Aims. Molecules that trace the high-density regions of the interstellar medium have been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity. Methods. The HCN(1−0), HNC(1−0), HCO + (1−0), CN(1−0) and CN(2−1), CO(2−1), and CS(3−2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions were augmented with data available in the literature. Results. The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities were used for a first-order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiation among the sources that relate to the initial conditions and the radiative excitation environment. Initial differentiation was introduced between the FUV radiation field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser activity have line ratios typical of photon-dominated regions.

We present high resolution (0. ′′ 4) IRAM PdBI and ALMA mm and submm observations of the (ultra) luminous infrared galaxies ((U)LIRGs) IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission from vibrationally excited (ν 2 =1) J=3-2 and 4-3 HCN. The emission is emerging from buried, compact (r <17-70 pc) nuclei that have very high implied mid-infrared surface brightness >5 × 10 13 L ⊙ kpc −2 . These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200 K) extreme starbursts. Vibrational, ν 2 =1, lines of HCN are excited by intense 14 µm mid-infrared emission and are excellent probes of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H 2 column densities exceed 10 24 cm −2 . It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, we show strong evidence that the ground vibrational state (ν=0), J=3-2 and 4-3 rotational lines of HCN and HCO + fail to probe the highly enshrouded, compact nuclear regions owing to strong self-and continuum absorption. The HCN and HCO + line profiles are double-peaked because of the absorption and show evidence of non-circular motions -possibly in the form of in-or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early-type spiral LIRGs, and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.

Using the IRAM 30 m telescope and the Plateau de Bure interferometer we have detected the C I( 3 P 2 → 3 P 1 ) and the CO 3−2, 4−3, 6−5, 7−6 transitions as well as the dust continuum at 3 and 1.2 mm towards the distant luminous infrared galaxy IRAS F10214+4724 at z = 2.286. The C I( 3 P 2 → 3 P 1 ) line is detected for the first time towards this source and IRAS F10214+4724 now belongs to a sample of only 3 extragalactic sources at any redshift where both of the carbon fine structure lines have been detected. The source is spatially resolved by our C I( 3 P 2 → 3 P 1 ) observation and we detect a velocity gradient along the east-west direction. The CI line ratio allows us to derive a carbon excitation temperature of 42 +12 −9 K. The carbon excitation in conjunction with the CO ladder and the dust continuum constrain the gas density to n(H 2 ) = 10 3.6−4.0 cm −3 and the kinetic temperature to T kin = 45−80 K, similar to the excitation conditions found in nearby starburst galaxies. The rest-frame 360 μm dust continuum morphology is more compact than the line emitting region, which supports previous findings that the far infrared luminosity arises from regions closer to the active galactic nucleus at the center of this system.

Using the IRAM 30 m telescope, we have detected the 12 CO J = 2−1, 4−3, 5−4, and 6−5 emission lines in the millimeter-bright, blank-field selected AGN COSMOS J100038+020822 at redshift z = 1.8275. The sub-local thermodynamic equilibrium (LTE) excitation of the J = 4 level implies that the gas is less excited than that in typical nearby starburst galaxies such as NGC 253, and in the high-redshift quasars studied to date, such as J1148+5251 or BR1202-0725. Large velocity gradient (LVG) modeling of the CO line spectral energy distribution (CO SED; flux density vs. rotational quantum number) yields H 2 densities in the range 10 3.5 −10 4.0 cm −3 , and kinetic temperatures between 50 K and 200 K. The H 2 mass of (3.6−5.4) × 10 10 M implied by the line intensities compares well with our estimate of the dynamical mass within the inner 1.5 kpc of the object. Fitting a two-component gray body spectrum, we find a dust mass of 1.2 × 10 9 M , and cold and hot dust temperatures of 42 ± 5 K and 160 ± 25 K, respectively. The broad MgII line allows us to estimate the mass of the central black hole as 1.7 × 10 9 M . Although the optical spectrum and multi-wavelength SED matches those of an average QSO, the molecular gas content and dust properties resemble those of known submillimeter galaxies (SMGs). The optical morphology of this source shows tidal tails that suggest a recent interaction or merger. Since it shares properties of both starburst and AGN, this object appears to be in a transition from a strongly starforming submillimeter galaxy to a QSO.