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

If you have actually ever taken an introductory astronomy course, among the first things you learned around galaxies is that elliptical galaxies are red, and spiral galaxies are blue. Two significant classifications or morphologies (plus irregulars and also lenticulars), two unique colors – done. Cshed textbook. We have actually an easy means to identify between galaxy types without looking too very closely at them, and we can relocate on through our resides. However before, it shows up that the genuine image may not be so simple: the exploration of a population of red spiral galaxies in the neighborhood universe (and some blue, elliptical-prefer spheriods, however we won’t get into that here) has led astronomers to begin to doubt this standard paradigm.

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The writer of this research study note sets off to investigate this odd populace of spiral galaxies and uncover what is, or is not, responsible for their rosy appearance.

Before we deserve to puzzle out the reason this seemingly abnormal group exists, we first should revisit the physical explanation for the optical colors of “normal” galaxies. In slightly even more technological terms than “blue” and “red”, astronomers say that spirals live in the “blue cloud” of a color-magnitude diagram, ellipticals live in the “red sequence”, and a couple of interlopers fall into the “green valley”. (See this astrobite for a diagram and a similar discussion). The galaxies in the blue cloud are proactively creating stars; their colors are overcame by emission from the youngest stars, which are the brightest and also hottest, and also therefore the bluest à la blackbody radiation (prefer the bottom of a flame). In contrast, galaxies in the red sequence are mostly devoid of gas and cannot create many kind of new stars. Because substantial, blue stars evolve and explode as supernovae fairly conveniently, only the long-lived, low mass stars – which are the dimmest and also coolest, and also therefore the a lot of red – are thought to reprimary in these forms of galaxies; for this reason, we say they are “red and also dead”.

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

And now, time for the strange galaxies. Citizen researchers contributing to Galaxy Zoo have actually helped astronomers uncover about 300 red spiral galaxies in the Sloan Digital Sky Survey (SDSS). (Keep in mind that individually classifying every one of the galaxies in the SDSS would certainly have been a near-difficult job for experienced astronomers without assistance – view Galaxy Zoo co-founder Kevin Schawinski’s guest short article around exactly how to get affiliated in citizen scientific research yourself!) To find out if these red galaxies are also dead, the writer of this paper adds ultraviolet (UV) information from the Galaxy Evolution Explorer (GALEX) in addition to infrared (IR) data from the Wide-field Infrared Survey Explorer (WISE) to the optical information from SDSS for 255 of the original galaxies. Ultraviolet emission can be used to infer the star formation price, discovering that the huge majority of the UV again originates from the youngest, hottest, the majority of massive stars, and also assuming some underlying circulation of masses for the stars that aren’t emitting in the UV (the Initial Mass Function, or IMF – e.g. these Sindicate measuring the UV emission isn’t sufficient, as some of this radiation is took in by dust grains alengthy the way. Luckily, the UV pholoads warm the dust grains, resulting in them to emit thermally in the infrared (IR); thus, if we have information in both the UV and the IR parts of the spectrum, we can incorporate them to infer how much UV emission tried to leave the galaxy in the initially place.

Figure 2b: Edge-on galaxy (SDSS)

Figure 2a: Face-on galaxy (SDSS)

Of course, there are two perfectly enough yet mundane reasons why spiral galaxies can be red. Happily, the authors of the original paper in which these data were publimelted (Masters et al. 2010) have actually currently pared down the sample to protect against such confusion. The first explanation is reddening of by dust, which scatters blue light more properly than red light, relocating blue light out of our line-of-sight. This might reason inherently blue galaxies to show up red. The same physics (Rayleigh scattering) defines why the sky is blue while sunsets are red; in the first case, we are looking at instraight, scattered light, and in the latter situation, we are looking directly at the Sun via a thick, light-scattering setting. To cut dvery own on dusty galaxies, the authors pick galaxies that are almost face-on (Figure 2a), as opposed to edge-on (Figure 2b), based on the proportion of their projected significant versus minor axes. An picture of our very own Milky Way in the infrared (e.g. from WISE, as disputed in this astrobite) reflects the logic in this technique – all of the dust lies in the plane of the Galaxy, so an edge-on galaxy will be much even more influenced by reddening than a face-on galaxy.

The second factor that could normally redden a spiral galaxy hregarding execute through the size of its central bulge relative to the star-creating disk. Ala lot of all spirals have actually this spherical, quiescent (and also red) component, and also in the instance wright here the bulge dominates the luminosity, the galaxy can hence quickly appear red.

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For each galaxy, the authors model the surconfront brightness profile – which explains how a lot light the galaxy emits as a duty of position, right here defined solely by the distance from the galaxy facility – by treating the disk and the bulge as two distinct components, using an exponential disk for the previous and also a de Vaucoulers law for the latter. They then compare the contributions from the 2 propapers, selecting just galaxies wright here the bulge contributes much less that 50% of the light, or fDeV 

Figure 3: Increasing bulge fraction from left to best (Masters et al. 2010). fDeV is the fractivity of the galaxy's luminosity contributed by the bulge; a greater fDeV leads to a a lot redder looking galaxy.

Figure 4: NUV - r color versus stellar mass. Contours show a distribution of regional galaxies, and red points are the red spirals in this sample.

Now that we have ruled out the uninteresting choices, we have the right to ask the question we are really after: are the red spiral galaxies forming stars? The answer, surprisingly, is yes! Not just are these red spirals still active, yet they seem to be developing stars at the exact same rate as other spirals in the local universe. The contours of Figure 4 show the underlying bimodal distribution of regional galaxies as categorized by their near-UV (NUV) to optical (r-band) shade (lower NUV – worths show bluer colors). From here, it is obvious that the majority of the spirals from this sample, which are plotted as red points in the number, autumn in line through the typical blue spirals. So, why do they appear red in the optical? The writer argues that the mass of the galaxy is the vital parameter. All of these red spirals are enormous (> 10^10 solar masses), and although they are forming stars at a normal price, a big, underlying population of older, red stars dominates the stellar mass content in these galaxies, shifting their optical shade to the red. Hence, we are left through a cautionary tale: at high masses, optical shade is no longer an excellent proxy for morphology, and also a red spiral galaxy might be nothing even more than a red herring!