Star Formation and Gas Kinematics of Quasar Host Galaxies at z~6: New insights from ALMA (1302.4154v2)

Abstract: We present ALMA observations of the [C II] 158 micron fine structure line and dust continuum emission from the host galaxies of five redshift 6 quasars. We also report complementary observations of 250 GHz dust continuum and CO (6-5) line emission from the z=6.00 quasar SDSS J231038.88+185519.7. The ALMA observations were carried out in the extended array at 0.7" resolution. We have detected the line and dust continuum in all five objects. The derived [C II] line luminosities are 1.6x10^{9} to 8.8x10^{9} Lsun and the [C II]-to-FIR luminosity ratios are 3.0-5.6x10^{-4}, which is comparable to the values found in other high-redshift quasar-starburst systems and local ultra-luminous infrared galaxies. The sources are marginally resolved and the intrinsic source sizes (major axis FWHM) are constrained to be 0.3" to 0.6" (i.e., 1.7 to 3.5 kpc) for the [C II] line emission and 0.2" to 0.4" (i.e., 1.2 to 2.3 kpc) for the continuum. These measurements indicate that there is vigorous star formation over the central few kpc in the quasar host galaxies. The ALMA observations also constrain the dynamical properties of the atomic gas in the starburst nuclei. The intensity-weighted velocity maps of three sources show clear velocity gradients. Such velocity gradients are consistent with a rotating, gravitationally bound gas component, although they are not uniquely interpreted as such. Under the simplifying assumption of rotation, the implied dynamical masses within the [C II]-emitting regions are of order 10^{10} to 10^{11} Msun. Given these estimates, the mass ratios between the SMBHs and the spheroidal bulge are an order of magnitude higher than the mean value found in local spheroidal galaxies, which is in agreement with results from previous CO observations of high redshift quasars.

• The paper presents ALMA observations of [C II] and dust continuum emissions, detailing star formation regions spanning 1.7–3.5 kpc in z~6 quasar hosts.
• It identifies clear velocity gradients indicative of rotating gas disks and derives dynamical masses between 10^10 and 10^11 M☉.
• The study highlights elevated SMBH-to-bulge mass ratios, suggesting rapid supermassive black hole growth during early cosmic evolution.

Star Formation and Gas Kinematics in z~6 Quasar Host Galaxies

The paper by Wang et al. presents an intensive paper of star formation and the kinematic properties of gas in quasar host galaxies situated at a redshift of approximately 6. This analysis is conducted using observations from the Atacama Large Millimeter/submillimeter Array (ALMA), focusing on the [C II] 158 µm fine structure line and dust continuum emissions from these distant and highly luminous astronomical objects. The research also incorporates complementary CO (6-5) line and 250 GHz dust continuum measurements from the IRAM 30m telescope, specifically targeting the quasar SDSS J231038.88+185519.7 (hereafter J2310+1855).

Observations and Measurements

The ALMA observations, operating at a resolution of 0.7 arcseconds, detected significant [C II] line and dust continuum emissions across all five examined z~6 quasars. The observed [C II] line luminosities range between 1.6×10^9 to 8.7×10^9 L_☉, while the [C II]-to-FIR luminosity ratios are within the range of 2.9-5.1×10^-4. These ratio values align with those found in other high-redshift quasar-starburst systems and local ULIRGs. Spatial observations reveal that the sources are only marginally resolved, with intrinsic major axis source sizes for the [C II] line and dust continuum sized between 1.7 to 3.5 kpc and 1.2 to 2.3 kpc, respectively. These observational parameters highlight regions of vigorous star formation within the central few kiloparsecs of the quasar host galaxies.

The intensity-weighted velocity maps show discernible velocity gradients in three of the observed sources, which could imply that the gases are in rotation and gravitationally bound. Assuming a simple rotating disk model for these gas components, the derived dynamical masses within the [C II]-emitting regions vary from 10^10 to 10^11 M_☉. These masses result in SMBH-to-bulge mass ratios significantly higher than those observed in local spheroidal galaxies, consistent with previous CO observations of high-z quasars.

Implications and Future Directions

The research furnishes crucial insights into the early growth stages of supermassive black holes (SMBHs) and their consequential evolution with the host galaxies. The elevated SMBH-to-bulge mass ratios suggest a potentially swift pace of SMBH accretion and an evolving relationship with the host galaxies in the universe's infancy, less than 1 Gyr after the Big Bang. This aligns with simulations forecasting significant SMBH-galaxy co-evolution during the epoch of cosmic reionization.

The methodological approaches in this paper also underscore the utility of combining multiple emission lines ([C II], CO) to probe diverse regions within the quasar hosts. This multi-line strategy offers rich information on the star formation activities and kinematic properties of dense gas in these distant galaxies.

In terms of future research directives, the paper highlights potential advantages from utilizing ALMA's higher spatial resolution and broader frequency coverage in forthcoming cycles. Such observations would allow researchers to more definitively ascertain the spatial distributions of gas and dust, disentangle AGN and star formation contributions to continuum emissions, and acquire robust velocity fields. This would improve mass estimates and understand the dynamical processes at play. Additionally, pursuits for serendipitous line emitters in the quasar environment can advance the understanding of galaxy formation and evolution during the earliest epochs in the universe.

By advancing the understanding of the star formation dynamics and gas kinematics in high-redshift quasar-host galaxies, this research nurtures a more comprehensive understanding of cosmic evolution processes during the epoch of reionization.