1.1 HOW-TO use Galactic extinction in Astro-WISE

We have implemented in Astro-WISE two Galactic extinction maps:

SFD:
Schlegel, D., Finkbeiner, D., & Davis, M., ApJ, 1998, 500, 525
Arenou:
Arenou F., Grenon M., Gomez A., Astron. Astrophys. 1992, 258, 104

In the case of SFD map we used an original IDL program rewritten in python
http://astro.berkeley.edu/~marc/dust/data/data.html

1.1.1 SFD extinction map: for extragalactic sources

To find the Galactic extinction towards an extragalactic object one can use the SFD map for which you have to provide galactic coordinates:

awe>longvec=45.0
awe>latvec=45.0
awe>from astro.util.extinction import extinction 
awe>ret=extinction(longvec,latvec)
awe>print ret
[0.0439861752093]

The returned value is an excess ratio (EB-V mathend000#) in the selected direction. You can use as well vectors for input coordinates:

awe>longvec=[0.0,45.0,90.0]
awe>latvec=[0.0,45.0,90.0]
awe>from astro.util.extinction import extinction 
awe>ret=extinction(longvec,latvec)
awe>print ret
[101.313850403, 0.0439861752093, 0.012209450826]

The Galactic extinction can be calculated with an interpolation between pixels closest to the desired direction:

awe>longvec=[0.0,45.0,90.0]
awe>latvec=[0.0,45.0,90.0]
awe>from astro.util.extinction import extinction 
awe>ret=extinction(longvec,latvec,interp=True)
awe>print ret
[99.697704474, 0.0443909994508, 0.0119094799738]

Note: The number precision used in the IDL code is lower than in the python implementation. This can cause differences in derived EB-V mathend000# between the two implementations in areas of highly varying extinction. Differences are < 0.1% mathend000# for 99.8% of the sky as table [*] illustrates:

Table: Differences in derived EB-V mathend000# between the IDL and python implementation for SFD
Absolute difference sky area fraction
< 0.1% mathend000# 99.8 %
< 1% mathend000# 99.8 %
> 5% mathend000# 00.02 %
> 10% mathend000# 00.004 %
> 50% mathend000# 00.0004 %

1.1.2 Arenou extinction map: inside the Galaxy

Arenou extinction model based on Hipparcos data and provides an extinction inside the Galaxy, i.e., for a selected distance. The user can provide a distance (in kpc), if the distance is omitted, an extinction for 15 kpc will be returned (according to the model).

awe>longvec=[0.0,45.0,90.0]
awe>latvec=[0.0,45.0,90.0]
awe>from astro.util.extinction import extinction 
awe>ret=extinction(longvec,latvec,source='Arenou')
awe>print ret
[0.51390040827009664, 0.017030418212218647, 0.032073867112540198]

or, for 100 pc distance,

awe>longvec=[0.0,45.0,90.0]
awe>latvec=[0.0,45.0,90.0]
awe>d=[0.1,0.1,0.1]
awe>from astro.util.extinction import extinction 
awe>ret=extinction(longvec,latvec,source='Arenou',dist=d)
awe>print ret
[0.08085090032154342, 0.017030418212218647, 0.0032773954983922873]

1.2 Coordinate transformation

A number of functions for coordinate transformations are available in Astro-WISE (which are based on the IDL astro library).

  1. glactc Convert between celestial and Galactic (or Supergalactic) coordinates.

    Input parameters:

    ra
    right ascension, hours (or degrees if degree=True is set), scalar
    dec
    declination, degrees, scalar
    year
    equinox of ra and dec, scalar
    degree
    if degree=True, both coordinates are in degree (overwise ra is in hours), degree=False by default
    fk4
    if fk4=True, then coordinates are assumed to be in FK4, if fk4=False (default), FK5 is assumed. By B1950 coordinates use year=1950 and fk4=True
    SuperGalactic
    SuperGalactic=False by default, if SuperGalactic=True, SuperGalactic coordinates are returned (deVaucouleurs et al. 1976), to account for the local supercluster. The North pole in SuperGalactic coordinates has Galactic coordinates l = 47.47, b = 6.32, and the origin is at Galactic coordinates l = 137.37, b= 0 )
    eqtogal
    direction of conversion, eqtogal=True by default, if eqtogal=False, the input coordinates (ra, dec) are galactic coordinates and returned coordinates are celestial ones

    Example: Convert coordinates (0.0,0.0) to galactic coordinates

    awe>from astro.util.idllib import glactc
    awe>glactc(0.0,0.0,2008.0)
    (96.112413056666824, -60.188305254568284)
    

    Convert galactic coordinates (0.0,0.0) to FK4 coordinates for epoch 2008.0, in degrees

    awe> from astro.util.idllib import glactc
    awe> glactc(0.0,0.0,2008.0,degree=True,fk4=True,eqtogal=False)
    (266.53170097124888, -28.938911978654406)
    

  2. precess Precess coordinates between two epochs

    Input parameters:

    ra
    right ascension, degrees (or radians if radian=True is set), scalar
    dec
    declination, degrees (or radians if radian=True is set), scalar
    equinox1
    original equinox of coordinates, numeric scalar
    equinox2
    equinox of precessed coordinate, numeric scalar
    fk4
    if fk4=True, then coordinates are assumed to be in FK4, if fk4=False (default), FK5 is assumed
    radian
    if radian=True, coordinates must be in radians, by default radians=False

    Example:

    awe> from astro.util.idllib import precess
    awe> ra=329.887720833
    awe> dec=-56.9925147222
    awe> precess(ra, dec, 1950.0,1975.0,fk4=True)
    (330.3144305415542, -56.871861264857067)
    



page generated Tue Apr 4 14:29:28 CEST 2017