CON-quest

Abstract

Context. Some luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) host extremely compact (r < 100 pc) and dusty nuclei. The high extinction associated with large column densities of gas and dust toward these objects render them hard to detect at many wavelengths. The intense infrared radiation arising from warm dust in these sources can provide a significant fraction of the bolometric luminosity of the galaxy and is prone to excite vibrational levels of molecules such as HCN. This results in emission from the rotational transitions of vibrationally excited HCN (HCN-vib); the brightest emission is found in compact obscured nuclei (CONs; ΣHCN − vib > 1 L⊙ pc−2 in the J = 3−2 transition). However, there have been no systematic searches for CONs, and it is unknown how common they are. Aims. We aim to establish how common CONs are in the local Universe (z < 0.08), and whether their prevalence depends on the luminosity or other properties of the host galaxy. Methods. We conducted an Atacama Large Millimeter/submillimeter Array survey of the rotational J = 3−2 transition of HCN-vib in a volume-limited sample of 46 far-infrared luminous galaxies. Results. Compact obscured nuclei are identified in 38−13+18% of the ULIRGs, 21−6+12% of the LIRGs, and 0−0+9% of the lower luminosity galaxies. We find no dependence on the inclination of the host galaxy, but strong evidence of lower IRAS 25 μm to 60 μm flux density ratios (f25/f60) in CONs (with the exception of one galaxy, NGC 4418) compared to the rest of the sample. Furthermore, we find that CONs have stronger silicate features (s9.7 μm), but similar polycyclic aromatic hydrocarbon equivalent widths (EQW6.2 μm) compared to other galaxies. Along with signatures of molecular inflows seen in the far-infrared in most CONs, submillimeter observations also reveal compact, often collimated, outflows. Conclusions. In the local Universe, CONs are primarily found in (U)LIRGs, in which they are remarkably common. As such systems are often highly disturbed, inclinations are difficult to estimate, and high-resolution continuum observations of the individual nuclei are required to determine if the CON phenomenon is related to the inclinations of the nuclear disks. Further studies of the in- and outflow properties of CONs should also be conducted to investigate how these are connected to each other and to the CON phenomenon. The lower f25/f60 ratios in CONs as well as the results for the mid-infrared diagnostics investigated (EQW6.2 μm and s9.7 μm) are consistent with the notion that large dust columns gradually shift the radiation from the hot nucleus to longer wavelengths, making the mid- and far-infrared “photospheres” significantly cooler than the interior regions. Finally, to assess the importance of CONs in the context of galaxy evolution, it is necessary to extend this study to higher redshifts where (U)LIRGs are more common.

A technique to select the most obscured galaxy nuclei

“…More recent works found that the HCN (3-2) line in its vibrationally excited ν 2 =1 state (HCN-vib) is an excellent tracer of CONs (e.g. Sakamoto et al 2010;Imanishi & Nakanishi 2013;Aalto et al 2015Aalto et al , 2019Falstad et al 2021). In particular, Falstad et al (2021) suggested that a high HCN-vib surface brightness (Σ HCN−vib >1 L pc −2 and/or L HCN−vib /L IR >10 −8 ) indicates that the core may be optically thick up to sub-mm wavelengths (see also e.g.…”

“…Sakamoto et al 2010;Imanishi & Nakanishi 2013;Aalto et al 2015Aalto et al , 2019Falstad et al 2021). In particular, Falstad et al (2021) suggested that a high HCN-vib surface brightness (Σ HCN−vib >1 L pc −2 and/or L HCN−vib /L IR >10 −8 ) indicates that the core may be optically thick up to sub-mm wavelengths (see also e.g. Sakamoto et al 2013Sakamoto et al , 2017Scoville et al 2017;Aalto et al 2019).…”

“…Using the HCN-vib molecu-lar gas transition, previous studies have revealed that 20-40% of the U/LIRGs in the local Universe harbour previously undetected CONs (e.g. Sakamoto et al 2010;Imanishi & Nakanishi 2013;Aalto et al 2015Aalto et al , 2019Falstad et al 2021). In particular, Falstad et al (2021) reported that CONs are more frequently found in ULIRGs (∼38%) than in LIRGs (∼21%) by using a volume-limited sample of 46 far-IR luminous galaxies (L FIR(40−400 µm) >10 10 L ).…”

“…Sakamoto et al 2010;Imanishi & Nakanishi 2013;Aalto et al 2015Aalto et al , 2019Falstad et al 2021). In particular, Falstad et al (2021) reported that CONs are more frequently found in ULIRGs (∼38%) than in LIRGs (∼21%) by using a volume-limited sample of 46 far-IR luminous galaxies (L FIR(40−400 µm) >10 10 L ). However, more studies using larger samples are needed to confirm or otherwise whether this is a commonly occurring feature in U/LIRGs, its dependence on luminosity and/or other properties of the host galaxy.…”

“…PAH emission has been also detected in the IR spectra of CONs (e.g. Falstad et al 2021) by using the relatively large aperture afforded by Spitzer (∼4 ). However, for the archetypal CON NGC 4418, the 11.3 µm feature was not detected in its subarcsecond angular resolution ground-based mid-IR spectrum (Roche, Alonso-Herrero, & Gonzalez-Martin 2015).…”