Major impact from a minor merger

Abstract

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T > 300 K) and high gas densities (n(H2) > 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

Planck’s Dusty GEMS

“…High-density tracer molecules are expected to reside in the inner core of giant, star-forming molecular clouds and could potentially probe an ISM component that is distinct from that probed in the low-to-mid-J CO, or [C i] lines. Different spatial distributions have been observed in low-redshift (U)LIRGs (see, e.g., Scoville et al 2015Scoville et al , 2017König et al 2018), where dense regions are located within the starburst cores and embedded in diffuse, colder gas reservoirs. High-redshift lensed SMGs could also have such configurations and, in this case, spatially integrated emission line fluxes and profiles could be strongly affected by differential magnification (Hezaveh et al 2012;Serjeant 2012), introducing high uncertainties on the diagnostics from line ratios.…”

“…Observational studies of the dense-gas reservoirs in lowredshift star-forming galaxies started about three decades ago (e.g., Nguyen et al 1992), followed by extended surveys of HCN line emission in luminous and ultra-luminous infrared galaxies (ULIRGs) and normal spirals (Gao & Solomon 2004a,b). Since then, HCN and HCO + detections have been extensively used to characterize the dense interstellar medium (ISM) of local (U)LIRGs (e.g., Graciá-Carpio et al 2008;Juneau et al 2009;García-Burillo et al 2012), including diagnostics for the physical conditions of the gas from line excitation modeling (see, e.g., Papadopoulos et al 2014, for NGC 6240 and Arp 193) and spatially-resolved studies with the NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA) interferometers (Chen et al 2017;König et al 2018;Imanishi et al 2019). The reliability of each molecule as a proxy of the overall dense-gas content was discussed in Papadopoulos (2007).…”

“…Recently, valuable constraints on the dense-gas components of galactic outflows have also allowed us to peer into the embedded star-formation activity and its role as a positive or negative feedback mechanism (e.g., Walter et al 2017;Michiyama et al 2018;Aladro et al 2018). Combining sub-arcsec resolution HCN and HCO + maps with traditional CO lines provides particularly detailed pictures of complex molecular gas flows and triggering mechanisms, as demonstrated in the local barred Seyfert galaxy NGC 1068 (García-Burillo et al 2014) and the Medusa (König et al 2018).…”

A Hard X-Ray Test of HCN Enhancements As a Tracer of Embedded Black Hole Growth

“…More representative studies have found starburst galaxies that have HCN/HCO + enhancements similar to the enhancements seen in AGN (e.g., Costagliola et al 2011;Privon et al 2015;Martín et al 2015;König et al 2018;Harada et al 2018), where the relative starburst-AGN strength has been primarily assessed using the mid-infrared (MIR) 6.2 µm polycyclic aromatic hydrocarbon (PAH) equivalent width (EQW). This suggests two possibilities: 1) the HCN/HCO + enhancements trace an alternate physical process than AGN activity, or 2) these MIR classified starbursts host AGN that are behind obscuring screens sufficiently thick to render MIR diagnostics ineffective.…”

Analysis of the Spatially Resolved V−3.6 μm Colors and Dust Extinction in 257 Nearby NGC and IC Galaxies

“…NGC 3801 could be at an earlier stage of an NGC 2685like merger (Hota et al 2012). The merging event ∼2-3 Gyr ago is presumably responsible for nuclear-ring in NGC 7742 (Martinsson et al 2018), while the minor merger ∼200 Myr ago (Knierman et al 2012(Knierman et al , 2013 is responsible for the central dust in NGC 2782 and in NGC 4194 (König et al 2018). Balcells (1997) argues that the dusty disk, two tidal tails, and shells in E-type NGC 3656 is the outcome of a disk-disk major merger.…”