[en] We analyze the far-infrared dust emission from the Galactic center region, including the circumnuclear disk (CND) and other structures, using Herschel PACS and SPIRE photometric observations. These Herschel data are complemented by unpublished observations by the Infrared Space Observatory Long Wavelength Spectrometer (ISO-LWS), which used parallel mode scans to obtain photometric images of the region with a larger beam than Herschel but with a complementary wavelength coverage and more frequent sampling with 10 detectors observing at 10 different wavelengths in the range from 46 μm to 180 μm, where the emission peaks. We also include data from the Midcourse Space Experiment at 21.3 μm for completeness. We model the combined ISO-LWS continuum plus Herschel PACS and SPIRE photometric data toward the central 2 pc in Sagittarius A* (Sgr A*), a region that includes the CND. We find that the far-infrared spectral energy distribution is best represented by a continuum that is the sum of three gray body curves from dust at temperatures of 90, 44.5, and 23 K. We obtain temperature and molecular hydrogen column density maps of the region. We estimate the mass of the inner part of the CND to be ∼5.0 x 10⁴ M_sun, with luminosities: L_cavity ∼ 2.2 x 10⁶ L_sun and L_CND ∼ 1.5 x 10⁶ L_sun in the central 2 pc radius around Sgr A*. We find from the Herschel and ISO data that the cold component of the dust dominates the total dust mass, with a contribution of ∼3.2 x 10⁴ M_sun; this important cold material had escaped the notice of earlier studies that relied on shorter wavelength observations. The hotter component disagrees with some earlier estimates, but is consistent with measured gas temperatures and with models that imply shock heating or turbulent effects are at work. We find that the dust grain sizes apparently change widely across the region, perhaps in response to the temperature variations, and we map that distribution.

[en] We present the results from a systematic search for galactic-scale, molecular (OH 119 μm) outflows in a sample of 52 Local Volume ($d<<!--\; <\; -->50$ Mpc) Burst Alert Telescope detected active galactic nuclei (BAT AGNs) with Herschel-PACS. We combine the results from our analysis of the BAT AGNs with the published Herschel/PACS data of 43 nearby ($z<<!--\; <\; -->0.3$) galaxy mergers, mostly ultra-luminous infrared galaxies (ULIRGs) and QSOs. The objects in our sample of BAT AGNs have, on average, $\sim <!--\; ???\; -->10\text{--}100$ times lower AGN luminosities, star formation rates, and stellar masses than those of the ULIRG and QSO samples. OH 119 μm is detected in 42 of our BAT AGN targets. Evidence for molecular outflows (i.e., OH absorption profiles with median velocities more blueshifted than −50 km s⁻¹ and/or blueshifted wings with 84% velocities less than −300 km s⁻¹) is seen in only four BAT AGNs (NGC 7479 is the most convincing case). Evidence for molecular inflows (i.e., OH absorption profiles with median velocities more redshifted than 50 km s⁻¹) is seen in seven objects, although an inverted P-Cygni profile is detected unambiguously in only one object (Circinus). Our data show that both the starburst and AGN contribute to driving OH outflows, but the fastest OH winds require AGNs with quasar-like luminosities. We also confirm that the total absorption strength of OH 119 μm is a good proxy for dust optical depth as it correlates strongly with the 9.7 μm silicate absorption feature, a measure of obscuration originating in both the nuclear torus and host galaxy disk.

