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NGC 6240
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Title: Giant H-alpha nebula surrounding the starburst merger NGC 6240
Author: Michitoshi Yoshida, Masafumi Yagi, Youichi Ohyama, Yutaka Komiyama, Nobunari Kashikawa, Hisashi Tanaka, Sadanori Okamura

We revealed the detailed structure of a vastly extended H-alpha-emitting nebula ("H-alpha nebula") surrounding the starburst/merging galaxy NGC 6240 by deep narrow-band imaging observations with the Subaru Suprime-Cam. The extent of the nebula is ~90 kpc in diameter and the total H luminosity amounts to ~1.6 x 10^42 erg s^-1. The volume filling factor and the mass of the warm ionised gas are ~10^-4--10^-5 and ~5 x 10^8 solar masses, respectively. The nebula has a complicated structure, which includes numerous filaments, loops, bubbles, and knots. We found that there is a tight spatial correlation between the H nebula and the extended soft X-ray-emitting gas, both in large and small scales. The overall morphology of the nebula is dominated by filamentary structures radially extending from the center of the galaxy. A large-scale bi-polar bubble extends along the minor axis of the main stellar disk. The morphology strongly suggests that the nebula was formed by intense outflows - superwinds - driven by starbursts. We also found three bright knots embedded in a looped filament of ionized gas that show head-tail morphologies in both emission-line and continuum, suggesting close interactions between the outflows and star forming regions. Based on the morphology and surface brightness distribution of the H nebula, we propose the scenario that three major episodes of starburst/superwind activities which were initiated ~102 Myr ago formed the extended ionized gas nebula of NGC 6240.

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Hubble Revisits Tangled NGC 6240

NGC 6240 lies 400 million light-years away in the constellation of Ophiuchus (The Serpent Holder). This galaxy has an elongated shape with branching wisps, loops and tails. This mess of gas, dust and stars bears more than a passing resemblance to a butterfly and a lobster.
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NGC 6240
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NGC 6240: Colossal Hot Cloud Envelopes Colliding Galaxies

ngc6240_w11.jpg

Scientists have used Chandra to make a detailed study of an enormous cloud of hot gas enveloping two large, colliding galaxies. This unusually large reservoir of gas contains as much mass as 10 billion Suns, spans about 300,000 light years, and radiates at a temperature of more than 7 million degrees Kelvin.
This giant gas cloud, which scientists call a "halo," is located in the system called NGC 6240. Astronomers have long known that NGC 6240 is the site of the merger of two large spiral galaxies similar in size to our own Milky Way. Each galaxy contains a supermassive black hole at its center. The black holes are spiralling toward one another, and may eventually merge to form a larger black hole.

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Title: Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240
Authors: Junfeng Wang, Emanuele Nardini, Giuseppina Fabbiano, Margarita Karovska, Martin Elvis, Silvia Pellegrini, Claire Max, Guido Risaliti, Vivian U, Andreas Zezas

We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the X-ray emission from the pair of active nuclei and the diffuse hot gas in great detail. We detect extended hard X-ray emission from kT~6 keV (~70 million K) hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks with velocity of ~2200 km/s. For the first time we obtain the spatial distribution of this highly ionised gas emitting FeXXV, which shows a remarkable correspondence to the large scale morphology of H_2(1-0) S(1) line emission and H\alpha filaments. Propagation of fast shocks originated in the starburst driven wind into the ambient dense gas can account for this morphological correspondence. With an observed L(0.5-8 keV)=5.3E+41 erg/s, the diffuse hard X-ray emission is 100 times more luminous than that observed in the classic starburst galaxy M82. Assuming a filling factor of 1% for the 70 MK temperature gas, we estimate its total mass (M_{hot}=1.8E+8 solar masses) and thermal energy (E_{th}=6.5E+57 ergs). The total iron mass in the highly ionised plasma is M_{Fe}=4.6E+5 solar masses. Both the energetics and the iron mass in the hot gas are consistent with the expected injection from the supernovae explosion during the starburst that is commensurate with its high star formation rate. No evidence for fluorescent Fe I emission is found in the CO filament connecting the two nuclei.

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Title: The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240
Authors: Emanuele Nardini, Junfeng Wang, Giuseppina Fabbiano, Martin Elvis, Silvia Pellegrini, Guido Risaliti, Margarita Karovska, Andreas Zezas

We report on a recent ~150-ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma confidence level over a diamond-shaped region with projected physical size of ~110x80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5x10^{-3} cm^{-3}, and a total mass of ~10^10 solar masses, resulting in an intrinsic 0.4-2.5 keV luminosity of 4x10^41 erg s^{-1}. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially-resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main alpha-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favourable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.

