same coordinates with different redshifts

Question

Hi,

I found several cases of objects with identical coordinates and different redshift in the zall-tilecumulative-edr-vac.fits catalog. How is it possible? Multiple passes ? And how to have 1 redshift only (possibly the best) for each object? Is there a flag for selecting it?

One example (ra,dec,z)

[[236.03794862  42.80611495   0.23968727]
 [236.03794862  42.80611495   0.23971426]
 [236.03794862  42.80611495   0.23971734]
 [236.03794862  42.80611495   0.23975127]
 [236.03794862  42.80611495   0.23981638]

One more

 [236.04066257  42.81004426   1.85358045]
 [236.04066257  42.81004426   1.85346945]
 [236.04066257  42.81004426   1.85236989]
 [236.04066257  42.81004426   1.85387375]]

Thanks, S.

Answer 1

Hello S,

You should look at the TILEID column. If those objects are on different tiles, then indeed, same object, different pointings. The ZCAT_PRIMARY column is set to True for the best spectrum of a given object within that file.

Answer 2

The redshifts are not accurate to more than 2 or 3 decimal places because of limitations in radial velocity precision from both the instrument and line formation at the source galaxy.