Chris Peterson wrote:
Ann wrote:There can be no doubt that NGC 7497 is reddened by dust in our galaxy, so it is interesting to check out the colors of NGC 7497. Its B-V index is 0.720 and its U-B index is 0.150.
Color indices are pretty useless for galaxies reddened by intervening dust. That's because you can only use the ratio or difference of a pair of colors to yield temperature information when you are working with a blackbody (as true for galaxies as for stars). That requires no filtering or selective scattering interfering with the light we receive. Otherwise, you need to look at the full spectrum and do a more sophisticated analysis.
Of course, these days color indices aren't much used, anyway, except as screening tools to suggest targets that might be interesting for further study.
Well, the (effective) color indices for IC 342 are 0.280 (U-B) and 1.100 (B-V). That is clearly redder than the colors for NGC 7479, although the intrinsic
colors of IC 342 might well be bluer than for NGC 7497. My impression is that the yellow population of IC 342 is not as widespread as the yellow population of NGC 7497.
IC 10 is even more reddened. Its total B-V index is 1.430, while its effective B-V index is 1.280. Its total U-B index is 0.300, while its effective U-B index is 0.360. Fascinatingly, its apparent (unreddened by external factors) B-V index is given as 0.906, which is pretty red, while its apparent U-B index is -0.244, which is pretty blue (or rather, it is pretty ultraviolet). We know that IC 10 is a starburst galaxy that is going to be bright in ultraviolet light, but it will also be dusty. Clearly, however, our own galaxy contributes a lot of reddening to the light that reaches us from IC 10.
As for Maffei 2, a nearby spiral galaxy, its light is so reddened by dust in our own galaxy that Simbad Astronomical database
only gives its B magnitude as 16 (with no decimals), while its J, H and K infrared magnitudes are given as seventh, sixth and fifth magnitude (with two or three decimals). Maffei 1, a nearby elliptical (the nearest elliptical to the Local Group) is fainter than Maffei 2 in B magnitude (17 according to Simbad), while its infrared magnitudes are a little brighter than those of Maffei 2.
So I find color indices very interesting and quite revealing.