1
00:00:00,012 --> 00:00:06,604
Let us know take a look at the structural
or photometric properties of spiral

2
00:00:06,604 --> 00:00:11,171
galaxies.
First, about spiral galaxies in general,

3
00:00:11,171 --> 00:00:16,905
they do have a lot of diversity in their
structure and components.

4
00:00:16,905 --> 00:00:22,241
First of all, there is the degree of, in
which their spiral arms are prominent,

5
00:00:22,241 --> 00:00:27,981
which was, of course, the basis of Hubble
classification, as well as the ratio bulge

6
00:00:27,981 --> 00:00:31,333
to disk.
And because bulge and disk have different

7
00:00:31,333 --> 00:00:36,415
stellar populations, bulge being an old
one, disk being younger, there are all

8
00:00:36,415 --> 00:00:40,739
these differences in star-formation rates,
and colors, and so on.

9
00:00:40,739 --> 00:00:46,321
The disks, of course, have interstellar
material called gas and dust, from which

10
00:00:46,321 --> 00:00:50,497
stars are forming.
Also, spirals tend to avoid dense regions

11
00:00:50,497 --> 00:00:56,097
of the large scale structure, because this
is where galaxy mergers can happen, or do

12
00:00:56,097 --> 00:00:59,408
happen.
And those spirals have been turning into

13
00:00:59,408 --> 00:01:03,442
ellipticals.
Recall the trends that we discussed about

14
00:01:03,442 --> 00:01:07,911
Hubble sequence.
Spirals, of course, participate in all of

15
00:01:07,911 --> 00:01:10,866
them.
Most of the trends are, is along the

16
00:01:10,866 --> 00:01:17,024
spiral branch, and they are listed here.
But basically, as we go from early Hubble

17
00:01:17,024 --> 00:01:23,069
types towards the late ones, there is an
increasing amount of star formation,

18
00:01:23,069 --> 00:01:28,433
increasing amount of gas and increase in
the disk to bulge ratio.

19
00:01:28,433 --> 00:01:34,925
All of this manifest itself in colors,
they get bluer and other measured for the

20
00:01:34,925 --> 00:01:39,577
metric properties.
So, let's look at individual subcomponents

21
00:01:39,577 --> 00:01:43,254
of spiral galaxies.
First, there are the disks and they're

22
00:01:43,254 --> 00:01:47,958
characterized by exponential distribution.
We'll show that in a moment.

23
00:01:47,958 --> 00:01:50,789
And there may be more than one kind of
disks.

24
00:01:50,789 --> 00:01:54,556
There are thin disks within which star
formation occurs.

25
00:01:54,557 --> 00:02:00,683
That's where the interstellar material is.
But they may be embedded into thicker

26
00:02:00,683 --> 00:02:05,035
disks, composed mainly of intermediate to
old aged stars.

27
00:02:05,035 --> 00:02:09,546
The bulges are essentially ellipticals in
middle of spirals.

28
00:02:09,546 --> 00:02:15,405
Not all spirals have bulges, some simply
have disks that go all the way to the

29
00:02:15,405 --> 00:02:21,178
middle, maybe rounded off, but no distinct
elliptical-like component.

30
00:02:21,178 --> 00:02:27,520
Now, unlike Baade's original idea of
Population II stars that's supposed to be

31
00:02:27,520 --> 00:02:33,760
metal-poor, stars in halo are metal poor,
those in galactic bulge as well as in

32
00:02:33,760 --> 00:02:39,637
ellipticals are actually metal-rich.
They did result from a substantial amount

33
00:02:39,637 --> 00:02:44,227
of chemical evolution, but very early on.
And again, there is the important

34
00:02:44,227 --> 00:02:47,935
difference in dynamics.
The disks are rotationally supported

35
00:02:47,935 --> 00:02:52,466
against self-gravity, and bulges are
pressure supported, random motions.

36
00:02:52,466 --> 00:02:58,228
About half of all spirals contain bar-like
feature, and you've seen pictures,

37
00:02:58,228 --> 00:03:02,036
sometimes spiral arms begin at the ends of
the bar.

38
00:03:02,036 --> 00:03:07,073
Bars are similar to bulges in their
composition, but dynamically, they're

39
00:03:07,073 --> 00:03:10,508
distinct.
In their very centers, some people argue

40
00:03:10,508 --> 00:03:15,327
that spirals contain an additional
component concentric with the bulge

41
00:03:15,327 --> 00:03:20,383
itself, which can be very dense,
sometimes, may contain supermassive black

42
00:03:20,383 --> 00:03:25,904
hole, which may or may not be active and
there is usually some star formation.

43
00:03:25,904 --> 00:03:32,210
If you remember, numerical experiments and
collisions of spiral galaxies, the gas

44
00:03:32,210 --> 00:03:36,264
tends to sink to the center, because it
loses energy.

45
00:03:36,264 --> 00:03:41,706
And if you accumulate a lot of gas with
high density, it'll tend to make stars.

46
00:03:41,706 --> 00:03:47,151
In contrast to that, the most extended
stellar component is the stellar halo.

47
00:03:47,151 --> 00:03:51,777
And that is composed out of metal-poor
stars, the kind of stars that make

48
00:03:51,777 --> 00:03:55,797
[unknown] galaxies.
So, we believe that all of the stellar

49
00:03:55,797 --> 00:04:01,690
content of the galactic stellar halo, is
from disrupted, merged [unknown] galaxies

50
00:04:01,690 --> 00:04:05,160
that have been torn apart by tidal, tidal
forces.

51
00:04:05,160 --> 00:04:09,043
And there are simply, they are, they're
populated the halo.

