Authors: L. CorteseFirst Author’s Institution: European southerly Observatory, Garching, Germany

If you have ever taken an introduce astronomy course, one of the an initial things you learned around galaxies is that elliptical galaxies room red, and spiral galaxies are blue. Two significant classifications or morphologies (plus irregulars and lenticulars), two unique colors – done. Nearby textbook. We have an easy way to distinguish between galaxy species without looking too carefully at them, and also we have the right to move on through our lives. However, it shows up that the real picture may not be therefore simple: the discovery of a population of red spiral galaxies in the neighborhood universe (as well as some blue, elliptical-like spheriods, however we won’t obtain into that here) has led astronomers to begin to doubt this traditional paradigm.

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The author of this research note sets turn off to inspection this odd population of spiral galaxies and also uncover what is, or is not, responsible for your rosy appearance.

Before we can puzzle the end the reason this watch abnormal group exists, we first need come revisit the physics explanation because that the optical colors of “normal” galaxies. In slightly an ext technical terms 보다 “blue” and “red”, astronomers say that spirals live in the “blue cloud” that a color-magnitude diagram, ellipticals live in the “red sequence”, and a couple of interlopers loss into the “green valley”. (See this astrobite because that a diagram and a comparable discussion). The galaxies in the blue cloud are actively forming stars; their colors are conquered by emissions from the youngest stars, which space the brightest and also hottest, and also therefore the bluest à la blackbody radiation (like the bottom the a flame). In contrast, galaxies in the red succession are greatly devoid that gas and also cannot type many brand-new stars. Due to the fact that massive, blue stars evolve and also explode as supernovae relatively quickly, just the long-lived, short mass stars – which space the dimmest and also coolest, and therefore the many red – space thought to remain in these varieties of galaxies; thus, we say they room “red and dead”.

Figure 1: Red spirals (top) and normal, blue spirals (bottom) (Masters et al. 2010).

And now, time because that the weird galaxies. Citizen scientists contributing come Galaxy Zoo have actually helped astronomers uncover around 300 red spiral galaxies in the Sloan Digital sky Survey (SDSS). (Note the individually classifying every one of the galaxies in the SDSS would have actually been a near-impossible task for expert astronomers without help – check out Galaxy Zoo co-founder Kevin Schawinski’s guest post around how to get associated in citizen scientific research yourself!) To find out if these red galaxies are likewise dead, the author of this paper adds ultraviolet (UV) data indigenous the Galaxy development Explorer (GALEX) together with infrared (IR) data native the Wide-field Infrared Survey explorer (WISE) come the optical information from SDSS for 255 of the original galaxies. Ultraviolet emission have the right to be offered to infer the star formation rate, discovering that the vast majority of the UV again comes from the youngest, hottest, most massive stars, and assuming some underlying circulation of masses because that the stars the aren’t create in the UV (the Initial mass Function, or IMF – e.g. these Simply measuring the UV emission isn’t enough, as few of this radiation is absorbed by dust grains follow me the way. Luckily, the UV photons warmth the dust grains, leading to them to emit thermally in the infrared (IR); thus, if we have actually data in both the UV and also the IR parts of the spectrum, us can incorporate them come infer just how much UV emissions tried to leaving the galaxy in the an initial place.

Figure 2b: Edge-on galaxy (SDSS)

Figure 2a: Face-on galaxy (SDSS)

Of course, there space two perfectly enough yet mundane reasons why spiral galaxies can be red. Happily, the authors of the original file in which these data were released (Masters et al. 2010) have already pared under the sample to avoid such confusion. The an initial explanation is reddening of by dust, i beg your pardon scatters blue light more effectively than red light, relocating blue light the end of ours line-of-sight. This could reason intrinsically blue galaxies to appear red. The same physics (Rayleigh scattering) explains why the skies is blue if sunsets space red; in the very first case, we are looking at indirect, scattered light, and in the last case, we room looking straight at the sun through a thick, light-scattering atmosphere. To reduced down top top dusty galaxies, the authors choose galaxies the are virtually face-on (Figure 2a), as opposed to edge-on (Figure 2b), based upon the ratio of their projected major versus young axes. An image of our very own Milky way in the infrared (e.g. From WISE, as questioned in this astrobite) shows the logic in this method – all of the dust lies in the aircraft of the Galaxy, so one edge-on galaxy will be much much more affected by reddening 보다 a face-on galaxy.

The 2nd factor that could naturally redden a spiral galaxy has to do with the size of its central bulge family member to the star-forming disk. Practically all spirals have actually this spherical, quiescent (and red) component, and in the case where the bulge constrain the luminosity, the galaxy could thus easily show up red.

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For each galaxy, the authors model the surface ar brightness profile – which explains how lot light the galaxy emits together a function of position, here defined solely by the distance from the galaxy facility – by dealing with the disk and the bulge as two distinct components, utilizing an exponential disk because that the former and also a de Vaucoulers law because that the latter. They then to compare the contribute from the 2 profiles, selecting only galaxies wherein the bulge contributes much less that 50% the the light, or fDeV 

Figure 3: increasing bulge fraction from left to ideal (Masters et al. 2010). FDeV is the fraction of the galaxy's luminosity contributed by the bulge; a higher fDeV leads to a lot redder feather galaxy.

Figure 4: NUV - r shade versus stellar mass. Contours display a distribution of neighborhood galaxies, and also red points space the red spirals in this sample.

Now the we have actually ruled out the uninteresting options, we can ask the inquiry we space really after: are the red spiral galaxies creating stars? The answer, surprisingly, is correctly! Not just are this red spirals tho active, however they seem to be creating stars at the same rate as various other spirals in the regional universe. The border of figure 4 display the basic bimodal circulation of local galaxies as categorized by your near-UV (NUV) come optical (r-band) color (lower NUV – values indicate bluer colors). Native here, that is noticeable that the bulk of the spirals from this sample, which room plotted together red clues in the figure, fall in line with the usual blue spirals. So, why perform they show up red in the optical? The author argues that the fixed of the galaxy is the an essential parameter. All of these red spirals are massive (> 10^10 solar masses), and also although castle are forming stars in ~ a typical rate, a large, underlying populace of older, red stars dominates the mainly mass content in this galaxies, changing their optical shade to the red. Thus, we are left with a cautionary tale: in ~ high masses, optical color is no much longer a an excellent proxy for morphology, and a red spiral galaxy might be nothing an ext than a red herring!