Infrared 2–4 Micrometer Spectroscopy and Millimeter Interferometric HCN and HCO+ Observations of the Individual Merging Components of Arp 299

Abstract

Abstract We present ground-based infrared $K$–(2–2.5$\, {\mu \mathrm {m}}$) and $L$-band (2.8–4.1$\, {\mu \mathrm {m}}$) spectroscopy, as well as interferometric observations at $\lambda \sim 3 \,\mathrm{mm}$, for the individual merging components (A, B, and C) of the luminous infrared galaxy Arp 299. The presence and location of a buried active galactic nucleus (AGN) are investigated. Our sub-arcsec-resolution infrared spectra clearly reveal that the putative buried AGN resides in the nucleus B1 (a subcomponent of B), based on a very low equivalent width of 3.3$\, {\mu \mathrm {m}}$ polycyclic aromatic hydrocarbon emission, a weak 2.3$\, {\mu \mathrm {m}}$ CO absorption feature, and a large time variation of the $K$–and $L$-band continuum fluxes. Our interferometric observations simultaneously obtain HCN ($J = 1 \hbox{--} 0$) and $\mathrm{HCO}^{+}$ ($J = 1 \hbox{--} 0$) emission lines with $\sim 4 {{}^{\prime \prime }}$ resolution, and we find the HCN to $\mathrm{HCO}^{+}$ brightness-temperature ratios to be as low as those found in starburst nuclei in all of the major merging components of Arp 299. The low ratio even in the AGN-hosting nucleus B may be due to the presence of a large amount of high-density molecular gas whose chemistry is dominated by coexisting starbursts and/or shocks, rather than by the central strong X-ray-emitting AGN.