Redshifted formaldehyde from the gravitational lens B0218+357

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: njethava$@$mpifr-bonn.mpg.de
² National Radio Astronomy Observatory, Socorro, New Mexico, 87801, USA
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 42, Cambridge, MA 02138, USA

Received: 26 February 2007
Accepted: 8 June 2007

Abstract

Context.Little is known about individual molecular clouds at high redshifts. The gravitational lens toward B0218+357 offers the unique possibility to study cool moderately dense gas with high sensitivity and angular resolution in a cloud that existed half a Hubble time ago.

Aims.This non-CO molecular multi-level study of a significantly redshifted cloud aims at a better definition of the physical properties of molecular gas in a kind of interstellar environment that is rarely studied in the Galaxy or in the nearby extragalactic space.

Methods.Observations of the radio continuum and six formaldehyde (H₂CO) lines were carried out with the VLA, the Plateau de Bure interferometer and the Effelsberg 100-m telescope.

Results.Three radio continuum maps indicate a flux density ratio between the two main images, A and B, of ~. Within the errors the ratio is the same at 8.6, 14.1, and 43 GHz. The 1 line of para-H₂CO is shown to absorb the continuum of image A. Large Velocity Gradient radiative transfer calculations are performed to reproduce the optical depths of the observed two cm-wave “K-doublet” and four mm-wave rotational lines. These calculations also account for a likely frequency-dependent continuum cloud coverage. Confirming the diffuse nature of the cloud, an n(H₂) density of <1000 cm^-3 is derived, with the best fit suggesting n(H₂) ~ 200 cm^-3. The H₂CO column density of the main velocity component is ~¹³ cm^-2, to which about ¹² cm^-2 has to be added to also account for a weaker feature on the blue side, 13 km s^-1 apart. N(H₂CO)/N(NH₃) ~ 0.6, which is four times less than the average ratio obtained from a small number of local diffuse (galactic) clouds seen in absorption. The ortho-to-para H₂CO abundance ratio is 2.0–3.0, which is consistent with the kinetic temperature of the molecular gas associated with the lens of B0218+357. With the gas kinetic temperature and density known, it is found that optically thin transitions of CS, HCN, HNC, HCO⁺, and N₂H⁺ (but not CO) will provide excellent probes of the cosmic microwave background at redshift .