From erik Tue Feb 15 09:54:51 2000 Subject: IWG_OPM_003.txt meeting at Paris Observatory To: gaia-sag@astro.estec.esa.nl (GAIA SAG), straizys@itpa.lt (Vytas Straizys), wladas@itpa.lt (Vladas Vansevicius), Frederic.Arenou@obspm.fr (F. Arenou), Carine.Babusiaux@obspm.fr (Carine Babusiaux), brown@bufadora.astrosen.unam.mx (A. Brown), indus (Jens Knude), makarov@astro.ku.dk (V.V. Makarov), cf (Claus Fabricius), BERNACCA@ASTRAS.pd.astro.it (P.L. Bernacca), cflynn@astro.utu.fi (C. Flynn) Date: Tue, 15 Feb 2000 09:54:51 +0100 (MET) X-Mailer: ELM [version 2.5 PL2] Content-Length: 9966 Status: OR Michael, For your information, especially on galaxies : Regards erik -------------------------------------------------- GAIA meeting at Paris Observatory 04.02.2000 -------------------------------------------- Report by E. Hoeg and F. Arenou 15.02.2000 IWG_OPM_003 Participants: F. Arenou (FA), C. Turon (CT), C. Babusiaux (CB), N. Robichon (NB), A. Brown (AB), X. Luri (XL), E. Hoeg (EH) We discussed the following agenda and defined some actions. The Actions 3,4,5 and the Appendix on Galaxies may be of interest for the red report, and for the Action 13.2 of SAG. === Agenda with comments --GAIA photometry, sampling, Action 1 simulations, Action 2 --Galaxy detection and SWA Action 3, Action 4, Action 5, Appendix --Duplicity detection and SWA Action 6 --Kinematic tracers, see CUO_70 Action 7 --Tycho-2 Catalogue --Further use of the Tycho raw observations We have a plan at CUO, called Tycho-3 project, "Close up of 2.5 million stars", including technicalities of some interest for the future use of GAIA data. A description, 9 pages, may be found on the Tycho-2 website. --On-board photometry; low resolution spectrophotometry Question: Does anyone miss these options, which have been dropped in the present GAIA base line ? Answer was: No --High resolution spectrophotometry Will be discussed in a separate note. --Next meeting Action 8 --------------------------------------------------------------------- === Actions 1. A description of the focal planes and the sampling. The Sects.3.3-3.6 in version 1.5 of the RR (red report) should be used. CUO_67 describing the medium-band photometry (MBP) should also be consulted. It is up-to-date, except that it uses the previous terminology NBP instead of MBP, and F41 should have 2*3 s integration time. EH agreed to write more if required, within a few months. 2. Photometric simulations. Simulations of BBP observations and LSQ analysis as described in SAG_CUO_68.A2 which is identical to the Red Report, Sect.9.4.2. AB will do this soon. 3. Galaxy detection. Simulations of galaxies will be analysed by the APM algorithm. What is the limiting magnitude which still gives a low false-detection rate ? This false rate should not exceed 1 million galaxies/sky = 25 gal/sq.deg on a sky background of uV=22.5 mag/sq.arcsec. CB+FA will investigate this. For these tests, the APM algorithm will be used. The false rate at a sky background of uV=21.0 mag/sq.arcsec should be equally low since according to the present science case for galaxies it is very important that galaxies be detected at low galactic latitudes. According to Table 6.8, about 1400 sq.deg at low galactic latitudes have uR<20.3. The false rate due to pure photon noise from a constant background and readnoise is fairly simple to estimate with a given algorithm. But at the angular scales of interest the real sky is lumpy for various reasons and it is crucial to estimate this effect on the false detection rate. Faint galaxies and stars of all magnitudes will have an influence, and also Galactic nebulosities. Even more difficult to estimate is the influence from non-ideal behaviour of the CCD as CTE and cosmic hits. Realistic simulations of all effects and development of the chosen algorithm are required to minimize the false rate expected from real observations with GAIA. In CUO_61 is predicted that typical S and E galaxies of total magnitude I=16.5 mag (or V=17.5) can be detected with SNR=10 in ASM1. This corresponds to about 1.5 million galaxies on the sky. A study of other galaxy types, e.g. compact galaxies, is pending. 4. Scientific value of high-resolution photometry of galaxies What is the scientific value of high-resolution broad-band photometry of the 1 million brightest galaxies in the sky ? The Appendix gives the technical background. The observation of a larger number of galaxies is being discussed, but the detection of the fainter galaxies is yet doubtful, and the telemetry rate becomes a serious issue. Therefore the scientific value of only the 1 million brightest galaxies should be estimated separately. An important characteristic will be a high stability of the photometric system, it is well calibrated, and covers the entire sky. It should be discussed whether equivalent observations may be obtained by other means before the GAIA photometry would become available about 2015. The GAIA photometry may be called a by-product of GAIA, but the implementation onboard the satellite and the later data analysis represent significant efforts which require a very good scientific justification. NR will investigate this question. 