Galactic Surveys Astrometry and photometry

Carlos Allende PrietoIAC

Galactic Surveys Astrometry and photometry

Overview

• Astronometry: Hipparcos and Gaia• Photometry: DSS, SDSS, 2MASS …• Fitting data to models

Tuesday, August 27, 2013

Basic astronomical measurements (from light)

Astrometry > positions of stars in the sky, proper motions, parallaxes

Photometry > colors and brightness (stellar

properties)

Spectroscopy > radial velocities, line strengths,

stellar properties

Gaia’s Three ElementsAstrometry (V < 20)

completeness to 20 mag ⇒ 109 starsaccuracy: 10–25 µarcsec at 15 mag scanning satellite, two viewing directionsprinciples: global astrometric reduction (as for

Hipparcos)

Photometry (V < 20)Low-dispersion spectrophotometry 0.3 - 1 µm

Radial velocity (V < 16–17)slitless spectroscopy near Ca II triplet (847–874 nm)

third component of space motion,dynamics, population studies, binariesspectra: chemistry, rotation

Astrometry• Positions and motions of stars provide full 3D

maps of the near universe around us – first the solar neighborhood, now more distant parts of the Milky Way and even the nearest local group galaxies

• First parallax measured in 1838 by Bessel (61 Cygni, 0.3 arcsec)

• The Hipparcos mission measured parallaxes for 1e5 stars with mas precision and 1e6 stars with lower precision between 1989 and 1993.

• Gaia is Hipparcos successor, with all-around enhancements Tuesday, August 27,

2013

Hipparcos Gaia

Magnitude limit 12 20 magCompleteness 7.3 – 9.0 20 magBright limit 0 6 magNumber of objects 120 000 26 million to V = 15

250 million to V = 181000 million to V = 20

Effective distance limit

1 kpc 50 kpcQuasars None 5 x 105

Galaxies None 106 – 107

Accuracy 1 milliarcsec 7 µarcsec at V = 1010-25 µarcsec at V = 15300 µarcsec at V = 20

Photometry photometry

2-colour (B and V) Low-res. spectra to V = 20

Gaia: Complete, Faint, Accurate

Stellar Astrophysics Parallaxes and photometry imply a

comprehensive luminosity calibrationdistances to 1% for ~10 million stars to 2.5

kpcdistances to 10% for ~100 million stars to

25 kpcparallax calibration of all distance indicators

e.g. Cepheids and RR Lyrae to LMC/SMC

accurate parallaxes imply accurate surface gravities and age

Stellar Astrophysics An unbiased survey implies a detailed

Galactic census

solar neighbourhood mass function and luminosity function

e.g. white dwarfs (~200,000) and brown dwarfs (~50,000)

initial mass and luminosity functions in star forming regions

rare stellar types and rapid evolutionary phases in large numbers

Statistics on variability across the board (~40 (RVS) - 100 (AS,XP) visits per object)

One Billion Stars in 6-d will Provide …

in our Galaxy … the distance and velocity distributions of all stellar populations a rigorous framework for stellar structure and evolution theories a large-scale survey of extra-solar planets (~20,000) a large-scale survey of Solar System bodies (~ few 100,000)

… and beyond definitive distance standards out to the LMC/SMC rapid reaction alerts for supernovae and burst sources (~20,000) QSO detection, redshifts, microlensing structure (~500,000) fundamental quantities to unprecedented accuracy: γ to 10-7 (10-5

present)

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1 / 0 7 / 0 2∆ δ(∀ )

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Π λα ν τε : ρ = 100 µ α σ Π = 18 µ οισ

Exo-Planets: Expected Discoveries

Astrometric survey: monitoring of hundreds of thousands of FGK stars to ~200 pc detection limits: ~1MJ and P < 10 years

masses, rather than lower limits (m sin i) multiple systems measurable, giving relative inclinations

Results expected: ~20,000 exo-planets (~10 per day) orbits for ~5000 systems masses down to 10 MEarth to 10 pc

>1000 photometric transits

Figure courtesy François Mignard

Asteroids etc.: deep and uniform (20 mag) detection of all moving objects ~ few 100,000 new objects expected (357,614 with orbits presently) taxonomy/mineralogical composition versus heliocentric distance diameters for ~1000, masses for ~100 orbits: 30 times better than present Trojan companions of Mars, Earth and Venus Kuiper Belt objects: ~300 to 20 mag (binarity, Plutinos)