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Title: Evidence for CO shock excitation in NGC 6240 from Herschel SPIRE spectroscopy
Authors: R. Meijerink, L. E. Kristensen, A. Weiss, P. P. van der Werf, F. Walter, M. Spaans, A. F. Loenen, J. Fischer, F. P. Israel, K. Isaak, P. P. Papadopoulos, S. Aalto, L. Armus, V. Charmandaris, K. M. Dasyra, T. Diaz-Santos, A. Evans, Y. Gao, E. Gonzalez-Alfonso, R. Guesten, C. Henkel, C. Kramer, S. Lord, J. Martin-Pintado, D. Naylor, D. B. Sanders, H. Smith, L. Spinoglio, G. Stacey, S. Veilleux, M. C. Wiedner

We present Herschel SPIRE FTS spectroscopy of the nearby luminous infrared galaxy NGC 6240. In total 20 lines are detected, including CO J=4-3 through J=13-12, 6 H2O rotational lines, and [CI] and [NII] fine-structure lines. The CO to continuum luminosity ratio is 10 times higher in NGC 6240 than Mrk 231. Although the CO ladders of NGC 6240 and Mrk 231 are very similar, UV and/or X-ray irradiation are unlikely to be responsible for the excitation of the gas in NGC 6240. We applied both C and J shock models to the H2 v=1-0 S(1) and v=2-1 S(1) lines and the CO rotational ladder. The CO ladder is best reproduced by a model with shock velocity v_s=10 km s^-1 and a pre-shock density n_H=5 * 10^4 cm^-3. We find that the solution best fitting the H2 lines is degenerate: The shock velocities and number densities range between v_s = 17 - 47 km s^-1 and n_H=10^7 - 5 * 10^4 cm^-3, respectively. The H2 lines thus need a much more powerful shock than the CO lines. We deduce that most of the gas is currently moderately stirred up by slow (10 km s^-1) shocks while only a small fraction (< 1 percent) of the ISM is exposed to the high velocity shocks. This implies that the gas is rapidly loosing its highly turbulent motions. We argue that a high CO line-to-continuum ratio is a key diagnostic for the presence of shocks.

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Title: Metallicity in the merger Seyfert galaxy NGC 6240
Authors: M. Contini

We have calculated the physical conditions throughout the NLR of the merger Seyfert galaxy NGC 6240 by modelling the observed optical and infrared line ratios. We have found that the optical spectra are emitted by clouds photoionised by the power-law radiation flux from the AGN (or AGNs), and heated mainly by the shock accompanying the propagation of the clouds outwards. The infrared line ratios are emitted from clouds ejected from a starburst which photoionises the gas by the black-body radiation flux corresponding to a stellar colour temperature of about 50,000 K. Both the flux from the AGN and the ionisation parameters are low. The most characteristic physical parameters are the relatively high shock velocities (>400 km/s) and low preshock densities (about 40-60 cm-3) of the gas. The C/H, N/H, O/H relative abundances are higher than solar by a factor lower or about 1.5. We suggest that those high relative abundances indicate trapping of H into H2 molecules rather than high metallicities. Adopting an initial grain radius of 1 micron, the dust temperatures calculated in the clouds reached by the power-law radiation flux and by the black-body radiation flux are 81 K and 68 K, respectively.

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NGC 6240
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The two supermassive black holes, which show up as two points of light in the center of the galaxy NGC 6240, are only 3,000 light-years apart. Astronomers think the two will eventually combine into a single, larger black hole.

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See also Abell 400 black holes

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NGC 6240: Black Holes Go "Mano A Mano"

ngc6240_w1.jpg
Credit       X-ray (NASA/CXC/MIT/C.Canizares, M.Nowak); Optical (NASA/STScI)

This image of NGC 6240 contains new X-ray data from Chandra (shown in red, orange, and yellow) that has been combined with an optical image from the Hubble Space Telescope originally released in 2008. In 2002, the discovery of two merging black holes was announced based on Chandra data in this galaxy. The two black holes are a mere 3,000 light years apart and are seen as the bright point-like sources in the middle of the image.
Scientists think these black holes are in such close proximity because they are in the midst of spiraling toward each other - a process that began about 30 million years ago. It is estimated that the two black holes will eventually drift together and merge into a larger black hole some tens or hundreds of millions of years from now.

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Triangulum galaxy
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Queen's physicist unlocking the mysteries of neighbouring galaxies
An international team of astronomers, including Queen's University physicist Larry Widrow, have uncovered evidence of a nearby cosmic encounter. Their study indicates that the Andromeda and Triangulum galaxies, the two galaxies closest to our own, collided about two to three billion years ago.

"The encounter forever changed the structure of the galaxies. The collision between the galaxies appears to have caused millions of stars to be ripped from the Triangulum disk to form a faint stream visible in the PAndAS data" - Dr. Widrow, a professor of Physics, Engineering Physics and Astronomy at Queen's.


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