5. Scientific value of low-resolution photometry of compact galaxies Compact galaxies with a point-like core may be detected as stars in ASM and may not be recognised as galaxies on-board. They will then be measured in the astrometric and broad-band fields of Astro-1 and Astro-2. The BBP measurements of Astro-2 will allow photometry of point-like sources, i.e. the core itself and other sources within 1 arcsec of the core (see Figure 3.7 of Version 1.5) and as faint as V=22 mag. The field is first mapped in white light by analysis of all observations from AF17 (previously called PSM) during the mission. The angular resolution will be about 0.2 arcsec along the radius, but not as good in tangential direction. The BBP measurements of Astro-1 will have a higher resolution in radial direction, but will only reach a radial distance of 0.4 arcsec. These measurements will allow the correction for chromaticity of the astrometric observations of point sources. They will also have a separate astrophysical value for point sources. The question is what value may these measurements have for extended sources ? 6. Duplicity detection by means of ASM1 and ASM3 CB+FA will discuss this with V. Makarov during his stay in Paris in March. 7. Kinematic tracers The Table 2 in CUO_70 was discussed, and its relation to the choice of photometric system, especially the MBP system. The precision of photometric distances should be discussed for each tracer in the table of CUO_70, especially for those where a poor relative precision is obtained by trigonometric parallaxes. Note that the relative precision in Column 12 is valid only for the magnitude V_1 given in Column 7, but the natural spread of abolute magnitudes around that given in Column 2, and the interstellar absorption (Column 6) require that photometric distances to fainter stars be determined. The possible precision with a given photometric system, implemented in MBP+BBP, should be discussed. The precision of various systems, especially the F-system and the Stromvil-based G-system should be discussed. The tracers in Table 2 may not be optimal, as discussed in the Appendix of CUO_70. This discussion should be pursued. Who will volunteer ? EH will take this into account in the planned evaluation of the photometric systems with the CUO/Vilnius groups. But this is a big task in total. 8. Next meeting on 14 Sep 2000 TBC in Paris --------------------------------------------------------------------- === APPENDIX Photometry of galaxies with GAIA E. Hoeg 09.02.2000 ABSTRACT: Photometry of detected galaxies in four broad bands (BBP) and at high angular resolution is foreseen. Simulations have been presented in CUO_69. They show that the cores can be observed at a resolution of about 350 mas. This is obtained from samples from the Astro-2 telescope covering the galaxy and which are transmitted to the ground where they are analysed, e.g. by superposition. - The telemetry rate is estimated showing that the raw data rate for 1 million galaxies up to V=17.0 is about 50 kbits/s. Each samples consist of 6*4 pixels with the area As As=6*4 pixels=6*0.037*4*0.111 arcsec^2=0.099 arcsec^2 GAIA scans at a rate of v=120 arcsec/s * 0.66 deg = 285120 arcsec^2/s Statistics of galaxies is given in CUO_61. It is given as function of the total magnitude of the galaxy in units of I mag. The statistics may be approximately transformed to other bands by assuming the colour indeces of galaxies to be B-I=1.7 and V-I=1.0 mag. We thus obtain Table 1 from Tables 4 and 5 of CUO_61. The data rate must be added to that given in Table 3.11 which contains e.g. 845 kbits/s for AF01-16, and 352 kbits/s for the BBP of Astro-1. The extra rate is far from negligible and should be included in Table 3.11 if galaxies of I>17.0 should be measured. The telemetry will be smaller if an area corresponding to R1 instead of R2 would be sufficient. On the other hand no overhead due, e.g., to identification of samples and false detections has been included in T2. The total number of galaxies brighter than V=20.0 according to Table 1 is 44 million. The transmission of data for 100 million galaxies as suggested in Section 1.8.6 of Version 1.5 would require about 1 Mbits/s of raw data. Definitions: N_cum : the number of galaxies per deg^2 brighter than I r_hl : the typical half-light radius of a galaxy, i.e. core radius R_hl : the fraction of sky area occupied by the cores of galaxies Rx : the fraction of sky inside r_hl+x [arcsec] Tx : raw data rate, 16 bits/sample, from areas inside Rx for 4 colour bands where Tx = 16*4*Rx*v/As = Rx*184000 kbits/s Table 1. The number of galaxies, their size, and the raw data rate required to transmit samples from an area with a radius 2 arcsec larger than the core. V I N_cum r_hl R_hl R1 R2 T2 mag mag /deg^2 arcsec fraction of sky kbits/s 17.0 16.0 27 2.49 0.00014 0.00020 0.00027 50 18.0 17.0 97 1.58 0.00020 0.00034 0.00052 96 19.0 18.0 350 1.00 0.00029 0.00064 0.00120 221 20.0 19.0 1100 0.66 0.00041 0.00120 0.00260 478 -------------------------------------------------------***************