Near-Earth Objects: Amors, Apollos and Atens (2249, 2643, 406 known today) ~1600 Earth-crossers >1 km predicted (937 currently known) detection limit: 260–590 m at 1 AU, depending on albedo

Studies of the Solar System

Satellite and System

• ESA-only mission• Launch date: late 2013 • Launcher: Soyuz–Fregat• Orbit: L2• Lifetime: 5 years• Ground station: New Norcia and Cebreros• Downlink rate: 4–8 Mbps

• Mass: 2120 kg (payload 700 kg)• Power: 1720 W (payload 735 W)

Figures courtesy EADS-Astrium

Payload

Payload and TelescopeTwo SiC primary mirrors1.45 × 0.50 m2 at 106.5°

SiC toroidalstructure

(optical bench)

Basic anglemonitoring system

Combinedfocal plane

(CCDs)

Rotation axis (6 h)

Figure courtesy EADS-Astrium

Superposition of two Fields of View

(FoV)

Focal Plane

Star motion in 10 s

Total field: - active area: 0.75 deg2

- CCDs: 14 + 62 + 14 + 12 - 4500 x 1966 pixels (TDI) - pixel size = 10 µm x 30 µm

= 59 mas x 177 mas

Astrometric Field CCDs

Blue Photometer CCDs

Sky Mapper CCDs

104.26cm

Red Photometer CCDs

Radial-Velocity Spectrometer

CCDs

Basic Angle

Monitor

Wave Front Sensor

Basic Angle

Monitor

Wave Front Sensor

Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discriminationAstrometry: - total detection noise: ~6 e-

Photometry: - spectro-photometer - blue and red CCDsSpectroscopy: - high-resolution spectra - red CCDs

42.35cm

Figure courtesy Alex Short

On-Board Object DetectionRequirements:

unbiased sky sampling (mag, colour, resolution)all-sky catalogue at Gaia resolution (0.1 arcsec) to

V~20

Solution: on-board detection:good detection efficiency to V~21 magFPA CCDs generate Gbps thus windows needed

Sky Scanning Principle

Spin axis 45o to SunScan rate: 60 arcsec/sSpin period: 6 hours

45o

Figure courtesy Karen O’Flaherty

Astrometric Data Reduction Principles

Sky scans(highest accuracy

along scan)

Scan width: 0.7°

1. Object matching in successive scans2. Attitude and calibrations are updated3. Objects positions etc. are solved4. Higher terms are solved5. More scans are added6. System is iteratedFigure courtesy Michael Perryman

Light Bending in Solar System

Movie courtesy Jos de Bruijne

Light bending in microarcsec, after subtraction of the much larger effect by the Sun

Gaia imaging91 CCDs (4000 x 2000 pixels each)Distances for 1.000.000.000 sources!

The Radial Velocity SpectrometerTDI spectroscopy!

Stellar motions

Photometry Measurement Concept

Figures courtesy EADS-Astrium

Blue photometer:330–680 nm

Red photometer:640–1000 nm

Photometry Measurement Concept

Figures courtesy Anthony Brown

Blue photometer

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wavelength (nm)

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spectral dispersion per pixel (nm) .

Red photometer

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wavelength (nm)

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spectral dispersion per pixel (nm) .

RP spectrum of M dwarf (V=17.3)Red box: data sent to ground

White contour: sky-background levelColour coding: signal intensity

Ideal testsShot, electronics (readout) noiseSynthetic spectraLogg fixed (parallaxes will constrain

luminosity)

G=18.5

G=20

S/Nper pixel

Bailer-Jones 2009GAIA-C8-TN-MPIA-CBJ-043

(Spectro-)photometryILLIUM algorithm (Bailer-Jones 2008). Dwarfs:G=15 ([Fe/H])=0.21

(Teff)/Teff=0.005G=18.5 ([Fe/H])=0.42

(Teff)/Teff=0.008G=20 ([Fe/H])=1.14

(Teff)/Teff=0.021G=20

RVS S/N ( per transit and ccd)3 window types: G<7, 7<G<10 (R=11,500),

G>10 (R~4500) √ (S + rdn2)Most of the time RVS is working with S/N<1End of mission spectra will have S/N > 10x

higher

G magnitude

Allende Prieto 2009, GAIA-C6-SP-MSSL-CAP-003

Sample RVS spectra (mission end, black line)

