Study of Galactic Structure Using Ultra-Violet Surveys

Devendra Ojha

TIFR, Mumbai

IWSSL 2011 December 08, 2011 IWSSL 2011 December 08, 2011

Galactic Structure Flat disk:

•1011 stars (Pop.I)

• ISM (gas, dust)

• 5% of the Galaxy mass, 90% of the visible light

• Active star formation since 10 Gyr.

Central bulge:

• moderately old stars with low specific angular momentum. • Wide range of metallicity• Triaxial shape (central bar)• Central supermassive BH

Stellar Halo:

• 109 old and metal poor stars (Pop.II)

• 150 globular clusters (13 Gyr)

• <0.2% Galaxy mass, 2% of the light

•Dark Halo

Galactic Structure – Stellar ComponentsGalactic Structure – Stellar Components

• The structure of the stellar components of the Galaxy can be inferred from distributions of stars obtained from deep surveys of specific areas.

• The interpretation of these star counts is eased by making statistical comparisons with stellar population synthesis models.

• So far the modeling is done mainly in the visible and near-infrared domains, where data are readily available.

• From these data it is generally admitted that the Galaxy can be modeledassuming four main stellar components - thin disk, thick disk, stellar halo (spheroid) and the outer bulge.

• Structural parameters (including stellar densities, scale height, scalelength, etc) can be estimated from observed star counts towards different directions (Galactic latitude, longitude).

Galactic Structure - UV SurveysGalactic Structure - UV Surveys• A large volume of ultraviolet (UV) imaging data

are now available. For example, GALEX (Galaxy Evolution Explorer) is conducting an all-sky survey of the Galaxy in UV waveband.

• In coming years, UVIT (The Ultra Violet Imaging Telescope) on ASTROSAT (India's first multiwavelength astronomy satellite to be launched in ~ 2012), aims to provide flux calibrated images of the sky at a spatial resolution of ~ 1.5 second of arc in two UV channels.

Main Science Interests (UV Surveys):Main Science Interests (UV Surveys):

• Develop a model – star count predictions in the UV, for analysis of data fromthe GALEX satellite and to prepare the Indian space mission ASTROSAT byproducing simulations for the instrument UVIT.

• Trace the spiral structure which contains mainly very young stars – constrain the Initial Mass Function for massive stars as well as the recent star formation history.

• Trace very blue populations like white dwarfs from all components and blue horizontal branch stars deep in the spheroid population, which are also good tracers for streams and relics of ancient accretions in the Milky Way halo.

√ Need for a whole grid of stars with temperature from 2000 to 100000 K,log g from -1 to +8 and a range of metallicity – UV Stellar Library

√ Teff vs. UV colors calibration ?

GALEX

• GALEX: Galaxy Evolution Explorer• NASA mission, led by Caltech• LAUNCH: April 28th, 2003• DURATION: > 29 months• The first all-sky imaging and spectroscopic surveys in

the space ultraviolet• Wavelength: 1350-2750 Angstrom

GALEX Instruments

GALEX Performance

ASTROSAT is an Indian-national scientific satellite to be launched in ~2012.Polar orbit (sun-synchronous) offers long integrations only in CVZ.Operation to be targeted to interesting sources.

Ultra-Violet Imaging Telescope (UVIT)Ultra-Violet Imaging Telescope (UVIT)

• Two Ritchey-Chretien Telescopes : ~ 38 cm Diameter

• FOV ~ 0.5 square degree

• Simultaneous Observations in : FUV (1300-1800 Angstrom); NUV (1800-3000 Angstrom); Visible (3200-5300 Angstrom)

• Designed with Spatial Resolution ~ 1.5 arc-seconds FWHM

• Micro Channel Plate (MCP) based intensified CMOS Photon Counting Detectors.