[en] In Tombesi et al., we reported the first direct evidence for a quasar accretion disk wind driving a massive (>100 M _⊙ yr⁻¹) molecular outflow. The target was F11119+3257, an ultraluminous infrared galaxy (ULIRG) with unambiguous type 1 quasar optical broad emission lines. The energetics of the accretion disk wind and molecular outflow were found to be consistent with the predictions of quasar feedback models where the molecular outflow is driven by a hot energy-conserving bubble inflated by the inner quasar accretion disk wind. However, this conclusion was uncertain because the mass outflow rate, momentum flux, and mechanical power of the outflowing molecular gas were estimated from the optically thick OH 119 μm transition profile observed with Herschel. Here, we independently confirm the presence of the molecular outflow in F11119+3257, based on the detection of ∼±1000 km s⁻¹ blue- and redshifted wings in the CO(1−0) emission line profile derived from deep ALMA observations obtained in the compact array configuration (∼2.″8 resolution). The broad CO(1−0) line emission appears to be spatially extended on a scale of at least ∼7 kpc from the center. Mass outflow rate, momentum flux, and mechanical power of (80–200) $R_7^{-<!--\; ???\; -->1}$ M _⊙ yr⁻¹, (1.5–3.0) $R_7^{-<!--\; ???\; -->1}$ L _AGN/c, and (0.15–0.40)% $R_7^{-<!--\; ???\; -->1}$ $L_{\mathrm{A}\mathrm{G}\mathrm{N}}$, respectively, are inferred from these data, assuming a CO-to-H₂ conversion factor appropriate for a ULIRG (R ₇ is the radius of the outflow normalized to 7 kpc, and L _AGN is the AGN luminosity). These rates are time-averaged over a flow timescale of 7 × 10⁶ yr. They are similar to the OH-based rates time-averaged over a flow timescale of 4 × 10⁵ yr, but about a factor of 4 smaller than the local (“instantaneous”; ≲10⁵ yr) OH-based estimates cited in Tombesi et al. The implications of these new results are discussed in the context of time-variable quasar-mode feedback and galaxy evolution. The need for an energy-conserving bubble to explain the molecular outflow is also reexamined.

[en] We report initial results from the far-infrared fine structure line observations of a sample of 44 local starbursts, Seyfert galaxies, and infrared luminous galaxies obtained with the PACS spectrometer on board Herschel. We show that the ratio between the far-infrared luminosity and the molecular gas mass, L_FIR/M_H2, is a much better proxy for the relative brightness of the far-infrared lines than L_FIR alone. Galaxies with high L_FIR/M_H2 ratios tend to have weaker fine structure lines relative to their far-infrared continuum than galaxies with L_FIR/M_H2 , or approx. 80 L_.M_.^-1. A deficit of the [C II] 158 μm line relative to L_FIR was previously found with the Infrared Space Observatory, but now we show for the first time that this is a general aspect of all far-infrared fine structure lines, regardless of their origin in the ionized or neutral phase of the interstellar medium. The L_FIR/M_H2 value where these line deficits start to manifest is similar to the limit that separates between the two modes of star formation recently found in galaxies on the basis of studies of their gas-star formation relations. Our finding that the properties of the interstellar medium are also significantly different in these regimes provides independent support for the different star-forming relations in normal disk galaxies and major merger systems. We use the spectral synthesis code Cloudy to model the emission of the lines. The expected increase of the ionization parameter with L_FIR/M_H2 can simultaneously explain the line deficits in the [C II], [N II], and [O I] lines.

[en] We report a tentative correlation between the outflow characteristics derived from OH absorption at 119 μ m and [C ii] emission at 158 μ m in a sample of 22 local and bright ultraluminous infrared galaxies (ULIRGs). For this sample, we investigate whether [C ii] broad wings are a good tracer of molecular outflows, and how the two tracers are connected. Fourteen objects in our sample have a broad wing component as traced by [C ii], and all of these also show OH119 absorption indicative of an outflow (in one case an inflow). The other eight cases, where no broad [C ii] component was found, are predominantly objects with no OH outflow or a low-velocity (≤100 km s⁻¹) OH outflow. The FWHM of the broad [C ii] component shows a trend with the OH119 blueshifted velocity, although with significant scatter. Moreover, and despite large uncertainties, the outflow masses derived from OH and broad [C ii] show a 1:1 relation. The main conclusion is therefore that broad [C ii] wings can be used to trace molecular outflows. This may be particularly relevant at high redshift, where the usual tracers of molecular gas (like low-J CO lines) become hard to observe. Additionally, observations of blueshifted Na i D λλ 5890, 5896 absorption are available for 10 of our sources. Outflow velocities of Na i D show a trend with OH velocity and broad [C ii] FWHM. These observations suggest that the atomic and molecular gas phases of the outflow are connected.

