Starship Asterisk*

A Stranger in the Crowd
ESA/HEIC Hubble Picture of the Week | 2013 Jul 15

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The constellation of Virgo (The Virgin) is the largest of the Zodiac constellations, and the second largest overall after Hydra (The Water Snake). Its most appealing feature, however, is the sheer number of galaxies that lie within it. In this picture, among a crowd of face- and edge-on spiral, elliptical, and irregular galaxies, lies NGC 4866, a lenticular galaxy situated about 80 million light-years from Earth.

Lenticular galaxies are somewhere between spirals and ellipticals in terms of shape and properties. From the picture, we can appreciate the bright central bulge of NGC 4886, which contains primarily old stars, but no spiral arms are visible. The galaxy is seen from Earth as almost edge-on, meaning that the disc structure — a feature not present in elliptical galaxies — is clearly visible. Faint dust lanes trace across NGC 4866 in this image, obscuring part of the galaxy’s light.

To the right of the galaxy is a very bright star that appears to lie within NGC 4886’s halo. However, this star actually lies much closer to us; in front of the galaxy, along our line of sight. These kinds of perspective tricks are common when observing, and can initially deceive astronomers as to the true nature and position of objects such as galaxies, stars, and clusters.

This sharp image of NGC 4866 was captured by the Advanced Camera for Surveys, an instrument on the NASA/ESA Hubble Space Telescope. A version was entered into the Hubble’s Hidden Treasures image processing competition by contestant Gilles Chapdelaine.

Credit: NASA/ESA/Hubble
Acknowledgement: Gilles Chapdelaine

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Good one! I can't believe I never came across this particular object. Most of the lenticular galaxies in Hubble's archive are either face on or a lot more spherical in shape.

It is an interesting picture and an interesting galaxy. There is a hint of a peanut-shaped bulge, suggesting a bar, and the "brightness profile" of the galaxy in my software suggests the same thing. However, my software classifies NGC 4866 as an S0-a galaxy, not an SB0-a one. In other words, according to my software, NGC 4866 isn't a barred galaxy. And other pictures of this galaxy suggest the same thing, namely, it is an unbarred galaxy.

The colors of NGC 4866 are unremarkable. Its total B-V index is 0.920 and its total U-B index is 0.490. Its effective B-V index is 0.995 and its effective u-B index is 0.560. That's definitely redder than an average spiral galaxy, whose B-V and U-B indexes may be around 0.70 and 0.10. But that is because the average spiral galaxy contains a moderate to large amount of star formation, whereas lenticulars are more or less barren.

On the other hand, our large neighbour, the Andromeda Galaxy, definitely contains young blue stars and star formation, but M31 is nevertheless marginally redder than NGC 4866. The total B-V of M31 is 0.920, just like NGC 4866, but its U-B index is as red as 0.500. And the effective B-V index of M31 is as red as 1.010.

So how can M31, a starforming spiral of Hubble class Sb, be so red? I'd say it's because M31 contains such a fantastic profusion of old red stars in its bright bulge. Visually, the bulge of M31 is bright, but the outer blue parts of the galaxy are faint.

Thanks to its bright yellow bulge, M31 is very bright overall. According to my software, it is about three times brighter than the Milky Way. We have every reason to think that the Milky Way contains at least as many young blue stars as Andromeda, so the difference in brightness between our galaxy and M31 must be due to the profusion of old red stars in Andromeda.

NGC 4866 almost certainly contains far fewer blue stars than the Andromeda Galaxy. I would guess that the dust lanes of NGC 4866 contain a few moderately young stars of spectral classes A and F, but unlike M31, they probably hold extremely few (if any) B stars and no O-type stars at all. My guess is that the dust lanes of NGC 4866 would have shown up slightly blue if the galaxy had been imaged by the now defunct GALEX ultraviolet telescope. I would say that a blue population is there in NGC 4688, but it obviously contains far fewer stars than the blue population of M31. On the other hand, NGC 4866 clearly contains far fewer red stars than M31, too. According to my software, NGC 4966 is about equally bright as the Milky Way, whereas M31 is three times brighter.

So the ratio between red and blue stars appears to be the same in M31 and in NGC 4866. M31 just has more of everything, more blue and red stars.

Ann