Surveys and Catalogs

Summary:

• What is a survey? What surveys can do?

• Survey keywords (completeness, area, mag.limit, …)

• Type of surveys

• Survey among the surveys

• Data Processing

• Data Storage and access

Alessandro Spagna & Richard L. Smart

INAF - Osservatorio Astronomico di Torino

Torino, 5 febbraio 2002

What is a survey?

“A survey is a large, systematic assemblage of data for which some one else will get more glory than the people who did the assembling and systematizing.”

(Panel discussion of IAU Symposium n.179 (1996) on New Horizons from Multi-Wavelenght sky surveys)

What surveys can do?

• Images of celestial objects; sky Maps and Atlas (DSS)

• Generate Catalogs (reference systems, …, )

• Discovery of “rare” objects (WDs, subdwarfs, extra-solar planets, ...)

• Detection of rare events (SN, microlensing, …)

• Monitor of variable objects (AGN, variable stars, …)

• Multi-wavelengths cross identification ( X opt.-IR Radio )

• Selection of targets for spectroscopic follow up (efficient use of large and expensive telescopes)

• Studies based on large and homogeneous statistical sample (large scale structure of the Universe, galaxy evolution, galaxy clusters, Galactic structure, stellar populations)

What surveys can do?Cent. II B.C, Tolomeus’s Almagesto. Position and magnitude of 1080 bright stars, based on the 850 stars listed by Ipparcus.

June 2001: GSC 2.2, position and magnitude of 455 million stars and galaxies, complete down to R<18.5 and BJ < 19.5.

What surveys can do?

Hertzsprung-Russell (MV, B-V) diagram for the 16631 single stars from the Hipparcos Catalogue withrelative distance precision sigma_pi/pi < 0.1 and sigma_(B-V) less than or equal to 0.025 mag. Colours indicatenumber of stars in a cell of 0.01 mag in (B-V) and 0.05 mag in V magnitude (M_V).

803 refereed papers since 1997 which mention Hipparcos measurements (309 in the title).

What surveys can do?

GSC 2.2 optical counterparts in M45 (B<19.5 or R<18.5)

2Mass NIR counterparts in M45 (J<17 or H<16 or K<15)

What surveys can do?

What surveys can do?Microlensing results

~20% of the galactic halo is made of compact objects of ~ 0.5 M

MACHO: 11.9 million stars toward the LMC observed for 5.7 yr 13-17 events 8%-50% (C.L. 95%) of halo made of 0.15-0.9 M compact objects.

EROS-2: 17.5 million stars toward LMC for 2 yr 2 events (+2 events from EROS-1) less that 40% (C.L. 95%) of standard halo made of objects < 1 M

Candidate MACHOs:

• Late M stars, Brown Dwarfs, planets

• Primordial Black Holes

• Ancient Cool White Dwarfs

What surveys can do?Count the objects!

0

2)(),()( drrrDrMmA r

)(log55 rarmM

Fundamental Equation of the Stellar Statistics(Integral Fredholm’s equation of the first kind).

Distance Modulus

(M)=Luminosity function D(x,y,z)=density distribution

What surveys can do?NGP

Starcounts (i.e. n.ro of stars as a function of the magnitude, color & l,b) depends directly on the properties of the galactic components.

The properties of the galactic components (their spatial structure, kinematics, metallicity, LF) constrain the formation and evolution processes of the Galaxy.

What surveys can do?

Left panel: SDSS galaxy counts (dots). Solid line (r<16) shows the 100.6m distribution expected for a homogeneous galaxy distribution in an “Euclidean” universe. Crosses indicate the starcounts.

Right panel: galaxy counts normalized to the Euclidean distribution compared with 3 different cosmological models . (Yasuda et al, 2001, AJ)

Survey keywords

• spectral range

• area coverage

• magnitude limit, sensitivity

• accuracy level, spectral resolution

• angular resolution

• n.ro of objects and completeness

Survey keywords Spectral coverage

1-4 m. Stars - low extinction

0.3-0.8 m Stars - strong extinction

12, 60,100 m. Dust termal emission

7-10 m. Red giants, PNs, YSOs, clouds

0.25,0.75,1.5 keV Hot gas - strong extinction

>300 MeV Diffuse cosmic Rays, Pulsars

H2 emission from Molecular clouds

2.4-2.7 GHz. Hot gas, H-II regions

21cm from H clouds

408 MHz synchrotron radiation

Survey keywords Area and Magnitude limit

Survey keywords

Area and Magnitude limit

DSS-2. Magnitude limit R=20.5

Telescope D=1.2m, Exp.Time=60min, DQE=2%

HDF (HST). Magnitude limit, I=30

Telescope D=2.5m, Exp.Time=10 days, DQE=30-40% (WFPC2)

Survey keywords

Spatial resolution

1 arcsec pixel: (DSS2, 2MASS)

0.05 arcsec pixel HST-WFPC2

Survey keywords

Completeness and Selection criteria

Types of surveys:

•Selective surveys. For examples, stellar samples selected on the basis of the chemical or kinematic properties (e.g. low metallicity and high proper motion stars Pop. II halo stars. Warning: “biased” results)

• Surveys with tracers. High luminosity objects which can be observed up to great distances, easy to identify and to measure their distance (e.g. globular clusters, giants, variable RR Lyrae, … ) . It is assumed that tracers are representative of the whole population.

• In situ surveys. These measure directly the bulk of the objects which constitute the target populations (e.g. dwarfs of the galactic Pop.I and Pop.II). These should guarantee “unbiased” results if systematic effects due to the magnitude threshold, photometric accuracy, angular resolution, etc. are properly taken into account.

Correction of selection effects

Malmquist bias

For magnitude-limited surveys the volume within we can see the most luminous objects is larger to than that within which we can see the faintest ones luminous objects are over-represented The measured <M> is brighter than the true mean of the population.

dm

mAdMM

)(ln20

Correction of selection effects

Lutz-Kelker bias

Magnitude-limited surveys contain a greater number of distant stars than nearby stars.

Thus, because of the measure error, , the observed parallax is - on average - higher than its true value distance underestimation luminosity underestimation

= 1/d

N( )

observedtrue

Survey keywords

Accuracy level

pcd Mm 15/)(10

/1d Trigonometric distance

Photometric distance

)()( 46.05

10lnMmMmd dd

Example:

=0.2 mag d/d = 0.10

// dd

Examples:

=1 mas, d=1 kpc d/d = 1

=10 as, d=10 kpc d/d = 0.10

Survey keywords

Accuracy level

Survey among Surveys

(Stellar) Surveys:

• Astrometric surveys Positions, Proper motions, Parallaxes (Hipparcos/Tycho, DIVA, GAIA, PPM*, NPM/SPM*, GSC-2*, 2MASS*, …) [*=no parallaxes]

• Photometric surveys UV, Optical, IR photometry of stars/galaxies (EIS, 2MASS, DENIS, SDSS, GSC2, NPM/SPM, GAIA, …)

• Spectroscopic surveys stars (spectral classification, radial velocity, metallicity), galaxies/QSO (redshift) (CfA RS , SDSS, ESO Slice RS, 2dF, GAIA, …)