Transcript GAIA A Stereoscopic Census of our Galaxy

C. Cacciari
INAF - Osservatorio Astronomico, Bologna
The primary objective of Gaia is the Galaxy: to observe the physical
characteristics, kinematics and distribution of stars over a large
fraction of its volume, with the goal of achieving a full understanding
of the MW dynamics and structure, and consequently its formation
and history.
(Concept and Technology Study Report, ESA-SCI(2000)4
Gaia in a nutshell
 all sky (i.e. ~ 40,000 deg2) survey complete to Vlim = 20
 ~ one billion sources
 high (μas) accuracy astrometry (parallaxes, positions, proper motions)
 optical spectrophotometry (luminosities, astrophysical parameters)
 spectroscopy (radial velocities, rotation, chemistry) to V = 16
Satellite and System
ESA-only mission (Airbus DS contractor)
Lifetime: 5 yr (+ 1yr possible extension)
Launched on 19 December 2013 from Kourou (French Guiana)
Launcher: Soyuz–Fregat
L2 (gravitational equilibrium
1.5 million km from Earth away from Sun )
Lissajous orbit around L2
Figure courtesy A. Buzzoni
Figures courtesy EADS-Astrium
Satellite and System
Figures courtesy EADS-Astrium
Payload and Telescope
Two SiC primary mirrors
1.45  0.50 m2 at 106.5°
Rotation axis (6 h)
FoV
Basic angle
monitoring system
SiC toroidal
structure
(optical bench)
1.7° x
0.6°
Superposition of
two Fields of
View (FoV)
Combined
focal plane
(CCDs)
Sky Scanning Principle
Spin axis: 45o to Sun
Scan rate: 60 arcsec/s
Spin period: 6 hr
FoV-1: t0
t0 + 6hr
FoV-2: t0 + 106.5m
t0 + 106.5m + 6hr
repeated 10-30 days later
29 revolutions of spin axis around solar direction in 5 yr :
Transit maps
Ecliptic coordinates
Galactic coordinates
End of mission (5 yr) sky-average number of transits: ~ 70
(max  200 at  = 45  10)
Scanning the entire sky
MOVIE
The instruments
R ~ 80 – 20
R ~ 90 – 70
Slitless
spectroscopy on
Ca triplet
(847–870 nm)
Resolution 11,500
Focal Plane
104.26cm
42.35cm
Wave
Front
Sensor
Red Photometer CCDs
Blue Photometer CCDs
Wave
Front
Sensor
Basic
Angle
Monitor
Basic
Angle
Monitor
Radial-Velocity
Spectrometer
CCDs
CCD X time = 4.4 s
Sky Mapper
CCDs
Astrometric Field CCDs – Total FOV ~ 40x40 arcmin
~ 4.4 ‘
~ 4.4 '
along-scan
Total field:
- active area: 0.75 deg2
- CCDs: 14 + 62 + 14 + 12
- each CCD: 4500x1966 px (TDI)
- pixel size = 10 µm x 30 µm
= 59 mas x 177 mas
Sky mapper:
- detects all objects to 20 mag
- rejects cosmic-ray events
- FoV discrimination
Astrometry:
- total detection noise: 6 e-
Photometry:
- spectro-photometer
- blue and red CCDs
Spectroscopy:
- high-resolution spectra
- red CCDs
Gaia spectro-photometric system
Internal calibration
External calibration
●
●
●
●
same principle as for classical spectrophotometry
much more complicated instrument model
~ 200 calibrators needed to model instrument response
mmag internal accuracy, a few % external accuracy
Figure courtesy A. Brown
BP/RP first results
Star HIP 86564
K5, V=6.64
● BP and RP low resolution
internally calibrated
spectral energy distributions
● single transit
ESA/Gaia/DPAC/Airbus
Radial-Velocity Measurement Concept
Courtesy David Katz
Field of view
RVS spectrograph
CCD detectors
Spectra of the star HIP 86564
Top: Gaia-RVS on FoV 0, CCD row 4
strip 15
Bottom: Narval on 2-m Bernard Lyot
Telescope (Pic du Midi), convolved to
the nominal resolving power of the
RVS: R=11500
Parallax – Parsec
Arcsecond = 1/60 arcmin = 1/3600 deg = 1/206265 radian = 1/1,296,000 circle
1˝ ≈ the angle subtended by a US dime coin (17.9 mm diameter) at a distance
of 4 km
Parsec (pc): distance at which
the average Earth-Sun distance
(1 AU=150 106 km) is seen under a
parallactic angle (parallax) of 1˝
1pc = 3.26ly
parallax (˝) = 1/distance (pc)
Astrometry: a historical perspective
1 degree
Large-angle
astrometry
1/60
1 arcminute
1/60
1 arcsecond
1/1000
1 milli-arcsecond
Small-angle
astrometry
150 BC
1500
Figure: Lennart Lindegren
1600
1700
Year
1800
1/100
1 microarcsecond
1900
2000
2100
Astrometry: data reduction principles
Scan width: 0.7°
Sky scans
(highest accuracy
along scan)
Figure courtesy Michael Perryman
1. Object matching in successive scans
2. Attitude and calibrations are updated
3. Objects positions etc. are solved
4. Higher terms are solved
5. More scans are added
6. System is iterated (Global Iterative Solution)
From Hipparcos to Gaia
Hipparcos (1997)
Gaia (2020-22)
Magnitude limit
Completeness
Bright limit
Number of objects
12
7.3-9.0
~0
1.2 105
20
~ 20
~ 3-5
1.0 109
Effective distance (Mv=-3)
Quasars
Galaxies
Accuracy
10 kpc
None
None
1 ~ mas
1 Mpc
~5 105
~ 106 - 107
~ 5-14 μas at V  12
9-26 μas at V=15
130-600 μas at V=20
2 (B and V)
None
None
3 (to V=20) + 1 (to V=16)
2 bands (B/R)
1-15 km/s to V=16
Pre-selected targets
Complete and unbiased
Broad band photometry
Spectrophotometry
Spectroscopy (CaT)
Observing programme
Astrometric accuracy: the Hyades
d= 46 pc = 151 ly
1960
1990
Figure courtesy ESA
2020
One billion stars in 5D (6D … 9D) will provide:
in the Galaxy …
distances, velocity distributions and astrophysical parameters (APs)
of all stellar populations in the MW to unprecedented accuracy
allowing to:
 map the spatial and dynamical structure of bulge, disk(s) and
halo(s)
 derive formation and chemical history (e.g. accretion and/or
interaction events) of the MW & star formation history
throughout
 obtain the trigonometric calibration of primary distance
indicators (RR Lyraes, Cepheids)  accurate and robust
definition of the cosmic distance scale
and beyond the Galaxy …
 QSO detection and definition of rest frame
 structure and stellar population studies in nearby (LG) galaxies
Stellar populations of the MW:
The Hertzprung-Russell diagram
BRIGHT
MOVIE
intrinsic
luminosities
need
distances
FAINT
HOT
Temperatures from colours (spectrophotometry)
COLD
Gravitational light bending
This is the dominating relativistic effect in Gaia astrometric measurements
Accurate measures of γ of Parametrized Post-Newtonian (PPN) formulation
of gravitational theories is of key importance in fundamental physics
Gaia will measure  to ~ 5x10-7 (10-4 - 10-5 present)
Data processing & distribution
 Data Processing and Analysis Consortium (DPAC): more than
500 people from 20 European countries and ESA
 Final catalogue ~ 2020–22
 Intermediate data releases
 Science alerts (e.g. SNe) data released immediately
 No proprietary data rights
more information on Gaia at