Masatoshi Imanishi - Academia.edu (original) (raw)

Papers by Masatoshi Imanishi

We report the results of interferometric HCN(1-0) and HCO+(1-0) observations of four luminous inf... more 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-tem...

We present the results of a ground-based, high spatial resolution infrared 18 µm imaging study of... more We present the results of a ground-based, high spatial resolution infrared 18 µm 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 µm) 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 µm 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 µm 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.

The results of 3-4 mu\mumum spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are... more The results of 3-4 mu\mumum spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8-13 mu\mumum spectra, all the sources have strong absorption-like features at sim\simsim10 mu\mumum, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 mu\mumum carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 mu\mumum PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 mu\mumum carbonaceous dust and 9.7 mu\mumum silicate dust absorption ($\tau_{3.4}$/$\tau_{9.7}$). The tau3.4\tau_{3.4}tau3.4/$\tau_{9.7}$ ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative ``dusty tori'' in the close vicinity of AGNs.

FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FM... more FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope, targeting H$\alpha$ emitters at zsim1.18z \sim 1.18zsim1.18--$1.54$ down to the sensitivity limit of H$\alpha$ flux sim2times10−16rmergcm−2s−1\sim 2 \times 10^{-16} \ \rm erg \ cm^{-2} s^{-1}sim2times10−16rmergcm−2s−1. The primary goal of the survey is to detect redshift space distortions (RSD), to test General Relativity by measuring the growth rate of large scale structure and to constrain modified gravity models for the origin of the accelerated expansion of the universe. The target galaxies were selected based on photometric redshifts and H$\alpha$ flux estimates calculated by fitting spectral energy distribution (SED) models to the five optical magnitudes of the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Wide catalog. The survey started in March 2012, and all the observations were completed in July 2014. In total, we achieved 121121121 pointings of FMOS (each pointing has a 303030 arcmin diameter circula...

ABSTRACT A mid-infrared (MIR) imager and spectrometer is under consideration for construction in ... more ABSTRACT A mid-infrared (MIR) imager and spectrometer is under consideration for construction in the first decade of the Thirty Meter Telescope's (TMT) operation. When combined with a MIR adaptive optics system, the instrument will afford 15 times higher sensitivity, 4 times better spatial resolution (0.08") than 8m-class telescopes, and 4.5 times better spatial resolution than the JWST. Additionally, its huge light gathering power opens a new window of high-dispersion spectroscopy in the MIR. We discuss the key science drivers, from star and planet formation to galaxies and black holes and cosmology; science drivers which are in close synergy with the recent Astro 2010 Decadal Survey report. We flow down our science cases to produce fundamental and optional instrument capabilities, including imaging, long-slit and IFU spectroscopy, and polarimetry.

The Astrophysical Journal, 2014

We present high angular resolution (∼0.5 ′′) MIR spectra of the powerful radio galaxy, Cygnus A, ... more We present high angular resolution (∼0.5 ′′) MIR spectra of the powerful radio galaxy, Cygnus A, obtained with the Subaru telescope. The overall shape of the spectra agree with previous high angular resolution MIR observations, as well as previous Spitzer spectra. Our spectra, both on and off nucleus, show a deep silicate absorption feature. The absorption feature can be modeled with a blackbody obscured by cold dust or a clumpy torus. The deep silicate feature is best fit by a simple model of a screened blackbody, suggesting foreground absorption plays a significant, if not dominant role, in shaping the spectrum of Cygnus A. This foreground absorption prevents a clear view of the central engine and surrounding torus, making it difficult to quantify the extent the torus attributes to the obscuration of the central engine, but does not eliminate the need for a torus in Cygnus A.

Annual Report of the National Astronomical Observatory of Japan Volume 6, 2005

We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic ar... more We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic aromatic hydrocarbon (PAH) emission was used to estimate the magnitude of compact (< a few hundred pc) nuclear starbursts. For three of the selected Seyfert 2 nuclei, the magnitudes of compact nuclear starbursts estimated from the 3.3 micro m PAH emission feature (with no extinction correction) were in satisfactory quantitative agreement with those based on UV observations after extinction correction, indicating that the observed 3.3 micro m PAH emission luminosity can be used to estimate the magnitude of compact nuclear starbursts in the nuclei of Seyfert 2s. We found that (1) except in one case, the observed nuclear 3--4 micro m fluxes were dominated by AGNs and not by starbursts, (2) compact nuclear starbursts were detected in 6 out of 17 Seyfert 2 nuclei, but cannot dominate the infrared dust emission luminosities in the majority of the observed Seyfert 2 galaxies, and (3) more powerful AGNs tended to be related to more powerful compact nuclear starbursts. In addition to those showing 3.3 micro m PAH emission, 3.4 micro m carbonaceous dust absorption was detected in two Seyfert 2 nuclei, and 3.05 micro m H2O ice absorption was detected in one nucleus, and possibly detected in one more.

