[AstroPy] missing distance in galactic coordinates from catalog

[Rodrigo Amestica]

Hi Adrian,

thanks for your response and description.

My reading of your reply is that there is no bullet-proof recipe to query
for the distance to an object. For that sort of information one needs to
know very well how the value is to be used and, eventually, consult papers
like the one linked before; that is, not as simple as just querying a
catalog.

Thanks,
 Rodrigo

ps: I recognize some colleagues from my previous professional life in that
paper's authors list.

On 2020-02-03T11:17:56-0500, Adrian Price-Whelan wrote:
> 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
> >   `----
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy at python.org
> > https://mail.python.org/mailman/listinfo/astropy
> >


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