dissociation and X-ray dominated regions. We find that the former provide better model fits to our data, assuming that the bulk of the emission arises from dense (n H > 10 4 cm −3 ) clouds with a column density N H ∼ 10 23 cm −2 , exposed to a radiation field with intensity G 0 ∼ 10 3 (in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in z ∼ 6 quasar host galaxies. It also highlights the power of combined 3 mm and 1 mm observations for quantitative studies of the dense gas content in massive galaxies at cosmic dawn.

On the Connection between Supermassive Black Holes and Galaxy Growth in the Reionization Epoch

“…The resulting gas  and   of our quasars at their respective dyn  are shown in Figure 1 (left panel). The gas mass is distributed between ∼10 10 -10 11 M e , which is consistent with recent direct measurements in z ∼ 6 quasars (e.g., Decarli et al 2022). The derived   is typically 0.2-0.5 dex smaller than dyn  .…”

“…This gives rise to a luminosity-dependent virial BH mass bias (hereafter the SE bias) as detailed in Shen (2013). In addition, the hosts of z ∼ 6 quasars are found to be gas rich (e.g., Decarli et al 2022), thus approximating   by dyn  is likely an overestimate. In this Letter, we model the selection and measurement biases for a z ∼ 6 quasar sample compiled in the literature in order to reveal the underlying connection between SMBH and galaxy growth in the early universe.…”

The [CII] and FIR properties of z>6 radio-loud quasars

“…Meijerink et al 2007). The observations of [CI] and [CII] emission lines in radio-quiet quasars point to the PDR origin of their [CII] emission (Venemans et al 2017;Novak et al 2019;Pensabene et al 2021;Meyer et al 2022;Decarli et al 2022), but the [CI] emission line has not yet been observed for any of the quasars in our sample.…”

“…We note that in deriving SFRs, we assumed that [CII] and IR luminosities are only related to the star formation in the host galaxies. This is a reasonable assumption for radio-quiet quasars at z 6 (Venemans et al 2017;Novak et al 2019;Pensabene et al 2021;Meyer et al 2022;Decarli et al 2022), but has not yet been tested for radio-loud ones. If AGN-related sources of [CII] and IR emission are significant, we are overestimating the SFRs.…”

On the Connection between Supermassive Black Hole and Galaxy Growth in the Reionization Epoch

“…For instance, the M BH of flux-limited quasar sample might be systematically overestimated by the single-epoch virial estimator (e.g., Vestergaard & Osmer 2009; hereafter VO09) because of uncorrelated variation between AGN luminosity and broad line width (especially for Mg II and C IV) that gives rise to a luminosity-dependent virial BH mass bias (hereafter the SE bias) as detailed in Shen (2013). In addition, the hosts of z ∼ 6 quasars are found to be gas-rich (e.g., Decarli et al 2022), thus approximating M by M dyn is likely an overestimate.…”