The variation of the dust temperature within late-type spiral galaxies

N95- 21766 /

i2i y

S75 772

THE VARIATION OF THE DUST TEMPERATURE
WITHIN LATE-TYPE SPIRAL GALAXIES

RH. EVANS

Dept. of Physics and Astronomy, University of Toledo, Toledo, OB 4-3606

ABSTRACT We use Hilles 60 and 100 /xm data to investigate the
variation of the dust temperature in a sample of 4 late-type spiral galax-
ies. We have investigated the radial variation of the azimuthally averaged
60 and 100 /an surface brightness profiles to see how the dust temperature
(or, more correctly, the relative strength of the two components) varies
as a function of radius within the galaxies. We find strong evidence for
a decrease in the dust temperature (or an increase in the relative con-
tribution of the 100 /Jin 11 ux compared to the 60 /xm flux) as a function
of radius. We discuss these results in the light of the continuing debate
as to whether massive star formation or the general interstellar radiation
field is the major heating source of the dust.

DISCUSSION

The profiles (Figs. 1 - 4) in general show a decrease in the relative contribution
of the 60 to 100 /xm flux, as one would expect from a cirrus component heated
by a weakening general interstellar radiation field. However, there is also clear
evidence for warmer regions, which can be tied in with spiral arm features seen in
the HiRes images. Because we have azimuthally smoothed the images we are not
able from these data to determine the relative temperatures of the "warm" and
"cool" dust components. However, by doing aperture photometry on individual
giant H II complexes, where we would expect the warm component to nearly
totally dominate the cool component, we hope to be able to better tie down the
warm dust component. The cooler component can then also be determined, as
it will be just the excess required to fit the observed 60 to 100 /xm flux ratios
in other regions. HiRes and KAO data thus provide a unique opportunity to
better determine the relative contributions of each of these components in both
individual galaxies and individual regions of galaxies.

Elsewhere in this workshop Dcvereux has shown 60 to 100 /xm ratio profiles
for the galaxies M101 and M81 which show a constancy of dust temperature
with increasing radius. In fact, Dcvereux (1993) has also found a constancy for
the same galaxies as in this study, a discrepancy which is disconcerting, but
could be due to the different default values used in the HiRes images used in
this studies and those in Devereux's. Clearly this vital question needs further
study.

S. Terebey and J. Mazzarella (eds.), Science with Hi^h-Spatial Resolution Far Infrared Data, 121-126,
1994, Jet Propulsion Laboratory, Pasadena, CA.

122

EVANS

IC 342

1Q pi in ii i i I i i m i r i i t I i i i i t ri'i i | i i r i i t r T'i I i i i i i

-4

+ 100/um
O 60/xm

+
O +

o

I 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 I 1 1 I t I I t 1 1 1

IC 342

35

30

i»

3

■4-1

u
a
a,

25

3
O

20

i 1 1 i i i i i i 1 i r ! i m i i i [ i i r 1 1 i 1 1 i ] i 1 1 ir i i r t 1 1 r i i i i 1 1

10

20 30

Pixels

40

50

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 > 1 1 1 1 1 1 I i ■ 1 1

10 20 30 40 50

Pixels

FIGURE 1 The 100 and 60 /an surface brightness profiles of IC 342, and
the "dust temperature" derived from the 60 to 100 /«n flux ratios, for the two
emissivity indices ft — 1 and 2. In addition to the increase in 60/100 ratio due to
the spiral arms, which can still be seen in the convolved 60 fim image of IC 342,
the underlying disk shows a decrease in temperature with radius. The global
"dust temperatures" (i.e., those found by measuring the total 60 and 100 /im
fluxes from the whole galaxy) are 32.2 K and 27.4 K in the ft = 1 and 2 cases,
respectively.

DUST TEMPERATURES WITHIN LATE-TYPE SPIRAL GALAXIES 123

M51

M51

-10

v

C

x:

M

'C
m

Si ~ 6

X

3

llil — i i i i l I i i i i | i i — i l | l i i ■

'""^V

x-

+ 100/xm
O 60/i.m

+

■'ooOo

o +
o +

o +

o

35

30

25

3
Q

, I i

o

. I .... I

I I I I I I I I I I I I I I I I I I I I I I I ''' '

0=1
/S=2

20

i I I 1 i i 1 i ■ i i 1 i I I I I L..I. I — L_

10

15
Pixels

20

25

30

10

15
Pixels

20

25

30

FIGURE 2 The 100 and 60 /tin surface brightness profiles of NGC 5194, and
the "dust temperature" derived from the 60 to 100 /mi flux ratios, for the two
emissivity indices ft - 1 and 2. NGC 5195 was removed from the image (using
the I RAF badpixel routine) before profiling the image. Apart from a prominent
spiral arm feature, which can be seen in the 60 /mi profile and is shown by the
two maxima in the full resolution 60 /un image of M51, the underlying disk
shows a decrease in temperature with radius. The global "dust temperatures"
are 33.7 K and 28.5 K in the ft - 1 and 2 cases, respectively.

124

EVANS

NGC 6946

. iq I — i — i — i — i — I — i — i — i — i — ] — i — i — i — i — 1 — i — i — r— i — i — i — i — |-

-B

c

*J

Si
60

m -6

<u
o
«J

I

3

in

-4

.^

+

o

X

+

+

+

+

'Os

+

100/rni
60yum

O

+

+

o +
o

_i I I I I I I I 1 1 L-

-I I I I I I l__i_

36

34

NGC 6946

26

24

~i — i — i — I — i — i — i — i — I — i — i — i — i — I — r

0=1

0=2

_l — 1 — 1 — L_

10 15

Pixels

20

25

10 15

Pixels

20

25

FIGURE 3 The 100 and 60 /an surface brightness profiles of NGC 6946, and
the "dust temperature" derived from the 60 to 100 /mi flux ratios, for the two
emissivity indices (i — 1 and 2. The sharp rise in the 60/100 ratio at the outer
radii is probably due to the two features which cause extensions either side of
the nucleus in the Hi Res image of NGC 6946. The global "dust temperatures"
are 32.9 K and 28.0 K in the ft — 1 and 2 cases, respectively.

DUST TEMPERATURES WITHIN LATE-TYPE SPIRAL GALAXIES 125

NGC 2403

NGC 2403

-6

-4

to

10

v
G
+j
fi

5°

'u

m
<o

o
<a

u
m

-2

i i i i I i i i i

I i i i i I i i i i I

i i i i i i i i i i i i i i

-f 100/u.m
O 60/j.m

o

+
o +

+

o

1 I I I I I ■ I I I I I I I I t I I I ■ I t ■ I I I I

5 10 15 20 25 30 35

Pixels

32

30

24

22

I i i i i I i i i i I i i i i I i ■ i ' I ' i i i I i

0=1

0=2

20 I ■ i ■ i I i ■ ■■ I ■ ■ i i I i i i i I i i i i I i i i i I i i i i
5 10 15 20 25 30 35
Pixels

FIGURE 4 The 100 and 60 /xm surface brightness profiles of NGC 2403, and
the "dust temperature" derived from the 60 to 100 //m flux ratios, for the two
emissivity indices fi - 1 and 2. The 60/100 flux ratio shows a near constant
behavior for the inner parts, after which it falls. The rise in 60/100 is due to
the feature picked up in the 60 //in profile at ~ 15-20 pixels, but not in the 100
//m profile. The global "dust temperatures" are 31.1 K and 26.6 K in the fi - 1
and 2 cases, respectively.

126 EVANS

REFERENCES

Devereux, N.A., 1993, Private communication