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NGC 6240

This new image of the Triangulum galaxy is being released as part of the 'Around the World in 80 Telescopes' event for the International Year of Astronomy.

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A new image from NASA's Spitzer Space Telescope offers a rare view of an imminent collision between the cores of two merging galaxies, each powered by a black hole with millions of times the mass of the sun.
The galactic cores are in a single, tangled galaxy called NGC 6240, located 400-million light years away in the constellation Ophiuchus. Millions of years ago, each core was the dense centre of its own galaxy before the two galaxies collided and ripped each other apart. Now, these cores are approaching each other at tremendous speeds and preparing for the final cataclysmic collision. They will crash into each other in a few million years, a relatively short period on a galactic timescale.

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Two nearby, gas-rich galaxies have been caught crashing into each other in unprecedented detail, pinpointing the location of the two colossal black holes within them. The new observations could shed light on how easy it will be for the black holes themselves to merge one day.
Two disc-shaped galaxies are thought to have brushed past one another before circling around and beginning to merge about 30 million years ago. But studying the resulting galaxy, which lies 320 million light years away and is known as NGC 6240, can be difficult because it is shrouded in patchy dust that blocks light at optical wavelengths.
Now, a team using the 10-metre Keck II telescope on Mauna Kea in Hawaii, US the largest infrared telescope in the world has revealed the location of the two supermassive black holes within the merging galaxies to unprecedented precision by the infrared glow of the dense dust and stars around them.

NGC 6240

Young star clusters appear as small blue dots in this Keck image of the material surrounding the two supermassive black holes in NGC 6240
Credit: C Max, G Canalizo and W de Vries


The new observations also show clumps of young stars that formed as a result of the merger (seen as aqua dots in the image). When the gas clouds from each galaxy crashed into one another, shockwaves formed that caused the gas to clump together, triggering intense star formation.

"They're very close to where the black holes are" - team leader Claire Max, an astronomer at the University of California in Santa Cruz, US.

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Title: Circumnuclear Star Clusters in the Galaxy Merger NGC 6240, Observed with Keck Adaptive Optics and HST
Authors: L. K. Pollack, C. E. Max, G. Schneider

We discuss images of the central ~10 kpc (in projection) of the galaxy merger NGC 6240 at H and K' bands, taken with the NIRC2 narrow camera on Keck II using natural guide star adaptive optics. We detect 28 star clusters in the NIRC2 images, of which only 7 can be seen in the similar-spatial-resolution, archival WFPC2 Planetary Camera data at either B or I bands. Combining the NIRC2 narrow camera pointings with wider NICMOS NIC2 images taken with the F110W, F160W, and F222M filters, we identify a total of 32 clusters that are detected in at least one of these 5 infrared (lambda > 1 micron) bandpasses. By comparing to instantaneous burst, stellar population synthesis models (Bruzual & Charlot 2003), we estimate that most of the clusters are consistent with being ~15 Myr old and have photometric masses ranging from 7E5 M_sun to 4E7 M_sun. The total contribution to the star formation rate (SFR) from these clusters is approximately 10 M_sun/year, or ~10% of the total SFR in the nuclear region. We use these newly discovered clusters to estimate the extinction toward NGC 6240's double nuclei, and find values of A_V as high as 14 magnitudes along some sightlines, with an average extinction of A_V~7 mag toward sightlines within ~3 arcsec of the double nuclei.

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Title: Detection of the Buried AGN in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope

Authors:
L. Armus, J. Bernard-Salas, H.W.W. Spoon, J.A. Marshall, V. Charmandaris, S.J.U. Higdon, V. Desai, L. Hao, H.I. Teplitz, D. Devost, B.R. Brandl, B.T. Soifer, J.R. Houck

Researchers present mid-infrared spectra of the nearby, Ultraluminous Infrared Galaxy NGC 6240 taken with the IRS on Spitzer. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and 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 6.7E6 solar masses at T=957K and 1.6E9 solar masses at T=164K, or about 15% of the total molecular gas mass in this system.

Notably, they have detected the (NeV) 14.3Ám emission line, with a flux of 5E-14 erg/cm^2/s, providing the first direct detection of the buried AGN in the mid-infrared. Modelling of the total SED from near to far-infrared wavelengths requires the presence of a hot dust (T=700K) component, which we also associate with the buried AGN. The small (NeV)/(NeII) and (NeV)/IR flux ratios, the relative fraction of hot dust emission, as well as the large 6.2 Ám PAH EQW, are all consistent with an apparent AGN contribution of only 3-5% to the bolometric luminosity.

However, correcting the measured (NeV) flux by the extinction implied by the silicate optical depth and their SED fitting, suggests an intrinsic fractional AGN contribution to the bolometric luminosity of about 20-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.

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The 13.8 magnitude galaxy in the constellation Ophiuchus, became well-known when the IRAS satellite found it to have an enormous far-infrared luminosity; this is one of the prototype "IRAS galaxies".
NGC 6240 is in the throes of a titanic galaxy - galaxy collision 400 million light-years away (redshift = 0.0245). The merging galaxies stream out distorted tidal tails of stars, gas, and dust and undergo frantic bursts of star formation.


Hubble optical

NGC 6240 is a prime example of a "starburst" galaxy in which stars are forming, evolving, and exploding at an exceptionally rapid rate due to this relatively recent merger (30 million years ago). Heat generated by this activity creates extensive multimillion degree Celsius gas.


Position(2000): RA: 16h53m00.5s Dec: +02o24'16"
This Chandra image of NGC 6240, shows the butterfly-shape that is the product of the collision of two smaller galaxies. It revealed that the central region of the galaxy (inset) contains not one, but two active giant black holes.


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