We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic ar... more We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic aromatic hydrocarbon (PAH) emission was used to estimate the magnitude of compact (< a few hundred pc) nuclear starbursts. For three of the selected Seyfert 2 nuclei, the magnitudes of compact nuclear starbursts estimated from the 3.3 micro m PAH emission feature (with no extinction correction) were in satisfactory quantitative agreement with those based on UV observations after extinction correction, indicating that the observed 3.3 micro m PAH emission luminosity can be used to estimate the magnitude of compact nuclear starbursts in the nuclei of Seyfert 2s. We found that (1) except in one case, the observed nuclear 3--4 micro m fluxes were dominated by AGNs and not by starbursts, (2) compact nuclear starbursts were detected in 6 out of 17 Seyfert 2 nuclei, but cannot dominate the infrared dust emission luminosities in the majority of the observed Seyfert 2 galaxies, and (3) more powerful AGNs tended to be related to more powerful compact nuclear starbursts. In addition to those showing 3.3 micro m PAH emission, 3.4 micro m carbonaceous dust absorption was detected in two Seyfert 2 nuclei, and 3.05 micro m H2O ice absorption was detected in one nucleus, and possibly detected in one more.

We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrar... more We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

Publications of the Astronomical Society of Japan, 2014

ABSTRACT

The Astrophysical Journal, 2007

We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared... more We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared from 5-35 µm. These observations allow us to investigate mid-IR emission mechanisms in the core of M87 and to establish that the flaring, variable jet component HST-1 is not a major contributor to the mid-IR flux. The Spitzer data include a high signal-to-noise 15-35 µm spectrum of the knot A/B complex in the jet, which is consistent with synchrotron emission. However, a synchrotron model cannot account for the observed nuclear spectrum, even when contributions from the jet, necessary due to the degrading of resolution with wavelength, are included. The Spitzer data show a clear excess in the spectrum of the nucleus at wavelengths longer than 25 µm, which we model as thermal emission from cool dust at a characteristic temperature of 55 ± 10 K, with an IR luminosity ∼ 10 39 erg s −1 . Given Spitzer's few-arcsecond angular resolution, the dust seen in the nuclear spectrum could be located anywhere within ∼ 5 ′′ (390 pc) of the nucleus. In any case, the ratio of AGN thermal to bolometric luminosity indicates that M87 does not contain the IR-bright torus that classical unified AGN schemes invoke. However, this result is consistent with theoretical predictions for low-luminosity AGNs.

Astron J, 2011

We present the results of a ground-based, high spatial resolution infrared 18 μm imaging study of... more We present the results of a ground-based, high spatial resolution infrared 18 μm 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 μm) 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 μm 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 μm 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.

We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrar... more We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

Spitzer Proposal, May 1, 2007

Ultraluminous infrared galaxies (ULIRGs) have the power output of quasars, yet emit nearly all th... more Ultraluminous infrared galaxies (ULIRGs) have the power output of quasars, yet emit nearly all their energy in the mid and far-infrared part of the spectrum. Over the last 10 years, much effort has focussed on identifying tracers of AGN and star formation activity in the infrared in order to quantify the origin of the infrared luminosity in these systems. Recently, we discovered the presence of warm (200-600K) dense (~10^6 cm-3) molecular gas through acetylene, hydrogen cyanide, and carbon dioxide gas-phase absorption lines in the IRS spectra of (U)LIRGs. In the Galaxy these lines are characteristic for the 'Hot Core' phase of deeply embedded protostars. Consequently, we attribute these lines to a phase of deeply embedded star formation. While these features are most readily recognized in the spectra of highly dust enshrouded (U)LIRGs, they are also seen in ULIRGs with unambiguous evidence for an AGN. Hence, such deeply embedded star formation seems to be a general characteristic of (U)LIRG nuclei. Here we propose to use the IRS on Spitzer to search for these molecular absorption lines in a wider sample of ULIRG nuclei to determine the frequency and importance of this hitherto unrecognized phase of hidden star formation and characterize the physical conditions of the absorbing dense gas. These spectra, combined with existing high signal-to-noise observations of (U)LIRGs will allow us to place our observations of hidden star formation in ULIRGs in the context of evolution of infrared galaxies as a whole.

