[AstroPy] missing distance in galactic coordinates from catalog

[Adrian Price-Whelan]

Hi Rodrigo --

The .from_name() functionality is designed to only pull sky coordinates for
sources, so there is currently no way to use this to retrieve distance
measurements.

For many sources (especially the ones you list in your code snippet), there
are often many different options for distance measurements, and choosing
the "best" one often depends on a given researcher's needs or preferences.
We therefore recommend that you construct the SkyCoord instances
explicitly, e.g.:
import astropy.coordinates as coord
import astropy.units as u
galcen = SkyCoord(l=359.94425934*u.deg, b=-0.04616236*u.deg,
distance=XX*u.kpc, frame='galactic')

In the case of the Galactic center, if you are looking for a reasonable,
recent measurement, you may be interested in this paper:
https://ui.adsabs.harvard.edu/abs/2018A%26A...615L..15G/abstract

best,
Adrian

On Mon, Feb 3, 2020 at 11:04 AM Rodrigo Amestica <ramestica at gmail.com>
wrote:

> Hi,
>
> I assumed that the code below would return galactic coodinates including a
> known distance to the object.
>
> However, what I get are the two galactic angles and the distance seems to
> always default to 1.0, which seems to convey no distance information at
> all.
>
> I suppose that that happens because the actual distance is not known with a
> given accurracy?
>
> I'm interested on querying for very well known Milky Way objects (like in
> the example below), to map their position in the galaxy; without much
> concern about accurracy. Is it there a different approach that I could
> follow to get complete galactic coordinates?
>
> Rodrigo
>
> from astropy.coordinates import SkyCoord
> for i in ['Galactic Center', 'Cygnus A', 'Eta Carinae']:
>     c = SkyCoord.from_name(i)
>     print(i)
>     print('\t', c.galactic, '\t', c.galactic.distance)
>
>   ,----
>   | Galactic Center
>   |      <SkyCoord (Galactic): (l, b) in deg
>   |     (359.94425934, -0.04616236)>     1.0
>   | Cygnus A
>   |      <SkyCoord (Galactic): (l, b) in deg
>   |     (76.18988033, 5.75538823)>       1.0
>   | Eta Carinae
>   |      <SkyCoord (Galactic): (l, b) in deg
>   |     (287.59678845, -0.62951118)>     1.0
>   `----
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>


-- 
Adrian M. Price-Whelan
Flatiron Institute, NYC
http://adrn.github.io
(he / him)
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