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R: Driver & Robotham (2010) cosmic variance calculator

cosvar

R Documentation

Driver & Robotham (2010) cosmic variance calculator

Description

The main cosmic variance calculator function taken from Driver & Robotham (2010). cosvarcar is an interface to the Cartesian coordinate version, whilst cosvarsph is a utility interface to give approximate cosmic variance for astronomy survey regions (usually defined by RA, Dec and redshift limits).

Usage

cosvarcar(aside = 50, bside = 50, cside = 50, regions = 1)
cosvarsph(long = c(129, 141), lat = c(-2, 3), zmax = 1, zmin = 0, regions = 1,
inunit='deg', sep=":")

Arguments

`aside`	The aside (shortest projected side) of the Cartesian box, must be defined using 737 cosmology.
`bside`	The bside (longest projects side) of the Cartesian box, must be defined using 737 cosmology.
`cside`	The cside (radial side) of the Cartesian box, must be defined using 737 cosmology.
`regions`	How many well separated regions of this size will there be?
`long`	Upper and lower longitude (RA) limits of interest in units of inunit. If of length 1 then the number specified is assumed to be the upper limit and the lower limit is set to 0.
`lat`	Upper and lower latitude (Dec) limits of interest in units of inunit. If of length 1 then the number specified is assumed to be the upper limit and the lower limit is set to 0.
`zmax`	Maximum redshift of comoving cone.
`zmin`	Minimum redshift of comoving cone.
`inunit`	The units of angular coordinate provided. Allowed options are deg for degress, amin for arc minutes, asec for arc seconds, rad for radians and sex for sexigesimal (i.e. HMS for RA and DMS for Deg).
`sep`	When inunit='sex', sep defines the type of separator used for the HMS and DMS strings (i.e. H:M:S and D:M:S would be sep=':', which is the default). See `hms2deg` and `dms2deg` for more details.

Details

These functions use the empircally motivated cosmic variance percentage formula provided in Driver & Robotham (2010) Eqn 4.

cosvarsph is a 'best effort' approximation of the comoving box subtended by the specified spherical coordinates using the following conversions:

CoDistLow = cosdistCoDist(z=zmin,H0=70,OmegaM=0.3) CoDistHigh = cosdistCoDist(z=zmax,H0=70,OmegaM=0.3) cside=CoDistHigh-CoDistLow area=skyarea(long = long, lat = lat, inunit = inunit, outunit='deg2')[1] volume=cosvol(area=area, zmax = zmax, zmin=zmin, H0 = 70, OmegaM = 0.3, inunit='deg2')[1] aside=cos(mean(lat)*pi/180)*(abs(diff(long))/360)*(CoDistLow+cside/2) bside=(abs(diff(long))/180)*(CoDistLow+cside/2) scale=sqrt(volume*1e9/(aside*bside*cside)) aside=aside*scale bside=bside*scale return(cosvarcar(aside=aside, bside=bside, cside=cside, subsets=subsets))

Value

The output is the approximate percentage cosmic (or sample) variance that is expected for the volume specified.

Note

Many people get upset at the term 'cosmic variance' and prefer 'sample variance'. Whilst I am sympathetic to the argument, more astronomers are familiar with the former term.

These cosmic variance estimates are defined using SDSS at z~0.1, caveats abound at higher redshifts, but these numbers should serve as a reasonably conservative (i.e. pessimistic) upper limit.

Author(s)

Aaron Robotham and Simon Driver

References

Driver & Robotham, 2010, MNRAS, 407, 2131

Examples

#Approximate CV of the GAMA equatorial regions:
cosvarsph(long=12, lat=5, zmax=0.5)*1/sqrt(3)
#Or using the GAMA sexigesimal coordinates (should be the same):
cosvarsph(long = c('11:36:0','12:24:0'), lat = c('-2:0:0','3:0:0'), zmax=0.5,
inunit='sex')*1/sqrt(3)
#Approximate CV of the SDSS:
cosvarsph(long=150, lat=100, zmax=0.3)