by Ann » Wed Jul 05, 2023 5:39 am
Understanding redshift when it comes to galaxies is not so hard. The more distant a galaxy is, the more redshift-reddened it will be. Of course, the optical color that we observe from a redshift-reddened galaxy also depends (sometimes heavily depends) on the intrinsic color of the galaxy. Nevertheless, it's a good rule of thumb to think of galaxies as more distant the more optically red-looking they are.
So in the APOD, the lower half of the slice mostly represents galaxies. The most nearby ones are mapped as blue because they are not very redshift-reddened at all. The yellow galaxies are moderately redshifted, and the red galaxies are very redshifted.
Quasars are different. They remain blue-looking out to large distances, and in their case we are not talking about mapped color, but about actual optical colors.
Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.
Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.
The spectra of the redshift-reddened quasars are quite different, but the quasars appear optically blue out to redshifts way past 3. But after redshift 4, the quasars start becoming optically red.
The color depends on what part of the quasar's total spectrum becomes optically visible because of its redshift. Quasars are intrinsically very blue - or rather, they produce copious amounts of hard shortwave radiation. At higher and higher redshifts, various parts of their total spectra become redshifted into the optical part of the electromagnetic spectrum. We keep seeing their optical colors as blue or bluish, until their redshift becomes larger than 4.
I highly recommend
this page from Sloan Digital Sky Survey, where you can see the the redshifted spectra and optical colors of quasars at different redshifts.
That is why the upper half of the "slice of the Universe" in today's APOD is so blue: It's because quasars remain optically blue up to quite high redshifts. Eventually, though, redshift does make their optical light red, which is why most of the uppermost dots in today's APOD are in fact red, from very highly redshifted quasars.
Ann
[img3="A Map of Galaxies and Quasars in the Observable Universe.
Image Credit & Copyright: B. Ménard & N. Shtarkman; Data: SDSS, Planck, JHU, Sloan, NASA, ESA"]https://apod.nasa.gov/apod/image/2307/MapObsUni_SDSS_960.jpg[/img3]
Understanding redshift when it comes to galaxies is not so hard. The more distant a galaxy is, the more redshift-reddened it will be. Of course, the optical color that we observe from a redshift-reddened galaxy also depends (sometimes heavily depends) on the intrinsic color of the galaxy. Nevertheless, it's a good rule of thumb to think of galaxies as more distant the more optically red-looking they are.
So in the APOD, the lower half of the slice mostly represents galaxies. The most nearby ones are mapped as blue because they are not very redshift-reddened at all. The yellow galaxies are moderately redshifted, and the red galaxies are very redshifted.
Quasars are different. They remain blue-looking out to large distances, and in their case we are not talking about mapped color, but about actual optical colors.
[float=left][attachment=1]Quasar gallery by redshift SDSS.png[/attachment][c][size=85][color=#0040FF]Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.[/color][/size][/c][/float] [float=right][attachment=0]Quasar gallery 2 by redshift SDSS.png[/attachment][c][size=85][color=#0040FF]Optical colors and spectra of quasars of different redshifts.
Credit: SDSS.[/color][/size][/c][/float]
[clear][/clear]
The spectra of the redshift-reddened quasars are quite different, but the quasars appear optically blue out to redshifts way past 3. But after redshift 4, the quasars start becoming optically red.
The color depends on what part of the quasar's total spectrum becomes optically visible because of its redshift. Quasars are intrinsically very blue - or rather, they produce copious amounts of hard shortwave radiation. At higher and higher redshifts, various parts of their total spectra become redshifted into the optical part of the electromagnetic spectrum. We keep seeing their optical colors as blue or bluish, until their redshift becomes larger than 4.
I highly recommend [url=https://classic.sdss.org/gallery/gal_zqso.html]this page[/url] from Sloan Digital Sky Survey, where you can see the the redshifted spectra and optical colors of quasars at different redshifts.
That is why the upper half of the "slice of the Universe" in today's APOD is so blue: It's because quasars remain optically blue up to quite high redshifts. Eventually, though, redshift does make their optical light red, which is why most of the uppermost dots in today's APOD are in fact red, from very highly redshifted quasars.
Ann