G=10.5 G=12.3 G=15.8

B5V

G2V

Metal-poor

K1III

Allende Prieto 2009

RVS produceRadial velocities down to V~17 (108 stars)Atmospheric parameters (including overall metallicity) down to V~ 13-14 (several 106 stars)

Chemical abundances for several elements down to V~12-13 (few 106 stars)

Extinction (DIB at 862.0 nm) down to V~13 (e.g. Munari et al. 2008)

~ 40 transits will identify a large number of new spectroscopic binaries with periods < 15 yr (CU4, CU6, CU8)

RV performance

Spec. for late-type stars

1 km/s at V<13

15 km/s down to V=17

Atmospheric parameters (Ideal tests)

Solid: absolute fluxDashed: absolute flux, systematic errors

(S/N=1/20)Dash-dotted: relative flux

Allende Prieto (2008)

Photometry• Gaia will not be the first full-sky photometric

survey• Palomar photographic plates (POSS)• HST needed a full-sky pointing catalog, which

was prepared from digitized photographic plates (DSS)

• 2MASS: first full-sky ground-based near-IR photographic survey (J H Ks filters)

• SDSS provided a large/area (14,000 sqr. deg) optical survey (ugriz system) using CCD detectors

• Others: GALEX (NUV), WISE (IR), UKIDS …


Usual photometric systems• Johnson (-Cousins) UBVRI • Ströngrem ubvy • Near-IR Y J H Ks• SDSS ugriz• GALEX FUV/NUV• …• System responses usually include (approximate) atmospheric extinction


SDSS• First massive solid-state optical photometric

survey (some 14,000 deg2 and 150 million stars down to r ~ 22 mag)

• 2% photometry – 1% in stripe 82• Highly-uniform observations (single

site/telescope/instrument)• Carefully designed filters (though issues with

u-band)


SDSS imaging• 6 x 5 CCDs• Running in TDI

The world’s biggest picture

• 26 Gigapixels!

2MASS• 2 automated 1.3m telescopes (one in Arizona,

one in Cerro Tololo, Chile)• 3 channel (J, H, Ks) cameras, each with a

256x256 HgCdTe detector• 7.8s exposures• 4 years of operation• PSC: ~ 300 million stars down to J/H/Ks of ~

16,15,14• About 1 million extended sources


UKIDSS

started in 2005 some 400 papers already published uses WFCAM on 4-m class UKIRT (four

2048x2048 Rockwell devices) 7500 sqr. Deg down to K~18


VHS

• 19,000 sqrt. deg• About 4 mag. deeper than 2MASS• Using ESO’s VISTA telescope


The Future: LSST

• A wide-field 8.4m telescope• A 3.2 Gpix camera• Imaging the whole (accessible sky) every few

nights• Starting in 2018• Tens of TB of data each night


The variable sky• Most of the transients are fairly near, but

most exciting ones are far away

The variable sky

The variable sky• We do not know what is out there• Lots of room for classification

algorithms

Zero-point• Absolute calibration of astronomical sources

is non-trivial• Good lab reference sources hard to observe

through telescopes as if they were at infinity• Atmospheric extinction/distortion gets in the

way• Traditional reference source is Vega, which

sets zero-point tied to lab sources (Tungsten lamps or black bodies; see Hayes 1985, Megessier 1995), but Vega is not easy to model

• Spectrophotometric calibration nowadays tied to DA white dwarf models (Bohlin 2010 and prev. refs.)


White dwarfs• DA white dwarfs are fairly simple: just two

parameters (Teff,logg), pure-H physics, NLTE but good agreement among models

Allende Prieto, Hubeny & Smith 2009

Examples From SDSS

HST DAs• 3 DA white dwarf stars constitute the basis

for HST calibration (see papers by Bohlin)• Good to 1-2%• Calibration consistent for VegaV=0.023 +/- 0.008


HST DAs analyzed with different models


A-type stars Not pure hydrogen, but spectrum

dominated by it in optical and IR (continuum and lines); exception FeII lines in UV

Three parameters (Teff,logg,[Fe/H]) Reddening needs to be accounted for (also

true for faint WDs) Brighter and more common than WDs

A-type stars

Not pure hydrogen, but spectrum dominated by it in optical and IR (continuum and lines)

Three parameters (Teff,logg,[Fe/H]) Reddening needs to be accounted for (also

true for faint WDs) Brighter and more common than WDs

Allende Prieto & del Burgo (in prep). Spectra from NGS (Gregg et al.)

