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Detection of H3+ in the interstellar medium of IRAS 08572+3915

The first detection of the molecular ion H3+ in an extragalactic object has been made toward the highly obscured ultraluminous galaxy, IRAS 08572+3915. Two absorption features near 3.9 microns in its spectrum are identified as the redshifted 3.668 microns doublet and 3.716 microns singlet lines of H3+, both previously detected in a number of galactic molecular and diffuse clouds. We discuss the probable location of the H3+ in this galaxy.


Introduction
H + 3 , the highly reactive molecular ion upon which interstellar gas phase chemistry is based [6,12], is important observationally for understanding dark and diffuse interstellar clouds [5]. In both types of clouds it is produced following cosmic ray ionization of H 2 to H + 2 , which quickly reacts with H 2 to form H + 3 . H + 3 is destroyed readily in dark clouds by reactions with neutral molecules (principally by CO) and atoms (mainly by O) and even more readily in diffuse clouds by dissociative recombination on electrons, which, due to the ionization of carbon, are much more abundant than in dark clouds. The absorption strengths of H + 3 lines provide basic information on the cloud dimensions and environment, in addition to temperature. Assuming a value for the ionization rate, the column density of H + 3 directly yields either the distance through the cloud to an embedded continuum source or the thickness of an intervening cloud or group of clouds. Alternatively, if an estimate for the line of sight distance through the absorbing material is available, the ionization rate can be determined from the column density. These inferences are possible because unlike most other molecules, the number density of H + 3 is a constant that depends only on whether a cloud is dark or diffuse. This unusual property of H + 3 comes about because its creation rate per unit volume is proportional to the first power of the cloud density, rather than the square.
H + 3 has been detected in both dark and diffuse clouds by virtue of its vibration-rotation transitions near 3.7 µm (an ortho-para doublet at 3.668 µm and a para singlet at 3.715 µm). The amounts of H + 3 that have been found in dark clouds are consistent with predictions based on the estimated production rate and measured destruction rates [3,9]. In diffuse clouds, however, there is a large difference between the observed and predicted column densities [4,10]; the cause may be the use of an inaccurate value for the rate of dissociative recombination on electrons. The detection of strong H + 3 absorption toward the center of the Galaxy [4] suggests that it is possible to detect H + 3 in the interstellar medium of suitable external galaxies -those with sufficiently bright and compact sources of infrared continuum radiation as well as long columns of interstellar molecular gas along their lines of sight. Although 8-10 m telescopes and their successors will be required for studying H + 3 in all but a very few of the most brightest candidates, at the United Kingdom 3.8 m Infrared Telescope (UKIRT) a search was initiated in one of the most promising galaxies, IRAS 08572+3915. This galaxy has a deep 10 µm silicate absorption [2] and the strongest 3.4 µm interstellar absorption feature known [13,7], nearly twice as strong as the one observed toward the Galactic center. The 3.4 µm feature is known to be associated with objects reddened by diffuse gas [1] and its strength in IRAS 08572+3915 tends to imply the existence of a long pathlength of molecular material along the line of sight to the nuclear infrared source.

Observations and results
In December 2000 a 3.82-3.98 µm spectrum of IRAS 08572+3915 was obtained at UKIRT, using the facility spectrograph CGS4. The spectrum covered the wavelength range in which the redshifted 3.7 µm lines of H + 3 should appear. A portion of the spectrum is shown in Fig. 1 at a resolving power of ∼ 5,000. Statistically significant absorption features are observed at the expected wave-lengths of the two H + 3 features, 3.8815 µm and 3.9317 µm, indicating that the molecular ion has been detected.
The 3.88 µm doublet, which is blended but which also would be unresolved at the resolution used, appears to be roughly twice the strength of the singlet, indicating that the doublet has roughly equal contributions from its ortho and para components. The column density of H + 3 derived from the lines is about 5 × 10 15 cm −2 , roughly twice that found towards the Galactic center. This result is only mildly sensitive to the temperature, which cannot be determined accurately because of an insufficient signal-to-noise ratio and the low spectral resolution, but which appears to be 100 K or less. The FWHMs of the absorptions cannot be determined accurately, but are probably 150±50 km s −1 , similar to that found toward the Galactic center.

Discussion and Conclusions
The velocity at which H + 3 is seen in IRAS 08572+3915 is the systemic velocity; hence the associated gas is neither falling into nor being expelled from the presumed AGN [2]. Presumably the H + 3 is mainly found in relatively quiescent interstellar medium far from the nucleus, as in the case of the Galactic center although in principle it could be located in a circumnuclear molecular disk. Currently it is uncertain whether the gas containing the detected H +