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RE: NGC 4418
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Title: An atomic hydrogen bridge fueling NGC 4418 with gas from VV 655
Author: E. Varenius, F. Costagliola, H.-R. Kl÷ckner, S. Aalto, H. Spoon, I. MartÝ-Vidal, J. E. Conway

The galaxy NGC 4418 harbours a compact (<20 pc) core with a very high bolometric luminosity (~10^11 solar luminosity). As most of the galaxy's energy output comes from this small region, it is of interest to determine what fuels this intense activity. An interaction with VV 655 has been proposed, where gas acquired by NGC 4418 could trigger intense star formation and/or black hole accretion in the centre. We aim to constrain the interaction hypothesis by studying neutral hydrogen structures around the two galaxies. We present observations at 1.4 GHz with the Very Large Array of radio continuum as well as emission and absorption from atomic hydrogen. Gaussian distributions are fitted to observed HI emission and absorption spectra. An atomic HI bridge is seen in emission, connecting NGC 4418 to VV 655. While NGC 4418 is bright in continuum emission and seen in HI absorption, VV 655 is barely detected in the continuum but show bright HI emission (MHI~10^9 solar masses). We estimate SFRs from 1.4 GHz of 3.2 solar masses yr^-1 and 0.13 solar masses yr^-1 for NGC 4418 and VV 655 respectively. Systemic HI velocities of 2202▒20 km s^-1 (emission) and 2105.4▒10 km s^-1 (absorption) are measured for VV 655 and NGC 4418 respectively. Redshifted HI absorption is seen towards NGC 4418, suggesting gas infall. Blueshifted HI-emission is seen north-west of NGC 4418, which we interpret as a continuation of the outflow previously discussed by Sakamoto et al. (2013). The morphology and velocity structure seen in HI is consistent with an interaction scenario, where gas was transferred from VV 655 to NGC 4418, and may fuel the activity in the centre.

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NGC 4355
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NGC 4355 (also NGC 4418, IRAS 12243-0036, MCG 0-32-12, UGC 7545 and PGC 40762) is a magnitude +13.2 spiral galaxy located 100 million light-years away in the constellation Virgo.

The galaxy was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 inch) f/13 speculum reflector at Clayhall Farm House in Old Windsoráon the 1st January 1786 and listed as NGC 4418.
The galaxy was rediscovered by American astronomer David Peck Todd using a 66.04 cm (26 inch) refracting telescope on the 5th February 1878 and relisted asáNGC 4355.

Right Ascension 12h 26m 54.6s, Declination -00░ 52' 39"



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Posts: 131433
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NGC 4418
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Title: Submillimeter Interferometry of the Luminous Infrared Galaxy NGC 4418: A Hidden Hot Nucleus with an Inflow and an Outflow
Authors: Kazushi Sakamoto, Susanne Aalto, Francesco Costagliola, Sergio Martin, Youichi Ohyama, Martina C. Wiedner, David J. Wilner

We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418 with subarcsec resolution at 860 and 450 micron for the first time to characterize its hidden power source. A ~20 pc (0.1") hot dusty core was found inside a 100 pc scale concentration of molecular gas at the galactic center. The 860 micron continuum core has a deconvolved (peak) brightness temperature of 120-210 K. The CO(3-2) peak brightness temperature there is as high as 90 K at 50 pc resolution. The core has a bolometric luminosity of about 10^{11} Lsun, which accounts for most of the galaxy luminosity. It is Compton thick (N_H >~ 10^{25} cm^{-2}) and has a high luminosity-to-mass ratio ~500 Lsun/Msun as well as a high luminosity surface density 10^{8.5▒0.5} Lsun pc^{-2}. These parameters are consistent with an AGN to be the main luminosity source (with an Eddington ratio about 0.3) while they can be also due to a young starburst near its maximum L/M. We also found an optical colour (reddening) feature that we attribute to an outflow cone emanating from the nucleus. The hidden hot nucleus thus shows evidence of both an inflow, previously seen with absorption lines, and the new outflow reported here in a different direction. The nucleus must be rapidly evolving with these gas flows.

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