49 accepted fits was 0.017 (in sky intensity units) and the means and

In document Photometry and kinematics of pure disk galaxies (Page 78-80)

Chapter IV gives the photometry, kinematics and analysis for the three Sculptor Group galaxies NGC7793, NGC 2hr( and NGC 300;

49 accepted fits was 0.017 (in sky intensity units) and the means and

modes were always less than 0.002.

The second test was done hy averaging strips of varying widths at position angle 0° and 90° either centered on the galaxy or offset from it. The resulting "scans" were reduced in the same way as the photoelectric drift scans (see next section). After excluding the galaxy channels (if necessary) a 2-a clipping routine was applied until no more points were rejected to exclude all the star images. A linear least squares fit was then performed to the remaining channels. Again, zero points and gradients (from one side of the plate to the other) of the accepted fits were always less than O.OlIg. This gives us confidence in our Schmidt profiles at low light levels.

(iv) 'Zero point determination'

In order to get our magnitude zero points, we used our photoelectric (pe) drift scans. Although this technique is very time consuming ( 1 night / color / galaxy), it provides, at the same time, a very good check for the density-intensity calibration. In fact, it is the drift scans which showed us the bad calibration of the U.K. Schmidt North Wedge and brought us to recalibrate it. Between 20 and h O scans were obtained for each color and for each galaxy.

We know that the pe surface magnitude and the pg log I are related by the following expression

50

In o r d e r t o g e t A ( s lo p e ) and B ( z e r o p o i n t ) , we w ro te a p ro g ram c a l l e d SIMDS w h ich s im u l a te s a c i r c u l a r a p e r t u r e r u n n in g th r o u g h th e pg d a t a a r r a y . The s a m p lin g r a t e / c h a n n e l was u s u a l l y a ro u n d 10. A c c u ra te p o s i t i o n s o f s t a r s in t h e pe scan w ere m easured by g a u s s ia n

f i t t i n g t o e n s u r e t h a t t h e s c a l e s w ere w e ll m atch ed . The pe and pg s c a n s w ere th a n c r o s s - c o r r e l a t e d t o g e t th e p r o p e r r e g i s t r a t i o n . A p l o t o f t h e p h o t o e l e c t r i c s u r f a c e m a g n itu d e s v e r s u s th e p h o to g r a p h ic - 2 . 5 x lo g I o f t h e s im u la te d sc a n i s g iv e n f o r one o f th e J p l a t e in f i g u r e 2 .7» The z e r o p o i n t s f o r e a c h p l a t e (B) a r e g iv e n in t a b l e 2 .2 . No s i g n i f i c a n t d e p a r t u r e from t h e t h e o r e t i c a l s lo p e A =1.0 was fo u n d . T h is c o n firm e d t h e r e l i a b i l i t y o f o u r d e n s i t y - i n t e n s i t y c a l i b r a t i o n . (v ) 'C o lo u r t r a n s f o r m a t i o n s '

In o r d e r t o com pare o u r r e s u l t s w ith th o s e o f o t h e r i n v e s t i g a t i o n s an d w ith t h e o r e t i c a l m o d e ls, we d e r iv e d t h e t r a n s f o r m a t i o n s betw een o u r c o l o u r ban d s UB^-R^I and t h e s ta n d a r d Jo h n so n UBV a n d C o u sin s c o l o r s . The f o llo w in g c o n v e r s io n

fo rm u la e w ere a d o p te d f o r t h i s s tu d y : B = Bj + 0 .0 6 x (B j-R p ) (5 ) (B-V) = 0.61) x (B j-R p ) (6 ) (U-B) = -0.01) + 0 . 9 1* x (U-B ) (7 ) (U-R) = - 0 .0 2 + 0 .9 7 x ( U-R ) (8 ) r

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In document Photometry and kinematics of pure disk galaxies (Page 78-80)