Thermal infrared photometry in the LLL- and M′M'M′-band and L−M′L - M'L−M′ colors of type-1 and type-2 ac... more Thermal infrared photometry in the LLL- and M′M'M′-band and L−M′L - M'L−M′ colors of type-1 and type-2 active galactic nuclei (AGNs) are presented. After combining our observations with photometric data at similar wavelengths taken from the literature, we find that the excess of L−M′L - M'L−M′ colors of type-2 AGNs (37 sources, 50 data points) relative to type-1 AGNs (27 sources, 36 data points), due to dust extinction, is statistically detectable, but very small. We next investigate the L−M′L - M'L−M′ colors of type-2 AGNs by separating less dust-obscured type-2 AGNs and highly dust-obscured type-2 AGNs. In both cases, the L−M′L - M'L−M′ colors are similar to the intrinsic L−M′L - M'L−M′ color of unobscured AGNs, and the L−M′L - M'L−M′ color excess of the latter highly dust-obscured type-2 AGNs due to dust extinction is much smaller than that expected from the Galactic dust extinction curve. Contamination from starbursts and the time lag of flux variation are unlikely to explain this small L−M′L - M'L−M′ color excess, which is best explained if the dust extinction curve in the close vicinity of AGNs is fairly flat at 3--5 mu\mumum as a result of a size increase of the absorbing dust grains through coagulation.

Mon Notic Roy Astron Soc, 2002

The results of 3-4 µm spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are rep... more The results of 3-4 µm spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8-13 µm spectra, all the sources have strong absorption-like features at ∼10 µm, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 µm carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 µm PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 µm carbonaceous dust and 9.7 µm silicate dust absorption (τ 3.4 /τ 9.7 ). The τ 3.4 /τ 9.7 ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative "dusty tori" in the close vicinity of AGNs.

We report the results of interferometric HCN(1-0) and HCO+(1-0) observations of four luminous inf... more 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-tem...

We present the results of a ground-based, high spatial resolution infrared 18 µm imaging study of... more We present the results of a ground-based, high spatial resolution infrared 18 µm 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 µm) 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 µm 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 µm 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.

The results of 3-4 mu\mumum spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are... more The results of 3-4 mu\mumum spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8-13 mu\mumum spectra, all the sources have strong absorption-like features at sim\simsim10 mu\mumum, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 mu\mumum carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 mu\mumum PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 mu\mumum carbonaceous dust and 9.7 mu\mumum silicate dust absorption ($\tau_{3.4}$/$\tau_{9.7}$). The tau3.4\tau_{3.4}tau3.4/$\tau_{9.7}$ ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative ``dusty tori'' in the close vicinity of AGNs.

FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FM... more FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope, targeting H$\alpha$ emitters at zsim1.18z \sim 1.18zsim1.18--$1.54$ down to the sensitivity limit of H$\alpha$ flux sim2times10−16rmergcm−2s−1\sim 2 \times 10^{-16} \ \rm erg \ cm^{-2} s^{-1}sim2times10−16rmergcm−2s−1. The primary goal of the survey is to detect redshift space distortions (RSD), to test General Relativity by measuring the growth rate of large scale structure and to constrain modified gravity models for the origin of the accelerated expansion of the universe. The target galaxies were selected based on photometric redshifts and H$\alpha$ flux estimates calculated by fitting spectral energy distribution (SED) models to the five optical magnitudes of the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Wide catalog. The survey started in March 2012, and all the observations were completed in July 2014. In total, we achieved 121121121 pointings of FMOS (each pointing has a 303030 arcmin diameter circula...

ABSTRACT A mid-infrared (MIR) imager and spectrometer is under consideration for construction in ... more ABSTRACT A mid-infrared (MIR) imager and spectrometer is under consideration for construction in the first decade of the Thirty Meter Telescope&#39;s (TMT) operation. When combined with a MIR adaptive optics system, the instrument will afford 15 times higher sensitivity, 4 times better spatial resolution (0.08&quot;) than 8m-class telescopes, and 4.5 times better spatial resolution than the JWST. Additionally, its huge light gathering power opens a new window of high-dispersion spectroscopy in the MIR. We discuss the key science drivers, from star and planet formation to galaxies and black holes and cosmology; science drivers which are in close synergy with the recent Astro 2010 Decadal Survey report. We flow down our science cases to produce fundamental and optional instrument capabilities, including imaging, long-slit and IFU spectroscopy, and polarimetry.

