NGC 1827: Hubble Image of an Edge-On Spiral Galaxy in Columba

A spiral seen from the side

NGC 1827 lies roughly 37 million light-years away. Catalogues class it as a barred late-type spiral (type Sc), with a small bulge and loosely wound arms. Its disk is tilted about 85 degrees from face-on, so it looks about five times as long as it is wide. It spans just over three arcminutes, roughly 35,000 light-years across at that distance.

Edge-on, the disk shows its profile: a pale band of starlight around a brighter central bulge, with a ragged line of reddish-brown dust and blue clumps along the middle. NASA identifies the blue clumps as places where new stars are likely forming; young, hot stars shine blue, while dust absorbs and reddens light from behind.

Did you know?The four spikes around the bright foreground star are an optical effect: starlight bends around the supports of Hubble's secondary mirror, giving every bright star in Hubble images the same cross.

Why astronomers study galaxies edge-on

A face-on view shows the spiral pattern; an edge-on view shows the third dimension: how far the stars reach above and below the midplane. A study of 34 edge-on galaxies found that their light is best explained by two components, a thin disk and a thick disk about twice as thick. In galaxies rotating more slowly than about 120 km/s, the thick disk supplies nearly half the light and most of the stellar mass, suggesting most of their stars are old even when these galaxies look blue.

Dust is also easiest to see edge-on, in silhouette against the starlight behind it. A survey of edge-on disks found that sharp, narrow dust lanes are common in galaxies rotating faster than about 120 km/s and largely absent in slower ones, where the cold gas and dust form a thicker, more turbulent layer. The gas in NGC 1827 rotates at about 77 km/s, and its dust here is patchy and broken rather than a single knife-edge lane, consistent with that trend.

In an edge-on disk, gas on one side moves almost straight towards us and on the other side away, so measured rotation speeds need little correction for tilt: for NGC 1827 the correction raises the speed from 76.9 km/s to only 77.2 km/s.

Part of a census of small galaxies

The image comes from Hubble program 18061, the Hubble Census of Nearby Satellites, led by Michael Jones (Caltech). It aims to count every satellite galaxy brighter than a set limit around 22 Milky Way-like host galaxies within about 32 million light-years, 260 satellites in all. Of these, 111 already had Hubble images; the other 149 get one Hubble orbit each. NGC 1827 is on the program's target list. Catalogues place it in the NGC 1808 group; its distance estimates, 36 to 37 million light-years, put it just beyond the survey's nominal edge.

Small galaxies test models of how matter clumps, which often struggle on small scales; until recently the only complete sample was the satellites of our own Galaxy and its neighbours. NGC 1827 was photographed with Hubble's Wide Field Camera 3 through two filters, F606W (a broad visible band centred near 589 nm) and F814W (near-infrared, centred near 804 nm). Hubble resolves individual stars, and two colours locate the tip of the red giant branch, the nearly fixed peak luminosity of red giants in an old population, which gives the distance.

Can you see NGC 1827?

NGC 1827 lies in Columba, at right ascension 5h 10m and declination −37°. Observers north of about 53° N never see it rise; from London (51.5° N) it reaches only about 1.5° above the southern horizon, too low to observe. From the Southern Hemisphere it is well placed: it stands highest in the middle of the night in December, and in the evening sky in January and February.

Catalogues give it a total magnitude of about 13, spread along a streak about three arcminutes long, so even in a telescope under dark skies it is a faint glow. The dust and the blue star-forming clumps show up only in long-exposure photographs.