The Galactic plane in the direction of Sag-Carina Arm

3-D Structure of the Galaxy from Star Counts

A.Vallenari, G. Bertelli, C.Chiosi, E.Nasi, S.Pasetto, L.Schmidtobreick

A systematic analysis of the stellar content of the Galaxy has been undertaken t o:

? s t u d y t h e Ga lact ic s t r u ct u r e;

? s t u d y t h e stellar extinct ion a long the line of sigh t ;

? obt a in information about the age of the various galactic components (halo, bulge and disc);

? der ive t h e pa s t h ist ory of star formation;

? sim u late the sky (magnitudes, colou r s, st a r cou n t s, pr oper m ot ion s a n d r a dia l velocit ies ) in p r epa r a t ion t o t h e GAIA Mission d ecla r ed ESA

Cor n erstone, which will give in for m a t ion a bou t m a gnitudes, posit ion s , metal con t e n t , r a dia l velocit ies for 10

s t a r s in the Milky Way.

We m odel t h e Ga laxy star cou n t s a t varying SFR, enrich m ent law, sca le heights, sca le lengths using the model descr ibed by Ber t elli e t a l (1995) a n d Va llen a r i e t a l (2000, 2001).

Modeling the Galaxy

Thin disk : age

Thick Disk age:

Spir a l Ar m age

m a ss dist r ibu t ion

3 com p on e n t s fr om 11 to 0.001Gyr

Z=0.08 t o 0.03

double exponential mass dist r ibu t ion

exp(-r /h )*sech2(z/h z)

10-12 Gyr Z=0.001 to Z=0.004

0.001-0.1 Gyr

Loga r it h m ic spir a l

The bulge component

The mass model

We stress the impor t a n ce of t a k ing into a ccou n t t h e m a ss dist r ibu t ion of t h e bulge (see t h e followin g). T h e bulge has no spherical symmetry. Several models are p r oposed:

? P ower law, as a depr oject ion of t h e de Va u cou ler law.

The fla t t ening of t h e sph e r oid perpendicu larly to t h e pla n e is a free parameter

? COBE -DIRBE obla t e sph e r oid (Dwek et a l 1995 G0).

I t is a Gaussian fu n ct ion with axial ratios 1:1:.56

? COBE -DIRBE t r iaxial model (Kent 1991, Dwek et al G2) wit h a xia l r a t ios 1:0.22:0.16

? Bissa n t z et a l (1997) t r iaxial model based on COBE d a t a wit h a xia l r a t ios 1:0.6:0.4 (B2)

? Bissa n t z et a l (2002) m odel wit h r a t ios 1:0.3:0.3

? Expon e n t ial triaxial function (Dwek et al G2) with axial ratios 1:0.18:0.39, again based on COBE data

? The posit ion a n gle of t h e ba r is going fr om 25 (B2) , 20 (G2) a n d 36 (E2)

The data sets

The available data con s ist of sever a l sets:

? ESO Danish 1.5m tel : 260< l < 305, -5 < b < 1.5 Sag- C a r ina Spiral Ar m ( wit h Schmidtobreick - ESO, C a r r a r o - P a dova )

? ESO WFI 2.2m : 20 fields in t h e direct ion of t h e Ba r 14<l<349 , -8< b <8 in low reddening region s

(wit h Lub, Omon t , Sch u lt h eis)

? H S T fields in the direct ion of t h e BW8, NGC 6553 (wit h Ortola n i)

? GSPC2 data (in collabor a t ion with L a t t a n zi, Spagna, Bucciarelli-Oss. Tor ino)

Feltzing & Gilm ore (2000) a n d Bea u lieu e t a l (2001) discu s s t h e n a t u r e of t h e “red fla r e”, t h e field population towards the bulge cluster NGC 6553. The turn-off of the red fla r e is visible a t V=20.5 in F ig. 2.

These papers sugges t t h e following hypot h eses:

a ) Met a l poor bulge b) Sa g i t t a r i u s t u r n off

We m a k e use of t h e da t a fr om Zocca li e t a l (2001) t o separate clu s t e r a n d field st a r s on t h e ba s is of t h e pr oper m ot ion s . Fig.2 represents the CMD of cluster and field s t a r s .

