Infrared Molecular Starburst Fingerprints in Deeply Obscured (Ultra)Luminous Infrared Galaxy Nuclei

Abstract

High resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C 2 H 2 , HCN, and CO 2 molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n 10 7 cm −3 ), warm (T ex = 200 − 700 K) molecular gas with high column densities of these molecules ranging from a few 10 15 − 10 17 cm −2 . Abundances relative to H 2 , inferred from the silicate optical depth, range from ∼ 10 −7 to 10 −6 and show no correlation with temperature. Theoretical studies show that the high abundances of both C 2 H 2 and HCN exclude a X-ray dominated region (XDR) associated with the toroid surrounding an AGN as the origin of this dense warm molecular gas. Galactic massive protostars in the so-called Hot Core phase have similar physical characteristics with comparable high abundances of C 2 H 2 , HCN, and CO 2 in the hot phase. However, the abundances of C 2 H 2 and HCN and the C 2 H 2 /CO 2 and HCN/CO 2 ratios are much higher toward the (U)LIRGs in the cooler (T ex 400 K) phase. We suggest that the -2warm dense molecular gas revealed by the mid-IR absorption lines is associated with a phase of deeply embedded star formation where the extreme pressures and densities of the nuclear starburst environment have inhibited the expansion of HII regions and the global disruption of the star forming molecular cloud cores, and 'trapped' the star formation process in an 'extended' Hot Core phase.Subject headings: infrared: ISM -ISM: evolution -ISM: galaxies -ISM: molecules -galaxies: nuclei 11 The IRS-SH slit width is 4.7", equal to the size of the PSF at 19.5µm.

Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H218O, OH,18OH, O I, HCN, and NH3

“…Our models for the nuclear region are, however, structureless, and more refined models are required to clarify the role of the ν 2 = 1 1 pumping for these lines. Finally, we note that our derived HCN column is a factor of 20 higher than that derived by Lahuis et al (2007) from the 14 μm absorption band; as the authors state, however, their analysis is based on a covering factor of unity of the mid-IR continuum and their columns are then lower limits.…”

“…In NGC 4418 and Arp 220, the low-lying lines at millimeter wavelengths have been observed by Aalto et al (2007a,b) and Wiedner et al (2004). The ν 2 = 1 1 vibrationrotation band at 14 μm was detected in both sources by Lahuis et al (2007), and rotational emission from the upper vibrational state in NGC 4418 has been detected by Sakamoto et al (2010). The ν 2 = 1 1 l-type HCN lines from J = 4−6 at centimeter wavelengths were detected in absorption toward Arp 220 (Salter et al 2008).…”

“…The ν 2 = 1 1 l-type HCN lines from J = 4−6 at centimeter wavelengths were detected in absorption toward Arp 220 (Salter et al 2008). Among the 15 extragalactic sources detected in the HCN band, the Lahuis et al (2007) analysis indicated that NGC 4418 has the second highest HCN column. Recently, Rangwala et al (2011) have reported the detection of several emission/absorption HCN lines with Herschel/SPIRE in Arp 220.…”

“…by Lahuis et al (2007) from the HCN 14 μm absorption in these and other (U)LIRGs, Martín et al (2011) for Arp 220 based on multispecies observations at 1.3 mm, and Aalto et al (2007a) and Costagliola & Aalto (2010) to explain the luminous HC 3 N emission in NGC 4418. HCN is well known to be enhanced in Galactic hot cores, as in the Orion hot core where the abundance is ∼10 −6 (Schilke et al 1992), about two orders of magnitude higher than in cold molecular clouds.…”

c2dSpitzerIRS spectra of embedded low-mass young stars: gas-phase emission lines

“…) where these molecules are characteristic of hot-core chemistry (Doty et al 2002;Rodgers & Charnley 2003). Toward ULIRGs absorption of C 2 H 2 , HCN and CO 2 has been observed by Lahuis et al (2007a) suggesting the presence of pressure confined massive star formation. The absence of signatures of C 2 H 2 and HCN absorption towards the low-mass sources in our sample is not surprising, however.…”