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NGC 4631
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NGC 4631 (also Whale Galaxy, Caldwell 32, Arp 281, UGC 7865 and PGC 42637) is a magnitude +9.8 edge-on barred spiral galaxy located between 25 and 30 million light-years away in the constellation Canes Venatici.
The galaxy can be found about 4.5 degrees NE of the star gamma Comae Berenices.
This galaxy is about the size of our Milky Way Galaxy. Telescopes of 200 mm aperture show the distorted edge-on wedge shape of the galaxy, making it look like a whale; a thin strip of dark dust traverses the disk. To the north is the small galaxy NGC 4627.
The galaxy is gravitationally interacting with the neighbouring dwarf elliptical galaxy NGC 4627: these two galaxies are listed as Arp 281 in the Atlas of peculiar galaxies by Halton Arp.
NGC 4631 and NGC 4627 are part of the poorly defined NGC 4631 Group, consisting of between 5 to 27 galaxies (according to various sources), and also a member of the Coma I galaxies, which is approaching the Virgo Cluster, and will end up merging with it in the distant future.

The galaxy was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 inch) f/13 speculum reflector at Windsor Road, Slough, on the 20th March 1787. 

Right ascension 12h 42m 08.0s, Declination +32° 32' 29"



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PGC 42637
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Title: Resolved stellar streams around NGC4631 from a Subaru/Hyper Suprime-Cam survey
Author: Mikito Tanaka (Tohoku Univ.), Masashi Chiba (Tohoku Univ.), Yutaka Komiyama (NAOJ)

We present the first results of the Subaru/Hyper Suprime-Cam (HSC) survey of the interacting galaxy system, NGC4631 and NGC4656. From the maps of resolved stellar populations, we identify 11 dwarf galaxies (including already-known dwarfs) in the outer region of NGC4631 and the two tidal stellar streams around NGC4631, named Stream SE and Stream NW, respectively. This paper describes the fundamental properties of these tidal streams. Based on the tip of red giant branch method and the Bayesian statistics, we find that StreamSE (7.10 Mpc in Expected a posteriori, EAP, with the 90% credible intervals of [6.22, 7.29] Mpc) and StreamNW (7.91 Mpc in EAP with the 90% credible intervals of [6.44, 7.97] Mpc) are located in front of and behind NGC4631, respectively. We also calculate the metallicity distribution of stellar streams by comparing the member stars with theoretical isochrones on the colour-magnitude diagram. We find that both streams have the same stellar population based on the Bayesian model selection method, suggesting that they originated from a tidal interaction between NGC4631 and a single dwarf satellite. The expected progenitor has a positively skewed metallicity distribution function with [M/H]_EAP=-0.92 with the 90% credible intervals of [-1.46, -0.51]. The stellar mass of the progenitor is estimated as 3.7 x 10e+8 Msun with the 90% credible intervals of [5.8 x 10e+6, 8.6 x 10e+9] Msun based on the mass-metallicity relation for Local group dwarf galaxies. This is in good agreement with an initial stellar mass of the progenitor presumed in the previous N-body simulation.

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NGC 4631
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Title: Exploring the Dust Content of Galactic Winds with Herschel. I. NGC 4631
Author: M. Meléndez, S. Veilleux, C. Martin, C. Engelbracht, J. Bland-Hawthorn, G. Cecil, F. Heitsch, A. McCormick, T. Müller, D. Rupke, S. H. Teng

We present a detailed analysis of deep far-infrared observations of the nearby edge-on star-forming galaxy NGC 4631 obtained with the Herschel Space Observatory. Our PACS images at 70 and 160 um show a rich complex of filaments and chimney-like features that extends up to a projected distance of 6 kpc above the plane of the galaxy. The PACS features often match extraplanar Halpha, radio-continuum, and soft X-ray features observed in this galaxy, pointing to a tight disk-halo connection regulated by star formation. On the other hand, the morphology of the colder dust component detected on larger scale in the SPIRE 250, 350, and 500 um data matches the extraplanar H~I streams previously reported in NGC 4631 and suggests a tidal origin. The PACS 70/160 ratios are elevated in the central ~3.0 kpc region above the nucleus of this galaxy (the "superbubble"). A pixel-by-pixel analysis shows that dust in this region has a higher temperature and/or an emissivity with a steeper spectral index (beta > 2) than the dust in the disk, possibly the result of the harsher environment in the superbubble. Star formation in the disk seems energetically insufficient to lift the material out of the disk, unless it was more active in the past or the dust-to-gas ratio in the superbubble region is higher than the Galactic value. Some of the dust in the halo may also have been tidally stripped from nearby companions or lifted from the disk by galaxy interactions.

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