19 November 2016 | Seashells of the Universe


Credit & copyright: Oleg Bryzgalov | Click image to enlarge


Like ripples in a pond cascading outwards from a stone being thrown into it, the shell elliptical galaxy NGC 474 anchors a small group of galaxies in the cosmic waters of the aquatic constellation of Pisces. This peculiar group of galaxies also includes the relatively serene and tranquil blue spiral galaxy NGC 470. Completing the group to the right of the bright star is the distorted elliptical galaxy NGC 467 with a distinctive and rare tidal umbrella structure. NGC 474 and NGC 470 are also collectively known as Arp 227.

Despite the undisturbed appearance of NGC 470, detailed multiwavelength studies by professional astronomers have made apparent its role in a gravitational tug of war with its chaotic neighbour NGC 474. The evidence for this isn’t visible in the optical but a bridge of neutral hydrogen gas connecting the two galaxies has been detected in radio observations. At an estimated distance of 103 million light years, the separation between the two galaxies is about 17,500 light years.

In the past, elliptical galaxies were considered to be the placid denouement of galactic evolution. However, the pioneering work of the revolutionary astronomer David Malin in the enhancement of faint features visible on photographic plates in the 1970’s via the at the time newly developed method of ‘photographic amplification‘ led to the dramatic discovery of shells surrounding a number of elliptical galaxies. The first system of shells was identified in the galaxy M89 followed by NGC 1344 and a carefully selected sample of galaxies was surveyed with a catalogue published in the 1980’s. However at the time, there was a slight disagreement amongst professional astronomers in the terminology used to describe these extended structures with some opting to refer to tidal shells as “ripples” instead. With the progress of time, this second terminology fell out of favour with shells being the most commonly accepted term.

NGC 474 is one of the most well known examples of a shell elliptical galaxy and this deep image also reveals a series of tidal plumes to the south and a faint narrow tidal tail can barely be distinguished to the north. Other examples of shell elliptical galaxies include NGC 3923 and NGC 7600. Shell ellipticals are generally accepted to be the product of a merger between two galaxies and some can display complex internal morphologies consisting of dust lanes. With the advent of CCD technologies being employed at professional observatories after the use of photographic plates were discontinued, tidal shell systems were discovered around a greater number of elliptical and even spiral galaxies such as NGC 4414. Despite this, the combination of digital contrast enhancement techniques with archival photographic plates has yielded an impressive collection of deep images of galaxies revealing faint tidal features. Not limited by their relatively modestly sized telescopes, even amateur astrophotographers have revealed extremely faint tidal streams around a number of galaxies!

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