SED science cases
kamp at stsci.edu
Tue Nov 21 06:13:02 PST 2006
sorry, it took me a little longer to get back to this one.
On Tue, 19 Sep 2006, Anita Richards wrote:
> In Moscow Doug asked what science users wanted from SEDs and I think the
> question deserves more attention. Here are some priorities drawn from use
> cases which have arisen from AstroGrid and VOTech science cases and
> user workshops, to get the ball rolling.
> Firstly, in general an SED is any sequence of flux density measurements
> along a spectral axis - there are already more precise VO definitions but
> the main point here is that high-resolution regularly-sampled spectra
> _can_ be thought of as SEDs but in many cases more dedicated spectral
> tools and standards are more suitable. However high-res spectra can be
> mixed with individual broad-band photometry points in an SED (with some
> caveats, see below).
> SEDs may come from a single data source or many.
SEDs may even come from different objects to create a "typical" SED e.g.
QSOs, YSOs, etc.
> The more fundamental requirements come first, but this does not mean that
> the tool components have to work in this order:
> The sources need to be cross-matched. That typically gives a table with
> many sources, with column labels for each waveband. I believe that
> current tools need this to be split and transposed with one table per
> source, with one wavelength (etc.) column with many values and one flux
> column with many values. (can be done using STILTS)
> The units need to be homogenised. This can be done easily to moderate
> accuracy (e.g. VOSpec methods, STILTS etc) in some regimes/some units; in
> the case of optical magnitudes or X-ray flux densities (and even nominal
> band centres) it may be model-dependent and is often largely manual at
> present. This is being worked on by several EuroVO partners (AstroGrid,
> ESO et al.). In general the current outputs are good enough for e.g.
> diagnostics using SEDs stretching from X-ray to radio, but not necessarily
> for <5% accuracy e.g. for calculating photmetric redshifts from optical
> data, unless the input data are all from a homogenous source.
> The SED needs error bars, if possible to take into account conversion
It also needs an indication for the bandpass, so that tools can be able to
display vertical "measurement bars". It might be interesting to see if a
value was derived from narrow band photometry or a broadband filter.
> For data taken with many instruments, aperture matching may or may not be
> a consideration - e.g. for a star, is it unresolved in each data set?
> Might ther be more than one source in low resolution observations?
And don't forget resolved targets such as disks, clouds, envelopes, etc.
The different instruments may have looked at different parts of a nebula
or circumstellar disk.
> Are there any issues in including high-res spectra, whether or not it
> contains lines?
There will be problems with the aperture, but otherwise I see nothing what
would prevent us from doing it.
> Finally, ESO are aworking on a tool to also include data extracted from
> images, and AstroGrid are working on a model-fitting tool (using e.g. the
> SVO models)
What about having an option to calculate fractional luminosities. That
would be very useful. Some people may want to know what the L_UV/L_star
is, or the L_IR/L_star. The user should be allowed to choose the
wavelength bands. Overplotting of synthetic stellar spectra is probably
already there as a functionality.
In working towards synthetic SEDs, it might be useful to have the option
to convolve the synthetic spectrum with the filter properties stated in
the metadata of the observed SED and obtain a direct comparison. This is
of course assuming that the data provider give a full characterization of
their filters. So, more than just J-band:)
With a tool that has a fitting algorithm, it should be straightforward to
allow the user to derive a magnitude/flux at any point he wants. This
might be useful for observation planing. If you have optical and near-IR
data and fit that assuming e.g. blackbodies, you should be able to get a
guess for the far-IR fluxes and magnitudes to stick into proposals and
exposure time calculators. Same if you have some gaps you want to fill
with new observations.
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Dr. Anita M. S. Richards, AstroGrid Astronomer
> MERLIN/VLBI National Facility, University of Manchester,
> Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
> tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
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