HERSCHEL-PACS OBSERVATIONS OF FAR-IR CO LINE EMISSION IN NGC 1068: HIGHLY EXCITED MOLECULAR GAS IN THE CIRCUMNUCLEAR DISK

Abstract

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 2 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 (LVG) model and derive n H2 ∼ 10 5.6 cm −3 , T kin ∼ 170 K, and M H 2 ∼ 10 6.7 M ⊙ for the ME component, and n H2 ∼ 10 6.4 cm −3 , T kin ∼ 570 K, and M H 2 ∼ 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 H 2 1-0 S(1) observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H 2 ring, but a better kinematic match is found with a clump of infalling gas ∼40 pc north of the 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 H 2 O 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 ∼ 10 25 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.

Physical conditions of molecular gas in the Circinus galaxy Multi-JCO and Ci3PP0observations

“…We tried to fit CO ladders in the central 45 with a single LVG component first, however, it does not produce a good fit. This is not surprising since the modeling results in previous studies (e.g., Kamenetzky et al 2012;Hailey-Dunsheath et al 2012;Rigopoulou et al 2013) have shown that the coexistence of multiple excitation gas components in nearby galaxies (also see Lu et al 2014). Therefore, we use two LVG components to model the gas excitation in the central 45 region.…”

“…Although Circinus has a much smaller distance (∼4 Mpc) than NGC 1068 (∼14.4 Mpc Bland-Hawthorn et al 1997), the angular sizes of the gas-rich region in these two galaxies are both about 40 (in diameter). Both of them have strong S-F activities in their centers, and are fed by large amounts of molecular material (e.g., Curran et al 2001;Hailey-Dunsheath et al 2012).…”

“…Between the S-F ring and the CND there is a gap region deficient in molecular gas (e.g., Helfer et al 2003;Schinnerer et al 2000;Tacconi et al 1997;Tsai et al 2012). Spinoglio et al (2012) modeled the excitation conditions with the CO SLED deduced from the Herschel observations and found an LE component (T kin = 120 K, n H 2 = 10 2.8 cm −3 ) associated with an extended source (the S-F ring), a medium excitation (ME) component (T kin = 100 K, n H 2 = 10 4.6 cm −3 ) associated with the CND, and a HE component (T kin = 150 K, n H 2 = 10 5.7 cm −3 ) possibly arises from the central few pc heated by the AGN (e.g., Hailey-Dunsheath et al 2012).…”

Molecular line emission in NGC 1068 imaged with ALMA

“…Usero et al 2004;Israel 2009;Kamenetzky et al 2011;Hailey-Dunsheath et al 2012;Aladro et al 2013). Interferometric observations of CO, as well as HCN, HCO + , CS, and SiO (e.g.…”

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

“…In combination with ground-based telescope data, Photodector Array Camera and Spectrometer (PACS; Poglitsch et al 2010), Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al 2010), and Heterodyne Instrument for the Far Infrared (HIFI; de Graauw et al 2010) spectroscopic observations have enabled us to construct CO spectral line energy distributions (SLEDs) from the upper level J u = 1 to 50. In the past several years, Herschel-based CO SLEDs have been extensively examined for a wide range of Galactic (e.g., photodissociation regions (PDRs): Habart et al 2010;Köhler et al 2014;Pon et al 2014;Stock et al 2015;protostars: Larson et al 2015) and extragalactic sources (e.g., infrared (IR)-bright galaxies: Rangwala et al 2011;Kamenetzky et al 2012;Meijerink et al 2013;Pellegrini et al 2013;Greve et al 2014;Lu et al 2014;Papadopoulos et al 2014;Rosenberg et al 2014;Schirm et al 2014;Mashian et al 2015;Wu et al 2015b; Seyfert galaxies: van der Werf et al 2010;Hailey-Dunsheath et al 2012;Israel et al 2014), revealing the ubiquitous presence of warm molecular gas (T k 100 K). Various heating sources, e.g., ultraviolet (UV) photons, X-rays, and cosmic-rays, have been invoked to explain the properties of this warm molecular gas and the emerging picture is that non-ionizing sources such as mechanical heating (e.g., shocks driven by merging activities, stellar winds, and supernova explosions) must play a critical role.…”