The effect of t h e mass dist r ibu t ion in the direct ion of NGC 6553

We poin t ou t t h a t t h e effect of t h e mass dist r ibu t ion on t h e colou r -magnitude diagram is to pr oduce a large spread at the turnoff a n d subgiant b r a n ch level.

The expected distribution of t h e st a r s a long the line of sigh t d ue to t h e effect of different mass dist r ibu t ion in the bulge is shown in Fig.3.

Isoch r ones of Z=0.019 a n d a ge 12 Gyr , dist a n ce 5,8,12 Kpc (solid line) a n d Z=0.008, dist a n ce 5 Kpc

(dot t ed line) on t h e da t a of t h e field population where the clu s t er h a s been r emoved

Sim u lation of the CMD including bulge and disk stars. The red

fla r e is a n a t u r a l con s equence of t h e dist r ibu t ion of t h e st a r s a lon g t h e line of sigh t . Du e t o t h e poor s t a t ist ics, a ll t h e models bu t E 2 a r e in a greement with the data

pin k : Bissa n t z et a l ligh t blue: G0

dark blue :E2 yellow :G2

We m a k e use of Hst a r ch ive d a t a on Bw8 (see F ig.5).

We der ive a r edden ing of Av=1.4.

We sim u late the disk popu lation following Va llen a r i e t a l (2000).

Con cer n ing the bulge simulation we make use of t h r ee populations, namely a) an old metal poor one of age 13-14 Gyr h a ving Z ranging from 0.001 to Z=0.05, b) an old metal r ich one with age 13-14 Gyr a n d Z=0.005-0.03, a n d fin a lly c) a you n g metal rich of age ranging from 10 to 12 Gyr a n d Z fr om 0.008 to 0.05.

We find a dependence of t h e t u r n off m a gnitude on t h e spa t ial dist r ibu t ion of t h e st a r s. Th e t e r m ination poin t of t h e m a in sequ e n ce ch a n ges of 0.3-0.4 m a g beca u se of t h is effect . Th e best fit is obt a ined with model c) a n d a mass dist r ibu t ion G2 ( ?

p r obability 0.97) and reddening Av=1.45. Model a ) a n d b) give a fit on ly for t h e mass dist r ibu t ion B 2 a n d a t oo h igh value of t h e reddening, Av=1.6 a n d Av=1.7 respect ively ( ?

p r obability 0.90) ,

• The Luminosity fu n ct ion in BW8 is well reproduced by a fla t IMF s lope x=1.35 (Sa lpeter x=2.35). F ig.6 represents the luminosity function of the data (dots) t a k e n a t various colou r r a n ges, in com p a r ison w i t h t h e m odel (solid line) ob t a ined with population c) and a flat IMF .

The Galactic plane in the direction of Sag-Carina Arm (Vallenari et al 2002)

Sever a l fields in the direct ion of t h e Sag-Car arm are a n a lyzed:

• 5 fields (Boch u m 9, 10, 11, NGC 3114, Coll 228) between l=283-288, | b| <2 .

• 4 fields h a ving (l=265,276,290,305,| b| <5)

F ig.8 s h ows t h e direct ion s of some of the fields superposed on t h e loga r it h m ic spir a l model by Va llée (2002)

The main results ca n be su m m a r ized:

T h e d a t a a r e in agreement with a loga r it h m ic fou r -a r m spir a l model.

We der ive a p it ch a n gle of 12.7 in a greement with Va llée (2002). Adopt ing a gaussian dist r ibu t ion of t h e st a r s in s ide t h e arm we fin d a s igm a _h=0.07 Kpc.

The field population in the direction of NGC 6553 ( l=5, b=-3) (Vallenari & Ortolani 2001)

The bulge in the Bw8

Fig.6 Fig.7

Fig1

Fig.2

Fig.3

Fig.4

Fig.5

The adopted populations

Fig.8

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S a m p le of t h e ESO 2.2m W F I d a t a