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GAIA and cosmology
 
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Anze Slosar



Joined: 24 Sep 2004
Posts: 204
Affiliation: Brookhaven National Laboratory

PostPosted: January 27 2005  Reply with quote

GAIA is an ESA mission mostly dedicated to stellar astrophysics, radial velocities, planets and similar... However, full sky in many colours can't hurt anybody...

In particular, I've found the following predictions (http://astro.estec.esa.nl/GAIA/Info_sheets_science_topics.html):

  • 500.000 quasars with photometric redshifts (?), 2000 expected to be lensed...
  • 30% of all SN Ia within 630 Mpc: very local, but you could really look into "standard candleness" problem
  • Microlensing: don't have time to go into it, but in my understanding they hope to break some degeneracies by having nost just the light curve, but also the astrometry with idiotic accuracy and thus measured the mass of the lens
  • lesing: the astrometry should be accurate enough to see the light bending by the solar system, so you need a full GR to do stuff properly. Tests of GR, maybe?
  • Not explicitly said there, but could you do anything useful with photometric redshifts for objects brighter than −20? Something like very shallow but full sky galaxy survey? (don't have feeling for that and need to sit down to do calculation of how this translated to redshift).
  • They do seem to have a spectrograph, but I am told its useless for cosmology, because it has very limited lambda range.


All in all, I couldn't find a single astro-ph paper on this theme, which is quite interesting.
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Gil Holder



Joined: 25 Sep 2004
Posts: 27
Affiliation: McGill University

PostPosted: January 27 2005  Reply with quote

Very interesting. I'd just assumed it was a star thing, but it looks like it does a lot.

I don't fully understand their SNe calculation. It seems like there must be significant chunks of sky that don't get visited for something approaching six months, and they don't spend much time on any one piece. I'm not so good in spinning, precessing, revolving space, so maybe I am missing a fundamental piece of the scan strategy, but it seems to me like they see each point a few times one day and then don't see it again for several months. There are parts of the sky that are covered much better, but it looks like you would need either a dedicated follow-up program or a big statistical enterprise to look at standard candle questions, and even then you would be getting a lot of late time behaviour that is cosmologically uninteresting.

Gil
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Sarah Bridle



Joined: 24 Sep 2004
Posts: 149
Affiliation: University College London (UCL)

PostPosted: January 28 2005  Reply with quote

Evans and Belkurov astro-ph/0210570 did some SN predictions. They say they took into account the scanning strategy and lightcurves.

I would like to know more about this though, as I get the impression that it will be really helpful to improve the low-z calibration of the distances to get good cosmology constraints from the high z SN.

See also http://www.cosmocoffee.info/viewtopic.php?t=15
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Gil Holder



Joined: 25 Sep 2004
Posts: 27
Affiliation: McGill University

PostPosted: January 29 2005  Reply with quote

Thanks for the pointer to the paper. I didn't realize they really only meant "detect" when they were using those numbers (basically one data point on the light curve at unknown phase). It looks like follow-up would involve several fully dedicated telescopes imaging the candidates every night in both hemispheres. Is there any other science that could be done with imaging a bunch of semi-randomly selected nearby galaxies every night that GAIA wouldn't already be doing or would the follow-up be purely a SN project?
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mattia vaccari



Joined: 24 Feb 2005
Posts: 1
Affiliation: Imperial College London

PostPosted: February 24 2005  Reply with quote

Over 1999−2000, during GAIA preliminary (Pre-Phase-A) studies and as my Master's Thesis, I explored the possibility and the expected performance of an all-sky galaxy imaging survey that GAIA could perform.

Brief technical reports describing are available at

http://astro.ic.ac.uk/~vaccari/research/#GGS

whereas more details are available at

http://astro.ic.ac.uk/~vaccari/research/#masterthesis

Bottom line : GAIA is expected to be able to detect all galaxies brighter than V~17, i.e. about 3 million galaxies over the 3/4 of the sky out of the Galactic Plane where galaxy observations might be carried out without prejudice to observations of stars. All detected galaxies will be observed with a 0.4 arcsec angular resolution and an all-mission accuracy in surface photometry of 0.2 mag/arcsec2 at 20.0 mag/arcsec2 in the V band. Multi-color (4−5 broad bands) and multi-epoch (~50 epochs) information will be available for all observed objects.

Since my study I've been involved in GAIA only to a limited extent, and in the meantime the mission design has been substantially altered to save money, _apparently_ with an extremely limited impact on its performance.

All of the above, however, doesn't come "for free", as the limited downlink rate available to the mission means that trade-offs will have to be established between diffferent science objectives, so that galaxy observations might well be dropped from telemetry if the community doesn't "shout" for them.

Mattia
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