[en] We report the detection of far-IR CO rotational emission from the prototypical Seyfert 2 galaxy NGC 1068. Using Herschel-PACS, we have detected 11 transitions in the J_upper = 14-30 (E_upper/k_B = 580-2565 K) range, all of which are consistent with arising from within the central 10'' (700 pc). The detected transitions are modeled as arising from two different components: a moderate-excitation (ME) component close to the galaxy systemic velocity and a high-excitation (HE) component that is blueshifted by ∼80 km s^–1. We employ a large velocity gradient model and derive n_H2 ∼ 10^5.6 cm^–3, T_kin ∼ 170 K, and M_H2 ∼ 10^6.7 M_☉ for the ME component and n_H2 ∼ 10^6.4 cm^–3, T_kin ∼ 570 K, and M_H2 ∼ 10^5.6 M_☉ for the HE component, although for both components the uncertainties in the density and mass are ±(0.6-0.9) dex. Both components arise from denser and possibly warmer gas than traced by low-J CO transitions, and the ME component likely makes a significant contribution to the mass budget in the nuclear region. We compare the CO line profiles with those of other molecular tracers observed at higher spatial and spectral resolution and find that the ME transitions are consistent with these lines arising in the ∼200 pc diameter ring of material traced by H₂ 1-0 S(1) observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H₂ ring, but a better kinematic match is found with a clump of infalling gas ∼40 pc north of the active galactic nucleus (AGN). We consider potential heating mechanisms and conclude that X-ray- or shock heating of both components is viable, while far-UV heating is unlikely. We discuss the prospects of placing the HE component near the AGN and conclude that while the moderate thermal pressure precludes an association with the ∼1 pc radius H₂O maser disk, the HE component could potentially be located only a few parsecs more distant from the AGN and might then provide the N_H ∼ 10²⁵ cm^–2 column obscuring the nuclear hard X-rays. Finally, we also report sensitive upper limits extending up to J_upper = 50, which place constraints on a previous model prediction for the CO emission from the X-ray obscuring torus.

[en] Emission from high-J CO lines in galaxies has long been proposed as a tracer of X-ray dominated regions (XDRs) produced by active galactic nuclei (AGNs). Of particular interest is the question of whether the obscuring torus, which is required by AGN unification models, can be observed via high-J CO cooling lines. Here we report on the analysis of a deep Herschel/PACS observation of an extremely high-J CO transition (40-39) in the Seyfert 2 galaxy NGC 1068. The line was not detected, with a derived 3σ upper limit of $2\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->17}\mathrm{W}{\mathrm{m}}^{-<!--\; ???\; -->2}$. We apply an XDR model in order to investigate whether the upper limit constrains the properties of a molecular torus in NGC 1068. The XDR model predicts the CO spectral line energy distributions for various gas densities and illuminating X-ray fluxes. In our model, the CO(40-39) upper limit is matched by gas with densities of $\sim <!--\; ???\; -->{10}^6-<!--\; ???\; -->{10}^7{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$, located at 1.6–5 pc from the AGN, with column densities of at least ${10}^{25}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->2}$. At such high column densities, however, dust absorbs most of the CO(40-39) line emission at $\mathrm{\lambda <!--\; ??\; -->}=65.69\mathrm{\mu <!--\; ??\; -->}\mathrm{m}$. Therefore, even if NGC 1068 has a molecular torus that radiates in the CO(40-39) line, the dust can attenuate the line emission to below the PACS detection limit. The upper limit is thus consistent with the existence of a molecular torus in NGC 1068. In general, we expect that the CO(40-39) is observable in only a few AGN nuclei (if at all), because of the required high gas column density, and absorption by dust.

