A Using the Sloan Digital Sky Survey to Extract Quasar Spectra and Fit Models

[quasarLie]

Hello,
I was wondering if anyone here had experience using the dataset of the Sloan Digital Sky Survey, i want to exract quasar's spectra (precise quasars of which I have the reference).
Thanks

 

[phyzguy]

If you know the positions or the names, you can just go to the SDSS skyserver. Here is an example of a quasar with a redshift of 0.84. To see the spectrum, just click on the spectrum in the lower right. If you click on "Quick Look", you can also download the spectrum as a CSV or FITS file, or explore it with the interactive spectrum tool.

http://skyserver.sdss.org/dr14/en/tools/chart/navi.aspx?ra=10.17383414&dec=0.09007143&scale=0.2This link also shows you how to search for quasars.

It's also fun just to pan around. If you turn on the "Objects with Spectra" then objects that have measured spectra will have boxes around them. The link below shows a bright broadline quasar at Z=2.5.

http://skyserver.sdss.org/dr14/en/tools/chart/navi.aspx?ra=10.14253&dec=0.23243&scale=0.2

 

[Chronos]

Vizier is a very good source for astronomical catalogues. It should have all the SDSS releases. You can even sort the data yourself, if so desired.

 

[quasarLie]

Thank you for your answer, i m trying to find spectra for this quasar for example. Can you hgelp me with it please there is a lot of parameters I am a bit lost

nomLQAC type A.D. Decl. redshift u_SDSS b_unknown g_Gaia v_unknown g_SDSS r_SDSS i_SDSS z_SDSS J_2MASS K_2MASS

LQAC_138+060_011 QUASAR 138.2684968514 60.0162492586 3.408 26.855 0.0 20.352 0.0 21.11 20.235 19.902 19.968 0.0 0.0

 

[phyzguy]

Thank you for your answer, i m trying to find spectra for this quasar for example. Can you hgelp me with it please there is a lot of parameters I am a bit lost

nomLQAC type A.D. Decl. redshift u_SDSS b_unknown g_Gaia v_unknown g_SDSS r_SDSS i_SDSS z_SDSS J_2MASS K_2MASS

LQAC_138+060_011 QUASAR 138.2684968514 60.0162492586 3.408 26.855 0.0 20.352 0.0 21.11 20.235 19.902 19.968 0.0 0.0

Here it is:
http://skyserver.sdss.org/dr14/en/t...ra=138.2684968514&dec=60.0162492586&scale=0.2

You just type the A.D. number (138.26849.. I'm not sure what A.D stands for, but this is the Right Ascension in degrees) in the ra line, and the Decl number (60.162...) in the dec line and hit search and it pops up. I'm sure this is the right object, because the redshift (z=3.408) and the magnitudes (like u_SDSS = 26.855 etc.) match.

The spectrum is in the lower right. This guy is a looong ways away. Note the LyAlpha line is shifted all the way from its rest wavelength of 1215 Angstroms into the visible at about 5400 Angstroms.

Do you understand what all of the parameters in your line are?

 

[quasarLie]

Here it is:
http://skyserver.sdss.org/dr14/en/t...ra=138.2684968514&dec=60.0162492586&scale=0.2

You just type the A.D. number (138.26849.. I'm not sure what A.D stands for, but this is the Right Ascension in degrees) in the ra line, and the Decl number (60.162...) in the dec line and hit search and it pops up. I'm sure this is the right object, because the redshift (z=3.408) and the magnitudes (like u_SDSS = 26.855 etc.) match.

The spectrum is in the lower right. This guy is a looong ways away. Note the LyAlpha line is shifted all the way from its rest wavelength of 1215 Angstroms into the visible at about 5400 Angstroms.

Do you understand what all of the parameters in your line are?

Thanks a lot, I did it but i wanted to be sure :p yes I understand what the parameters are. But why the LymanAlpha line is shifted ?

 

[phyzguy]

Thanks a lot, I did it but i wanted to be sure :p yes I understand what the parameters are. But why the LymanAlpha line is shifted ?

Do you understand what redshift is?

 

[quasarLie]

Yes I know what redshift is, and to correct it I have to use K correction??

 

[phyzguy]

Yes I know what redshift is, and to correct it I have to use K correction??

If you understand redshift, what did you mean when you asked why the Lyman alpha line is shifted? What is the rest wavelength of Lyman alpha? Can you tell me where you would expect to find the Lyman Alpha line in a source with z=3.408?

K-correction is used to correct the filter magnitudes. It isn't used when dealing with spectra.

 

[quasarLie]

OK thanks, and do you know why it is written STar in the object type while i am putting Quasar's informations??

 

[phyzguy]

OK thanks, and do you know why it is written STar in the object type while i am putting Quasar's informations??

I don't know for certain, but I think it was originally classified as a star when it was chosen as a target, and only later re-classified as a quasar after examining the spectrum.

 

[quasarLie]

I don't know for certain, but I think it was originally classified as a star when it was chosen as a target, and only later re-classified as a quasar after examining the spectrum.

Thanks, when i download the spectra i have 4 columns wavelength flux bestfit and skyflux. Do you know what they mean by BestFit and skyFlux please?

 

[phyzguy]

In addition to measuring the spectrum, they fit a model to it. The model includes the type of galaxy, the redshift, and other parameters. The BestFit column is the prediction of the model. In the plots you can see the model plotted in addition to the measured spectrum (I think the BestFit is in black and the measured flux is in gray). The skyFlux is the part of the measured flux which is not from the quasar, meaning it is from the sky background. The night sky is not perfectly dark - there is always some light coming from the sky even if there is no star or galaxy there. Ideally you want the object flux to be much larger than the skyFlux. The larger the skyFlux is compared to the object flux, the more uncertain you are about how much of the measured flux is actually from the quasar.