The Astrophysical Journal, 2014

We present high angular resolution (∼0.5 ′′) MIR spectra of the powerful radio galaxy, Cygnus A, ... more We present high angular resolution (∼0.5 ′′) MIR spectra of the powerful radio galaxy, Cygnus A, obtained with the Subaru telescope. The overall shape of the spectra agree with previous high angular resolution MIR observations, as well as previous Spitzer spectra. Our spectra, both on and off nucleus, show a deep silicate absorption feature. The absorption feature can be modeled with a blackbody obscured by cold dust or a clumpy torus. The deep silicate feature is best fit by a simple model of a screened blackbody, suggesting foreground absorption plays a significant, if not dominant role, in shaping the spectrum of Cygnus A. This foreground absorption prevents a clear view of the central engine and surrounding torus, making it difficult to quantify the extent the torus attributes to the obscuration of the central engine, but does not eliminate the need for a torus in Cygnus A.

Annual Report of the National Astronomical Observatory of Japan Volume 6, 2005

We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic ar... more We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic aromatic hydrocarbon (PAH) emission was used to estimate the magnitude of compact (< a few hundred pc) nuclear starbursts. For three of the selected Seyfert 2 nuclei, the magnitudes of compact nuclear starbursts estimated from the 3.3 micro m PAH emission feature (with no extinction correction) were in satisfactory quantitative agreement with those based on UV observations after extinction correction, indicating that the observed 3.3 micro m PAH emission luminosity can be used to estimate the magnitude of compact nuclear starbursts in the nuclei of Seyfert 2s. We found that (1) except in one case, the observed nuclear 3--4 micro m fluxes were dominated by AGNs and not by starbursts, (2) compact nuclear starbursts were detected in 6 out of 17 Seyfert 2 nuclei, but cannot dominate the infrared dust emission luminosities in the majority of the observed Seyfert 2 galaxies, and (3) more powerful AGNs tended to be related to more powerful compact nuclear starbursts. In addition to those showing 3.3 micro m PAH emission, 3.4 micro m carbonaceous dust absorption was detected in two Seyfert 2 nuclei, and 3.05 micro m H2O ice absorption was detected in one nucleus, and possibly detected in one more.

We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic ar... more We report on 3--4 micro m slit spectroscopy of 17 Seyfert 2 nuclei. The 3.3 micro m polycyclic aromatic hydrocarbon (PAH) emission was used to estimate the magnitude of compact (< a few hundred pc) nuclear starbursts. For three of the selected Seyfert 2 nuclei, the magnitudes of compact nuclear starbursts estimated from the 3.3 micro m PAH emission feature (with no extinction correction) were in satisfactory quantitative agreement with those based on UV observations after extinction correction, indicating that the observed 3.3 micro m PAH emission luminosity can be used to estimate the magnitude of compact nuclear starbursts in the nuclei of Seyfert 2s. We found that (1) except in one case, the observed nuclear 3--4 micro m fluxes were dominated by AGNs and not by starbursts, (2) compact nuclear starbursts were detected in 6 out of 17 Seyfert 2 nuclei, but cannot dominate the infrared dust emission luminosities in the majority of the observed Seyfert 2 galaxies, and (3) more powerful AGNs tended to be related to more powerful compact nuclear starbursts. In addition to those showing 3.3 micro m PAH emission, 3.4 micro m carbonaceous dust absorption was detected in two Seyfert 2 nuclei, and 3.05 micro m H2O ice absorption was detected in one nucleus, and possibly detected in one more.