[en] Mass outflows driven by stars and active galactic nuclei (AGNs) are a key element in many current models of galaxy evolution. They may produce the observed black-hole-galaxy mass relation and regulate and quench both star formation in the host galaxy and black hole accretion. However, observational evidence of such feedback processes through outflows of the bulk of the star-forming molecular gas is still scarce. Here we report the detection of massive molecular outflows, traced by the hydroxyl molecule (OH), in far-infrared spectra of ULIRGs obtained with Herschel-PACS as part of the SHINING key project. In some of these objects the (terminal) outflow velocities exceed 1000 km s^-1, and their outflow rates (up to ∼1200 M_sun yr^-1) are several times larger than their star formation rates. We compare the outflow signatures in different types of ULIRGs and in starburst galaxies to address the issue of the energy source (AGN or starburst) of these outflows. We report preliminary evidence that ULIRGs with a higher AGN luminosity (and higher AGN contribution to L_IR) have higher terminal velocities and shorter gas depletion timescales. The outflows in the observed ULIRGs are able to expel the cold gas reservoirs from the centers of these objects within ∼10⁶-10⁸ years.

[en] We report on the energetics of molecular outflows in 14 local ultraluminous infrared galaxies (ULIRGs) that show unambiguous outflow signatures (P Cygni profiles or high-velocity absorption wings) in the far-infrared lines of OH measured with the Herschel/PACS spectrometer. All sample galaxies are gas-rich mergers at various stages of the merging process. Detection of both ground-state (at 119 and 79 μm) and one or more radiatively excited (at 65 and 84 μm) lines allows us to model the nuclear gas (≲300 pc) and the more extended components using spherically symmetric radiative transfer models. Reliable models and the corresponding energetics are found in 12 of the 14 sources. The highest molecular outflow velocities are found in buried sources, in which slower but massive expansion of the nuclear gas is also observed. With the exception of a few outliers, the outflows have momentum fluxes of (2–5) × L _IR/c and mechanical luminosities of (0.1–0.3)% of L _IR. The moderate momentum boosts in these sources (≲3) suggest that the outflows are mostly momentum driven by the combined effects of active galactic nuclei (AGNs) and nuclear starbursts, as a result of radiation pressure, winds, and supernova remnants. In some sources (∼20%), however, powerful (10^10.5–11 L _⊙) AGN feedback and (partially) energy-conserving phases are required, with momentum boosts in the range of 3–20. These outflows appear to be stochastic, strong AGN feedback events that occur throughout the merging process. In a few sources, the outflow activity in the innermost regions has subsided in the past ∼1 Myr. While OH traces the molecular outflows at subkiloparsec scales, comparison of the masses traced by OH with those previously inferred from tracers of more extended outflowing gas suggests that most mass is loaded (with loading factors of $\stackrel{\dot{}<!--\; ??\; -->}{M}/\mathrm{S}\mathrm{F}\mathrm{R}=1\text{--}10$) from the central galactic cores (a few × 100 pc), qualitatively consistent with an ongoing inside-out quenching of star formation. Outflow depletion timescales are <10⁸ yr, shorter than the gas consumption timescales by factors of 1.1–15, and are anticorrelated with the AGN luminosity.

[en] We report the detection of far-infrared (FIR) CO rotational emission from nearby active galactic nuclei (AGNs) and starburst galaxies, as well as several merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using the Herschel Photodetector Array Camera and Spectrometer (PACS), we have detected transitions in the J_upp = 14–30 range. The PACS CO data obtained here provide the first reference of well-sampled FIR extragalactic CO spectral line energy distributions (SLEDs) for this range. We find a large range in the overall SLED shape, even among galaxies of similar type, demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source of a galaxy. Combining our data with low-J line intensities taken from the literature, we present a CO ratio–ratio diagram and discuss its value in distinguishing excitation sources and physical properties of the molecular gas. The position of a galaxy on such a diagram is less a signature of its excitation mechanism, than an indicator of the presence of warm, dense molecular gas. We then quantitatively analyze the CO emission from a subset of the detected sources with single-component and two-component large velocity gradient (LVG) radiative transfer models to fit the CO SLEDs. From these fits we derive the molecular gas mass and the corresponding CO-to-H₂ conversion factor, ${\mathrm{\alpha <!--\; ??\; -->}}_{\mathrm{C}\mathrm{O}}$, for each respective source. For the ULIRGs we find α values in the canonical range 0.4– 5M_⊙ (K km s⁻¹ pc²)⁻¹, while for the other objects, α varies between 0.2 and 14. Finally, we compare our best-fit LVG model results with previous studies of the same galaxies and comment on any differences.