Nobeyama Millimeter Interferometric HCN(1-0) and HCO+(1-0) Observations of Further Luminous Infrared Galaxies

We report the results of interferometric HCN(1-0) and HCO+(1-0) observations of four luminous infrared galaxies (LIRGs), NGC 2623, Mrk 266, Arp 193, and NGC 1377, as a final sample of our systematic survey using the Nobeyama Millimeter Array. Our survey contains the most systematic interferometric, spatially-resolved, simultaneous HCN(1-0) and HCO+(1-0) observations of LIRGs. Ground-based infrared spectra of these LIRGs are also presented to elucidate the nature of the energy sources at the nuclei. We derive the HCN(1-0)/HCO+(1-0) brightness-temperature ratios of these LIRGs and confirm the previously discovered trend that LIRG nuclei with luminous buried AGN signatures in infrared spectra tend to show high HCN(1-0)/HCO+(1-0) brightness-temperature ratios, as seen in AGNs, while starburst-classified LIRG nuclei in infrared spectra display small ratios, as observed in starburst-dominated galaxies. Our new results further support the argument that the HCN(1-0)/HCO+(1-0) brightness-temperature ratio can be used to observationally separate AGN-important and starburst-dominant galaxy nuclei.Comment: 25 pages (emulateapj.cls), 12 figures, accepted for publication in Astronomical Journal (March 2009 issue). Higher resolution version is available at http://optik2.mtk.nao.ac.jp/~imanishi/Paper/HCN3/HCN3emu.pd

Luminous buried AGNs as a function of galaxy infrared luminosity revealed through Spitzer low-resolution infrared spectroscopy

We present the results of Spitzer IRS infrared 5-35 micron low-resolution spectroscopic energy diagnostics of ultraluminous infrared galaxies (ULIRGs) at z > 0.15, classified optically as non-Seyferts. Based on the equivalent widths of polycyclic aromatic hydrocarbon emission and the optical depths of silicate dust absorption features, we searched for signatures of intrinsically luminous, but optically elusive, buried AGNs in these optically non-Seyfert ULIRGs. We then combined the results with those of non-Seyfert ULIRGs at z < 0.15 and non-Seyfert galaxies with infrared luminosities L(IR) < 10^12Lsun. We found that the energetic importance of buried AGNs clearly increases with galaxy infrared luminosity, becoming suddenly discernible in ULIRGs with L(IR) > 10{12}Lsun. For ULIRGs with buried AGN signatures, a significant fraction of infrared luminosities can be accounted for by detected buried AGN and modestly-obscured (Av < 20 mag) starburst activity. The implied masses of spheroidal stellar components in galaxies for which buried AGNs become important roughly correspond to the value separating red massive and blue, less-massive galaxies in the local universe. Our results may support the widely-proposed AGN-feedback scenario as the origin of galaxy downsizing phenomena, where galaxies with currently larger stellar masses previously had higher AGN energetic contributions and star-formation-originating infrared luminosities, and have finished their major star-formation more quickly, due to stronger AGN feedback.Comment: 21 pages (emulateapj.cls), 5 figures, accepted for publication in ApJ (April 2009 -1 v694 issue

Subaru and Gemini High Spatial Resolution Infrared 18 Micron Imaging Observations of Nearby Luminous Infrared Galaxies

We present the results of a ground-based, high spatial resolution infrared 18 micron imaging study of nearby luminous infrared galaxies (LIRGs), using the Subaru 8.2-m and Gemini South 8.1-m telescopes. The diffraction-limited images routinely achieved with these telescopes in the Q-band (17-23 micron) allow us to investigate the detailed spatial distribution of infrared emission in these LIRGs. We then investigate whether the emission surface brightnesses are modest, as observed in starbursts, or are so high that luminous active galactic nuclei (AGNs; high emission surface brightness energy sources) are indicated. The sample consists of 18 luminous buried AGN candidates and starburst-classified LIRGs identified in earlier infrared spectroscopy. We find that the infrared 18 micron emission from the buried AGN candidates is generally compact, and the estimated emission surface brightnesses are high, sometimes exceeding the maximum value observed in and theoretically predicted for a starburst phenomenon. The starburst-classified LIRGs usually display spatially extended 18 micron emission and the estimated emission surface brightnesses are modest, within the range sustained by a starburst phenomenon. The general agreement between infrared spectroscopic and imaging energy diagnostic methods suggests that both are useful tools for understanding the hidden energy sources of the dusty LIRG population.Comment: 17 pages, 3 figures, accepted for publication in AJ (No. 141, 2011 May issue). Higher resolution version is available at http://optik2.mtk.nao.ac.jp/~imanishi/Paper/20um/20um.pd

Infrared 2-4 Micron Spectroscopy and Millimeter Interferometric HCN and HCO+ Observations of the Individual Merging Components of Arp299

We present ground-based infrared K- (2-2.5 micron) and L-band (2.8-4.1 micron) spectroscopy, as well as interferometric observations at 3mm, for the individual merging components (A, B, and C) of the luminous infrared galaxy Arp 299. We investigate the presence and location of the putative buried active galactic nucleus (AGN) inferred from previous X-ray observations at E > 10 keV. 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 micron polycyclic aromatic hydrocarbon emission, a weak 2.3 micron CO absorption feature, and a large time variation of the K- and L-band continuum fluxes. In component C, we find strong 3.1 micron ice absorption at L and weak 2.3 micron CO absorption at K, as expected in a buried AGN; however, a centrally concentrated young super star cluster is an alternative possibility because of the modest infrared luminosity and non-galaxy-nucleus nature of this component. The infrared K- and L-band spectra of the infrared brightest nucleus, A, are typical of a normal starburst with no explicit AGN signatures. Our interferometric observations simultaneously obtain HCN (J=1-0) and HCO+ (J=1-0) emission lines with 4 arcsec resolution, and we find the HCN to HCO+ brightness-temperature ratios to be as low as those found in starburst nuclei in all 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.Comment: 20 pages, 8 figures, Accepted for publication in PASJ (Vol. 58, No. 5, 2006 Oct 25 issue