UVIT Filter throughput

GALEX Filters NUV

FUV

Classification of the UV sources (GALEX + SDSS)

Bianchi et al. 2007

Fundamental Equation of the Stellar Statistics

(von Seeliger 1989)

∫∞

⋅⋅=0

2)(),()( drrrDrMmA rψω

)(log55 rarmM −−+=(Integral Fredholm’s equation of the first kind).

ω

Ψ(M)=Luminosity function

D(x,y,z)=density distribution

Problem: inversion of the integral equation!

Galaxy Models

An alternative approach: integrate the Eqn of stellar statistics assuming some prior information concerning the stellar population. In practice,

•(1) They assume discrete Galactic components, each parametrized by specific spatial density, ρ(R,z; p), velocity ellipsoid and by a well defined LF/CMD consistent with the age/metallicity of each component.

•(2) Predicted star counts (i.e. N.ro of stars vs. magnitude, color, proper motion, radial velocity, etc.) are derived by means of the fundamental Eqn. of the stellar Statistics.

•(3) Comparisons against observations are used to confute or validate and improve the model parameters.

Besancon Model of Stellar Population Synthesis:(Robin et al. 1992a,b, 1996, 2003; Haywood et al. 1993, 1994, 1995; Ojha et al. 1994, 1996, 2001):

• 4 populations (thin disk, thick disk, spheroid, bulge) (each population is described by a SFR history, an IMF, an age or age-range, a set of evolutionary tracks, kinematics, metallicity characteristics, and includes a white dwarf population) • Density laws for the thin disk are constrained self-consistently by the potential via the Boltzmann equation and are age dependent. • The extinction is modeled by a diffuse thin disk.

• Model simulations are produced in the form of catalogues of pseudo-stars, from Monte-Carlo simulations. For each stars, observable parameters as well as intrinsic ones are given. tables of statistical distributions as a function of either observables (magnitudes, colors, proper motions, radial velocities) or intrinsic parameters (distances, spectral type, age, absolute magnitude). Integrated luminosity in any specified photometric band among UBVRIJHKL. Other photometric systems are being studied

Web-based Besancon Model of Stellar Web-based Besancon Model of Stellar Population Synthesis of the GalaxyPopulation Synthesis of the Galaxy

Galex/UVIT passbands

Preliminary Results:GALEX NUV Star Counts + Model Comparison(@ different latitudes, longitudes)

GALEX NUV Star CountsGALEX NUV Star CountsLatitude Variation forl = 45 deg & 90 deg

NUV and FUV NUV and FUV Star Counts &Star Counts &ColorsColors

(l = 80º, b = -55º)

Hot white dwarfs& BHB stars

Cool stars

Additional diskwhite dwarf population(hz = 275 pc) ?

Disk Scale Lengths (using star count ratio between the two fields, GC & GAC at

intermediate latitude)Density function:

ρ(R,z) = ρ(R0)exp(-(R-R0)/hR)exp(-|z|/hz)

Assuming the LF is the same & |R-R0| ≈ |z| for GC and

GAC, the star count ratio:

AGC(m)/AGAC(m) = exp(+2*|R-R0|/hR) = exp(√2 dlos/hR)

Scale length:

hR = √2 dlos / log(AGC(m)/logGAC(m))

GALEX fields (GC: l = 0 deg, b = +60 deg;

GAC: l = 180 deg, b = +60 deg)

Scale length (hR) = 4.0 kpc (thin disk) & 4.5 kpc (thick disk)

APC, 9 June 2010 22

StreamsIntégrale du mouvement

Amina’s figures

Streams in the

Galactic Halo(SDSS)

Accretionhistory

Spectroscopy – IUE white dwarf spectra (Holberg et al. 2003; Bica et al. 1996)

Conclusions• We plan to explore the comparison between the model and GALEX data in various directions and investigate the differences in order to improve model fitting and to ensure that the model is well suited for UV wavelengths and can be applied to predict counts for UVIT-ASTROSAT.

• Need for high Signal to Noise ratio of different cases of white dwarfs & BHB stars, to be used as templates for stellar population analyses.

Thank You