52
00:04:09,043 --> 00:04:12,592
And there is lot of good experimental
evidence for that.

53
00:04:12,592 --> 00:04:18,144
And of course, there is the dark halo.
Dark halos are more extended than visible

54
00:04:18,144 --> 00:04:21,929
parts.
There are some hints as to whether radial

55
00:04:21,929 --> 00:04:25,441
distribution would be, as well as their
shape.

56
00:04:25,441 --> 00:04:31,863
They're probably triaxial ellipsoids.
So, the way we can quantify distribution

57
00:04:31,863 --> 00:04:36,416
of visible material is through surface
odometry meaning.

58
00:04:36,416 --> 00:04:40,924
But before we look at the distribution of
stars, we first need to correct for the

59
00:04:40,924 --> 00:04:44,944
inclination effects, because phase on
spirals are more or less circular, so

60
00:04:44,944 --> 00:04:49,041
their apparent distance in the sky tells
you about the inclination is.

61
00:04:49,041 --> 00:04:52,933
Then, we have to correct for the
interstellar extinction, both in the

62
00:04:52,933 --> 00:04:57,355
galaxies themselves, but also in the Milky
Way and the direction that we look at

63
00:04:57,355 --> 00:05:01,388
stellars.
Usually, these radial surface brightness

64
00:05:01,388 --> 00:05:06,488
profiles are obtained by averaging in
circular or elliptical annuli.

65
00:05:06,488 --> 00:05:10,166
And so, that tends to average over the
spiral arms.

66
00:05:10,166 --> 00:05:15,479
As it turns out, typical surface
brightness profiles of spiral disks are

67
00:05:15,479 --> 00:05:19,788
exponential.
The projected surface brightness declines

68
00:05:19,788 --> 00:05:25,407
exponentially from the center out.
And the enfolding lengths or disk scale

69
00:05:25,407 --> 00:05:31,366
lengths served the order few kiloparsecs.
So, if you measure a surface brightness

70
00:05:31,366 --> 00:05:35,557
profile of a galaxy, then you'll see
something like this.

71
00:05:35,557 --> 00:05:40,235
If plotted on semi log plot, log surface
brightness versus linear radius, an

72
00:05:40,235 --> 00:05:44,907
exponential looks like a straight line,
and indeed that's what you see in the

73
00:05:44,907 --> 00:05:48,828
outer regions.
There is extra light in the middle, and

74
00:05:48,828 --> 00:05:53,338
that's due to the bulge.
And so, one can then fit an exponential

75
00:05:53,338 --> 00:05:59,160
disk plus the bulge component, using one
of the elliptical galaxy-fitting formulae

76
00:05:59,160 --> 00:06:04,408
that we will look at in the next chapter,
and does decompose the light in, into

77
00:06:04,408 --> 00:06:08,631
bulge and to disk.
So, this is now well-established procedure

78
00:06:08,631 --> 00:06:11,918
and many galaxies have been studied in
this way.

79
00:06:11,919 --> 00:06:16,937
Here are examples of a few.
Now, here are some contour maps of the

80
00:06:16,937 --> 00:06:21,916
surface brightness distribution of spiral
galaxies in the sky.

81
00:06:21,916 --> 00:06:28,508
And these are lines of equal brightness.
And to anticipate something I'll show in a

82
00:06:28,508 --> 00:06:31,936
moment, they do seem to stop at some
point.

83
00:06:31,936 --> 00:06:37,668
So, what about perpendicular structure?
I mean, along the radius from center out,

84
00:06:37,668 --> 00:06:41,202
their exponential.
But it turns out, the distribution of

85
00:06:41,202 --> 00:06:45,604
density in spiral galaxies perpendicular
to the plane of the disk, is also

86
00:06:45,604 --> 00:06:48,740
exponential.
And the typical scale height is of the

87
00:06:48,740 --> 00:06:53,589
order of hundreds of parsecs.
Much shorter than the disk scale length of

88
00:06:53,589 --> 00:06:57,983
several kiloparsecs.
So, one interesting thing about spiral

89
00:06:57,983 --> 00:07:02,557
galaxies is that, their disks have
cutoffs, they do have an edge.

90
00:07:02,557 --> 00:07:05,809
Past certain radius, there are no more
stars.

91
00:07:05,809 --> 00:07:11,935
However, hydrogen, from which stars are
made usually extends further and the dark

92
00:07:11,935 --> 00:07:15,363
halo, further yet.
So, that suggests that disks may be

93
00:07:15,363 --> 00:07:18,631
forming from the inside out, in terms of
star formation.

94
00:07:18,631 --> 00:07:22,462
So, if you measure the profile, you can
take the integral under it.

95
00:07:22,462 --> 00:07:27,225
And that gives you the total luminosity of
the galaxy, or its two components, the

96
00:07:27,225 --> 00:07:30,671
bulge and the disk.
Inclination correction tends to be the

97
00:07:30,671 --> 00:07:34,473
trickiest of them all.
But, now we know how to do that.

98
00:07:34,473 --> 00:07:41,011
And the extinction correction depends on
estimating the amount of dust along the

99
00:07:41,011 --> 00:07:45,420
line of sight.
Because interstellar extinction causes

100
00:07:45,420 --> 00:07:51,400
reddening, absorbing more blue light than
red light, using colors can give us

101
00:07:51,400 --> 00:07:57,500
indication how much reddening there is.
And from that, how much of total

102
00:07:57,500 --> 00:08:03,370
absorption there is.
Next time, we will talk about interstellar

103
00:08:03,370 --> 00:08:06,330
medium, gas in spiral galaxies.