Anne’s Picture of the Day: Spiral Galaxy NGC 3621

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February 1, 2013 

NGC 3621, an unusual spiral galaxy in Hydra

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com)

NGC 3621 is a bright spiral galaxy of some 93,000 light-years across, located about 22 million light-years away in the constellation of Hydra (The Sea Snake). It is moving away from us at approximately 727 kilometers per second.

NGC 3621 is a truly remarkable galaxy. Like other spirals, it has a flat disk permeated by dark lanes of material and with prominent spiral arms where young stars are forming in clusters (the blue dots seen in the image). But while most spiral galaxies have a central bulge — a large group of old stars packed in a compact, spheroidal region — this galaxy doesn’t. There is simply a brightening to the centre, but no actual bulge. Thus, NGC 3621 is a pure-disk galaxy.

The galaxy has a flat, elliptical pancake shape, indicating that it hasn’t yet come face to face with another galaxy as such a galactic collision would have disturbed the thin disk of stars, creating a bulge in its center.

NGC 3621 is also interesting as it is believed to have an active supermassive black hole at its center that is engulfing matter and producing radiation. This is somewhat unusual because most of these so-called active galactic nuclei exist in galaxies with prominent bulges. In this particular case, the supermassive black hole is thought to have a relatively small mass, of around 20,000 times that of the Sun.

Another interesting feature is that there are also thought to be two smaller black holes – intermediate black holes – with masses of a few thousand times that of the Sun, near the nucleus of the galaxy. Therefore, NGC 3621 is an extremely interesting object which, despite not having a central bulge, has a system of three black holes in its central region.

Its relative proximity allows astronomers to study a wide range of astronomical objects within it, including stellar nurseries, dust clouds, and pulsating stars called Cepheid variables.

Cepheid variables are very luminous stars — up to 30,000 times brighter than our Sun — whose brightness varies at regular intervals over several days, weeks or months. The period of this variation in luminosity is related to the star’s true brightness, known as its absolute magnitude. By knowing the absolute magnitude of the star, and measuring how bright it appears, astronomers can easily calculate its distance from Earth. Cepheid variables are therefore vital for establishing the scale of the Universe.

In the late 1990s, NGC 3621 was one of 18 galaxies selected for a project on the Extragalactic Distance Scale of the Hubble Space Telescope to observe Cepheid variables and measure the rate of expansion of the Universe to a higher accuracy than had been possible before. In the successful project, 69 Cepheid variables were observed in this galaxy alone.

NGC 3621 is comparatively bright and can be well seen in moderate-sized telescopes.

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