Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope
By Lee Armus, Jeronimo Bernard-Salas, H. Spoon, J. Marshall, Vassilis Charmandaris, S. Higdon, Vandana Desai, L. Hao, H. Teplitz, D. Devost and 3 more. 2005
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We present mid-infrared spectra of the nearby ultraluminous infrared galaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and polycyclic aromatic hydrocarbon (PAH) emission features. The H2 line fluxes suggest molecular gas at a variety of temperatures. A simple two-temperature fit to the S(0) through S(7) lines implies a mass of ~6.7 × 106 M☉ at T ~ 957 K and ~1.6 × 109 M☉ at T ~ 164 K, or about 15% of the total molecular gas mass in this system. Notably, we have detected the [Ne V] 14.3 μm emission line, with a flux of 5 × 10-14 ergs cm-2 s-1, providing the first direct detection of the buried active galactic nucleus (AGN) in the mid-infrared. Modeling of the total spectral energy distribution (SED) from near- to far-infrared wavelengths requires the presence of a hot dust (T ~ 700 K) component, which we also associate with the buried AGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relative fraction of hot dust emission, and the large 6.2 μm PAH equivalent width (EQW), are all consistent with an apparent AGN contribution of only 3%-5% to the bolometric luminosity. However, correcting the measured [Ne V] flux by the extinction implied by the silicate optical depth and our SED fitting suggests an intrinsic fractional AGN contribution to the bolometric luminosity of ~20%-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/07688471f633742cd3397b77144f4ec12b867ba9
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- MID-INFRARED SPECTROSCOPY OF SUBMILLIMETER GALAXIES: EXTENDED STAR FORMATION IN MASSIVE HIGH-REDSHIFT GALAXIES (2009, methodology): “We compare the mid-IR properties of the SMGs in our sample with those of starburst-dominated low-redshift galaxies (Brandl et al. 2006), select local ULIRGs from the Bright IRAS sample (Armus et al. 2006, 2007) and the mid-IR selected sources of Sajina et al. (2007) and Farrah et al. (2008).”
- H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “c shock NGC 6240! Starburst galaxy (ULIRG) Figure 6: Two examples of extragalactic mid-infrared spectra (taken with the Spitzer IRS) showing prominent rotational lines of H2. Top panel: spectrum from Armus et al. (2006) of NGC 6240, a nearby (z = 0:0245) merging galaxy that has a powerful starburst,”
- Towards a complete census of AGN in nearby Galaxies: a large population of optically unidentified AGN (2009, background): “32, 24.32 µm (97.1 eV), mid-IR spectroscopy provides an unambiguous indicator of AGN activity in nearby galaxies (e.g., Weedman et al. 2005; Armus et al. 2006).1 The relative optical depth at midIR wavelengths is also considerably lower than at optical wavelengths (Aλ14.”
- FUNDAMENTAL PARAMETERS AND SPECTRAL ENERGY DISTRIBUTIONS OF YOUNG AND FIELD AGE OBJECTS WITH MASSES SPANNING THE STELLAR TO PLANETARY REGIME (2015, methodology): “All MIR spectra were obtained with the InfraRed Spectrograph (IRS; Houck et al. 2004) on board the Spitzer Space Telescope using the Short wavelength Low resolution (SL) module, 11 of which had Long wavelength Low resolution (LL) orders stitched in as well.”
- Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. IV. The Controlling Parameters of the Starburst SED (2007, methodology): “Recently Armus et al. (2006) have directly detected the active nucleus via the [Ne V] 14.3 µm line using the IRS on the Spitzer Space Telescope.”
- H2O and OH Gas in the Terrestrial Planet-forming Zones of Protoplanetary Disks (2008, methodology): “…Short-High (SH) module spectrum of AS 205A was taken as part of the “cores to disks” (c2d) Legacy program (Evans et al. 2003; Kessler-Silacci et al. 2006), while the spectrum of DR Tau, observed as part of the guaranteed time observations (Houck et al. 2004), was obtained from the Spitzer archive.”
- Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. II. The IRAS Bright Galaxy Sample (2006, background): “As discussed in Armus et al. (2006) in the context of NGC 6240, the τW list in Table 6, which can be very large, may be a more accurate estimate of the optical depth at 10µm, since it does explicitly correct for silicate emission.”
- The Mid-Infrared Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy (2006, background): “NGC 6240 is a peculiar case with an intrinsic fractional AGN contribution to the bolometric luminosity of 20%Y24% (Armus et al. 2006).”
- A Multiwavelength Study of M17: The Spectral Energy Distribution and PAH Emission Morphology of a Massive Star Formation Region (2007, methodology): “Spitzer IRS Spectra Spitzer IRS observations were obtained as part of program ID 3697 (PI: M. Wolfire), to map the PDR in M17 SW in the IRS SH (short-high) and LH (long-high)modules (Houck et al. 2004).”
- Spitzer Quasar and ULIRG Evolution Study (QUEST). I. The Origin of the Far-Infrared Continuum of QSOs (2006, methodology): “…this feature was present but difficult to quantify in the presence of strong silicate emission, we have estimated the flux scaling from the flux of the well-detected 6.2 m PAH feature, using the 7.7/6.2 feature ratio measured in the starburst/AGN NGC 6240 (Lutz et al. 2003; Armus et al. 2006).”
- High-Resolution Mid-Infrared Spectroscopy of Ultraluminous Infrared Galaxies (2007, background): “32, leading to a double-peaked profile (see also Armus et al. 2006).”