Vega

Allende Prieto & del Burgo (in prep). HST spectrum Gilliland & Bohlin

Fast rotation

Zero-point• HST flux calibration: using zero-point V

magnitude for Vega (not quite zero, V= 0.023 mag) and 3 DA WDs models

• Vega STIS spectrophotometry calibrated in that way compares well with a model atmosphere for Vega and leads to a consistent zero-point based on photometry performed on the model

• System seems robust to 1-2% level• STIS spectrophotometry (calspec, NGSL) now

being used to set zero points for photometric systems


Halo turn-off stars F-type, metal-poor: H continuum + lines

and few metal lines (not so few in the blue/UV)

Again 3 parameters (+ reddning) but now higher impact of [Fe/H] due to electrons forming H-

Many of them (just leaving the main sequence), easy to pick up from colors

Choice used for SDSS BD +17 4708 is the prototype

Halo turn-off stars F-type, metal-poor: continuum H and H-, H

lines and few metal lines (not so few in the blue/UV)

Many of them (just leaving the main sequence), easy to pick up from colors

Choice used for SDSS BD +17 4708 is the prototype

Ramírez et al. 2009; HST data from Bohlin and colleagues

Fitting models to data• Understanding the structure of the Milky Way

is critical • Starcounts are the most fundamental (and

easy) measurement: just photometry and coarse astrometry (star positions)

• Distances must be estimated, but parallaxes to a few percent available for only some 1e5 stars (Hipparcos)

• Photometric parallaxes derived for dwarfs based on models or semi-empirical relationships (e.g. clusters)

M-m = 5 -5 log(d)


Color – absolute mag relationships for dwarfs

Tuesday, August 27, 2013 Juric et al. 2008

Standard candles• Dwarfs outnumber giants in most cases (not

always). Their luminosities depend on metal content but weak dependence on age at a given color

• An alternative is to use stars at specific evolutionary stages that can be identified with certainty, and with reliable theoretical (or semi-empirical) luminosities

• Examples include cepheids, red-clump stars, RR Lyrae


Standard candles• For example, red-clump giants have been

very useful in the obscured parts of the Milky Way

• An approximateextinction relationbetween colors and passbands can be adoptedCabrera-Lavers

et al. 2008

Babuiaux and Gilmore 2005Tuesday, August 27, 2013

Red-clumb giants in the bulge


Cabrera-Lavers et al. 2008

Large data sets• One can either fit the data with models, e.g.

Larsen & Humphreys (2003), Robin et al. (2003)

• Or derive density maps, which are subsequently fit to infer the model parameters, e.g. Juric et al. 2008

• Models involve a number of std. Milky Way stellar components: a disk (or two), a halo, and a bulge (plus other non-std. such as a bar or streams as needed)

• Nowadays, more complex orbit-family-type or numerical-simulations available, but parametric models provide a fast and useful path to start


Density maps• Photometric parallaxes are derived first• Positions on the sky and distances are used to create a binned density map (Juric et al. 2008)• Photometric [Fe/H] estimates can be used


Density maps• This approach allows to clean-up the density

maps before we fit radially symmetric models

Juric et al. 2008


Fitting models to data• Typical 3-4 component stellar Milky Way

models involve 6-8 parameters: relative densities, halo exponent, disk scale height(s) and length(s)

• Extinction needs to be included in disk and bulge• Parameters constrained by optimization algorithmalgorithm


Tools• Besançon model (Gaia universe model)• M. Cohen’s model• TRILEGAL (see Girardi’s lectures)• GALFAST (Juric)• Jordi Molgo’s simulator • …


What’s next• Back to Gaia…• Starcounts soon to be suplemented with trig.

Parallaxes (replacing photometric ones) • and spectrophotometric metallicities (Gaia)

plus spectroscopic metallicities and more detailed abundances for a fraction of the sample (APOGEE/SDSS, Gaia-ESO, GALAH…)

• Further work on map construction desirable• Idem for tools for evaluating simple,

parametric, Milky Way modelsTuesday, August 27, 2013

Galactic Surveys Astrometry and photometry

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