We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrar... more We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

Publications of the Astronomical Society of Japan, 2014

ABSTRACT

The Astrophysical Journal, 2007

We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared... more We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared from 5-35 µm. These observations allow us to investigate mid-IR emission mechanisms in the core of M87 and to establish that the flaring, variable jet component HST-1 is not a major contributor to the mid-IR flux. The Spitzer data include a high signal-to-noise 15-35 µm spectrum of the knot A/B complex in the jet, which is consistent with synchrotron emission. However, a synchrotron model cannot account for the observed nuclear spectrum, even when contributions from the jet, necessary due to the degrading of resolution with wavelength, are included. The Spitzer data show a clear excess in the spectrum of the nucleus at wavelengths longer than 25 µm, which we model as thermal emission from cool dust at a characteristic temperature of 55 ± 10 K, with an IR luminosity ∼ 10 39 erg s −1 . Given Spitzer's few-arcsecond angular resolution, the dust seen in the nuclear spectrum could be located anywhere within ∼ 5 ′′ (390 pc) of the nucleus. In any case, the ratio of AGN thermal to bolometric luminosity indicates that M87 does not contain the IR-bright torus that classical unified AGN schemes invoke. However, this result is consistent with theoretical predictions for low-luminosity AGNs.

Astron J, 2011

We present the results of a ground-based, high spatial resolution infrared 18 μm imaging study of... more We present the results of a ground-based, high spatial resolution infrared 18 μm 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 μm) 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 μm 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 μm 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.

We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrar... more We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

Spitzer Proposal, May 1, 2007

Ultraluminous infrared galaxies (ULIRGs) have the power output of quasars, yet emit nearly all th... more Ultraluminous infrared galaxies (ULIRGs) have the power output of quasars, yet emit nearly all their energy in the mid and far-infrared part of the spectrum. Over the last 10 years, much effort has focussed on identifying tracers of AGN and star formation activity in the infrared in order to quantify the origin of the infrared luminosity in these systems. Recently, we discovered the presence of warm (200-600K) dense (~10^6 cm-3) molecular gas through acetylene, hydrogen cyanide, and carbon dioxide gas-phase absorption lines in the IRS spectra of (U)LIRGs. In the Galaxy these lines are characteristic for the 'Hot Core' phase of deeply embedded protostars. Consequently, we attribute these lines to a phase of deeply embedded star formation. While these features are most readily recognized in the spectra of highly dust enshrouded (U)LIRGs, they are also seen in ULIRGs with unambiguous evidence for an AGN. Hence, such deeply embedded star formation seems to be a general characteristic of (U)LIRG nuclei. Here we propose to use the IRS on Spitzer to search for these molecular absorption lines in a wider sample of ULIRG nuclei to determine the frequency and importance of this hitherto unrecognized phase of hidden star formation and characterize the physical conditions of the absorbing dense gas. These spectra, combined with existing high signal-to-noise observations of (U)LIRGs will allow us to place our observations of hidden star formation in ULIRGs in the context of evolution of infrared galaxies as a whole.

Thermal infrared photometry in the LLL- and M′M'M′-band and L−M′L - M'L−M′ colors of type-1 and type-2 ac... more Thermal infrared photometry in the LLL- and M′M'M′-band and L−M′L - M'L−M′ colors of type-1 and type-2 active galactic nuclei (AGNs) are presented. After combining our observations with photometric data at similar wavelengths taken from the literature, we find that the excess of L−M′L - M'L−M′ colors of type-2 AGNs (37 sources, 50 data points) relative to type-1 AGNs (27 sources, 36 data points), due to dust extinction, is statistically detectable, but very small. We next investigate the L−M′L - M'L−M′ colors of type-2 AGNs by separating less dust-obscured type-2 AGNs and highly dust-obscured type-2 AGNs. In both cases, the L−M′L - M'L−M′ colors are similar to the intrinsic L−M′L - M'L−M′ color of unobscured AGNs, and the L−M′L - M'L−M′ color excess of the latter highly dust-obscured type-2 AGNs due to dust extinction is much smaller than that expected from the Galactic dust extinction curve. Contamination from starbursts and the time lag of flux variation are unlikely to explain this small L−M′L - M'L−M′ color excess, which is best explained if the dust extinction curve in the close vicinity of AGNs is fairly flat at 3--5 mu\mumum as a result of a size increase of the absorbing dust grains through coagulation.

Mon Notic Roy Astron Soc, 2002

The results of 3-4 µm spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are rep... more The results of 3-4 µm spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8-13 µm spectra, all the sources have strong absorption-like features at ∼10 µm, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 µm carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 µm PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 µm carbonaceous dust and 9.7 µm silicate dust absorption (τ 3.4 /τ 9.7 ). The τ 3.4 /τ 9.7 ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative "dusty tori" in the close vicinity of AGNs.