Galactic structure and star counts

Du cuihua

2005. 8. 11

BATC meeting, NAOC

Basic components of the Galaxy

BATC observation data

object classification and photometric parallaxes

Space density distribution

conclusions

Dark halo

Stellar halo

Thick disk

bulge

Thin disk

Basic components of the Galaxy

It is primary stellar component 1. The thin disk

a young thin disk, with mean [Fe/H] of 0.0 dex and an exponential scale height of 100 pc.

a old thin disk, with mean [Fe/H] of –0.3 dex and an exponential scale height of 300 pc

2. the thick disk

its exponential scale height is about 500-1400 pc,

with local normalization 2%-14%. its velocity dispersion is much higher than that of the thin disk more metal poor than the thin disk

( - 2.2 [Fe/H] - 0.5 )

our Galaxy have a second thicker disk component

3. The stellar halo is metal-poor : a very old and diffuse population of stars and globular clusters extending far beyond the disk

[Fe/H]<-1.0

Only about 1/50 of the mass of the disk but very significant because it contains the oldest (>12 Ga) stars in the Galaxy

4. The dark haloAlmost all galaxies have a dark halo .... in other spirals

We would like to know more about the properties and history of the galactic dark halo - but it is difficult to study

Most of our insights into the growth and structure of the dark halo come from numerical simulations.

Contribute at least 90% of galactic mass, is spheroid rather than disklike.

5. central bulge

Our galactic bulge is really a small bar

Age=6 ~ 12 Gyr, [Fe/H]=-1.0 ~ +0.5

BATC observation data

At first, try three fields to study the structure

T329: (l=170, b=50) TA01: (l=134,b=-62) T516: (l=124,b=-62 )

Recently, use 20 fields to continue

the previous work.

Object classification and photometric parallaxes

These fields have been observed by the Sloan Digital Space Survey (SDSS) and each object type (stars-galaxies-QSO) has been given.

For stars, use the stellar spectra library to obtain the

stellar type. (Pickles 1998 stellar library )

According to the stellar type，

the absolute magnitude Mv

can be obtained.

(Lang K.R., 1992,

Astrophysical Data I,

Plantes and stars)

According to the absolute magnitude from the stellar type, the stellar photometric parallaxes can be derived.

log( ) 0.2[ 5 ( )]V Vr m M A r

Space density distribution

Vertical density distribution for the disk:

The density law of the halo:

1/ 4( ) exp( )Halor br

1 1 2 2( ) exp( / ) exp( / )z n z h n z h

Using two exponential disks, we determined that the scale height of thin disk is pc, and thick disk scale height is pc, with 7.0% of the thin disk.

Adopting a law halo, the observed counts yield a axial ratio of c/a<0.6, implying a more flatted halo.

The stellar luminosity function of thin disk is also derived.

(Du cuihua, Zhou xu, Ma jun, Alfred-B-C Chen, et al. 2003, A&A, 407,541)

1415320

3032640

1/ 4r

Conclusion 1

Recent observation data:

As an example ,we list several fields as follows:U085, T521, T359, T534, T491,T193

Conclusions 2

(3) Our results suggest that the disk itself may be irregular with a scale height and density that are variable with direction.

(1) Using 20 fields, we found that the scale height of the thin disk is 210-320 pc, And thick disk scale height is 600-840 pc, with a corresponding density normalization 2~10% of the thin disk.

(2) The axial ratio of the stellar halo approximate from 0.4 –0.6. and the parameters for the halo are most applicable to the inner halo.

Thank you !