ALMA view of RX J1131-1231: Sub-kpc CO (2-1) mapping of a molecular disk in a lensed star-forming quasar host galaxy

Abstract

We present ALMA 2-mm continuum and CO (2-1) spectral line imaging of the gravitationally lensed z = 0.654 star-forming/quasar composite RX J1131−1231 at 240 to 400 mas angular resolution. The continuum emission is found to be compact and coincident with the optical emission, whereas the molecular gas forms a complete Einstein ring, which shows strong differential magnification. The de-lensed source structure is determined on 400-parsec-scales resolution using a Bayesian pixelated visibility-fitting lens modelling technique. The reconstructed molecular gas velocity-field is consistent with a large rotating disk with a major-axis FWHM ∼9.4 kpc at an inclination angle of i = 54• and with a maximum rotational velocity of 280 km s −1 . From dynamical model fitting we find an enclosed mass within 5 kpc of M(r < 5 kpc) = (1.46 ± 0.31) × 10 11 M . The molecular gas distribution is highly structured, with clumps that are co-incident with higher gas velocity dispersion regions (40-50 km s −1 ) and with the intensity peaks in the optical emission, which are associated with sites of on-going turbulent star-formation. The peak in the CO (2-1) distribution is not co-incident with the AGN, where there is a paucity of molecular gas emission, possibly due to radiative feedback from the central engine. The intrinsic molecular gas luminosity is L CO = 1.2 ± 0.3 × 10 10 K km s −1 pc 2 and the inferred gas mass is M H 2 = 8.3 ± 3.0 × 10 10 M , which given the dynamical mass of the system is consistent with a CO-H 2 conversion factor of α = 5.5 ± 2.0 M (K km s −1 pc 2 ) −1 . This suggests that the star-formation efficiency is dependent on the host galaxy morphology as opposed to the nature of the AGN. The far-infrared continuum spectral energy distribution shows evidence for heated dust, equivalent to an obscured star-formation rate of SFR = 69 +41 −25 × (7.3/µ IR ) M yr −1 , which demonstrates the composite star-forming and AGN nature of this system.

SHARP – VI. Evidence for CO (1–0) molecular gas extended on kpc-scales in AGN star-forming galaxies at high redshift

“…Sharon et al (2016) detected the CO (1-0) line with the VLA on 2.5 arcsec-scales, finding that the line profile has an asymmetric double-horn structure, as is typically seen in rotating gas discs with a strong differential magnification (e.g. Paraficz et al 2018). Note that this line profile was consistent with the higher order CO lines detected with CARMA.…”

“…This effect has also been seen in other low J-level molecular gas distributions around lensed AGN when observed at sufficiently high angular resolution (e.g. Paraficz et al 2018). The velocity distribution shows a clear gradient across the major axis, which can be interpreted as multiple velocity source components, or more likely, as a rotating molecular gas disc.…”

“…The CO (1-0) emission line has a more complicated profile than can be modelled by a single Gaussian function, having an asymmetric structure with one strong peak centred at −190 km s −1 and another fainter peak centred at +180 km s −1 relative to the systemic velocity of the AGN; this kind of line profile is consistent with a differential magnification of a double horn profile from a rotating gas disc (e.g. Popović & Chartas 2005;Banik & Zhao 2015;Rybak et al 2015b;Leung et al 2017;Paraficz et al 2018;Stacey & McKean 2018). A Gaussian fit to the profile gives a line peak of 0.9 ± 0.4 mJy, an FWHM of 683 ± 280 km s −1 , and a CO (1-0) line luminosity of μ CO × L CO = (20.2 ± 4.0) × 10 10 K km s −1 pc 2 (see Table 3).…”

A flux-ratio anomaly in the CO spectral line emission from gravitationally lensed quasar MG J0414+0534

“…as has been seen in the molecular gas emission from other lensed galaxies (Rybak et al 2015b;Paraficz et al 2018). A single Gaussian fit to the line profile from images A1 and A2, gives a line FWHM of 1080 ± 20 km s −1 .…”

Spatially Resolved Patchy Lyα Emission within the Central Kiloparsec of a Strongly Lensed Quasar Host Galaxy at z = 2.8

“…Even subtracting the quasar emission with a well-understood point-spread function (PSF) in the ultraviolet (UV) and optical can leave residual noise from the wings of the quasar emission that overwhelms the host galaxy. Recent ALMA observations at millimeter wavelengths-where the central AGN emission is less of an issue-can resolve the cold molecular gas content of distant quasar host galaxies on ∼1-2 kpc scales (Wang et al 2013;Paraficz et al 2017;Venemans et al 2017). More local, low-redshift studies (  z 0.4) using adaptive optics and HST can recover host galaxy morphologies in the optical and near-infrared and find that quasar hosts can be early-or late-type galaxies (Dunlop et al 2003;Guyon et al 2006) and can exhibit significant star formation (e.g., Young et al 2014).…”