The SchmidtV

The SchmidtV - such as having the wrong limiting flux can...

This preview shows page 1. Sign up to view the full content.

The Schmidt V/V m test (ApJ 151, 393, 1968) uses the fact that any selection technique has some limiting flux or magnitude, and that we would ordinarily expect to find objects anywhere within the associated volume unless they occur preferentially at larger distances (greater lookback times). If we see individual objects at distances R obs and expect to detect them to R limit , then for a nonevolving population we would have the quantity V/V m = ( R obs /R limit ) ³ uniformly distributed from 0 to 1 for whole samples. Any error in the assumed limiting magnitude can only reduce the mean value. Observed samples of QSOs give V/V m = 0.51 - 0.66, so there must be a real local decrease in quasar number density. In practice, various corrections for color and emission-line properties are necessary (see Weedman chapter 6). Selection effects
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: such as having the wrong limiting flux can only make the mean value of V/V m less than 0.5, so that higher values may be taken as either a strong geometrical effect or evidence for some kind of evolution in the luminosity function. If we were sure of our world model, we could use a luminosity-distance formulation instead of simple r in calculating V/V m . As set out by Schmidt 1968, we then use as a distance measure not just cz/H but multiply this by This and related techniques show a strong increase in the number of luminous QSOs with increasing redshift up to about z =2.5, with the comoving density varying approximately as (1+z) 5 ....
View Full Document

This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

Ask a homework question - tutors are online