M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission

Beichman, C.

Boulanger, F.

Persson, Carol J. Lonsdale

Rice, W.

Viallefond, F.

English

NASA Technical Reports Server

N87- 24309
Global Properties of the Nearby Spiral M101
C. Beichman, F. Boulanger, W. Rice, C. Persson
Infrared Processing and Analysis Center,
California Institute of Technology and
Jet Propulsion Laboratory
Pasadena, CA 91125
and
F. Viallefond
Institute d'Astrophysique, Paris
M101 (NGC 5457) is a classic Sc I spiral galaxy located
sufficiently nearby, 6.8 Mpc, (Aaronson, Mould and Huchra 1980)
that its structure can be studied even with the coarse angular
resolution of IRAS. This work addresses the global infrared
properties of MI01 including the radial dependence of its
infrared emission.
IRAS pointed observations were combined to construct maps
of M101 at 12, 25, 60 and 100 micron. The processing included
subtraction of a linear baseline from each detector stream to
remove residual electronic offsets and zodiacal emission (Rice
et al. 1986). The emission from the galaxy was integrated using
a circular area of 28' diameter to obtain the integrated fluxes
of M101 presented in Table I. The flux densities include small
color correction factors derived by fitting an intrinsic source
spectrum consisting of the sum of Planck functions at two
temperatures times an emissivity proportional to frequency. The
best fit temperatures are 200 K and 31 K.
Table 1. Global Properties of M101
A. Observations I B. Derived Properties
fv(12) 9 +_ 2 Jy F(IR) 2 5.74E-12 W m-2
fv(25) 12 +__ 2 Jy L(12-100 urn) 1.3E10 L o
f¥(60) 95 _+ 15 Jy IR/B3 0.40
f_(lO0) 260 + 40 Jy L(0.15-100 urn) 1.0Ell L o
Notes: 1)Color-corrected values measured in a 28' diameter
aperture; 2)flux between 40 and 120 micron as defined by
Helou et al. 1985; 3) see text.
A spectrum of the galaxy from the UV to the far-infrared is
presented in Fig. I. The UV data are from Code and Welch (1982),
the visible results from Okamura et al. (1976) and the near IR
from Elmegreen (1980) and all have been corrected for reddening
(Av=0.17 mag) within our galaxy. In making Figure I a geometric
correction factor was applied to the data to account for the
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 297
https://ntrs.nasa.gov/search.jsp?R=19870014876 2020-03-20T10:56:23+00:00Z
C.BEICHMANETAL.
-7
_.. -7.5
o
0'1
lao
--8
V
,.<
V
S
lao
o
'--' -8.5
-9
''1''''1''''1''''1''''1''''1''''1
+ OBSERVATIONS
........... DEREDDENED SPECTRUM
"'% _ _,
• J ;
I , /#sl ÷
+_
÷
,,l,,,,It,,,l,J,,l,i,,l,,,,l,,,,l
-1 -.5 0 .5 t 1.5 £
loglo X (tzm)
Figure 1. Spectral energy distribution of MI01 from UV to far
infrared wavelengths.
fact that observations at the various wavelengths were obtained
with different sized apertures. This factor was based on
extending an exponential disk at each wavelength to infinite
radius.
The determination of the radial dependence of the
infrared emission is complicated by the fact that the size of
the galaxy (-10') is comparable to the resolution of the IRAS
detectors (about 0.75'x4.5' at 12 and 25 micron, 1.5'x4.5' at 60
micron and 3'x5' at 100 micron) and because a number of giant
HII regions are bright at these wavelengths. The following
procedure was invoked to separate these two components. Data
from scans which crossed the galaxy in the same direction were
fitted to a model consisting of an axisymmetric exponential disk
of amplitude I and scale length h convolved with the detector
response; data from the vicinity of the HII regions were
excluded. Fits were made in the scan direction at a number of
different position angles and the results averaged. Table 2
lists the properties of the MI01 disk.
298
THE GIANT SPIRAL M101
Table 2. The M101 Disk
Wavelength Amplitude Scale Length
Disk
(_lm) (MJylsr) ( ')
12 3 . 1 3 . 0+__.0.2
25 3 • 4 3 . 2+0.2
60 18 3.11_+0.4
100 45 3.4+0.5
CO 3.61
Visual Light 2.2-2.6
Integrated Model
(kpc) (D>>28') (Jy)
5.9+_0.4 14
6.3+_0.4 1 8
6.7+0.8 110
6.7+_.1.0 280
Notes: 1) Solomon et al. 1982.
Figure 2 shows a 25 micron map of M101 before and after
subtraction of exponential disks. While the fitting procedure is
not yet perfect, as evidenced by the negative parts of the maps,
the procedure enables one to detect five of the major HII
regions in the galaxy, N5447, N5455, N5461, N5462 and N5471.
Even preliminary estimates of the brightnesses of the HII
regions allow one to state that the emission from MI01 is
dominated its disk. The influence of the major optical HII
regions is negligible at 12 and 25 micron and less than 20% of
the total at 60 and 100 micron.
Overall Properties
M101 is similar to other spirals detected by IRAS with an
infrared luminosity and 12/25 and 60/100 micron colors
consistent with those of a galaxy with modest amounts of star
formation (Helou, this conference). A number of authors have
used the IR to blue luminosity ratio, defined as _fv(80)/_f_(B)
where f_(80) is the average of the 60 and 100 micron flux
densities, as a measure of infrared activity. MI01 has an
IR/blue ratio of 0.4, similar to that inferred for our own
galaxy and close to the median for spirals detected by IRAS
(Soifer et al. 1984).
OriKin of the Infrared Emission
M101 is an average spiral galaxy and it is therefore
important to understand the nature of its infrared emission
mechanism and to distinguish it from the more luminous infrared
galaxies. We argue that much of the infrared emission from MI01
originates in the neutral medium, and not in the ionized
component.
First, the ratio of 12 to 25 micron flux densities (Table
3) implies a temperature of 200 K and, as shown by the scale
lengths in Table 2, is roughly constant across the face of the
galaxy. This emission accounts for approximately 25% of the 12-
100 micron luminosity of the galaxy. The existence of large
amounts of hot, 200 K, material outside of HII regions cannot
be accounted for within the context of equilibrium heating of
299
C. BEICHMAN ET AL.
grains in the interstellar radiation field of the galaxy.
Further, the lack of a radial temperature gradient runs counter
to the expectation that, for equilibrium heating of dust, the
temperature in the disk should decrease as the radiation field
falls off with increasing radius.
Table 3. M101 Colors
MI01 Disk Galactic Cirrus
log(f_(12)/f_(25)) -0.12 -0.2 to 0 1
log(fv(25)/f$(6 0)) -0.92 -0.65+_0.05 _
log(f_ (60)/f_(I00) ) -0.34 -0.81+0.022
Notes: 1) Boulanger et _al. 1985; 2) Weiland et al. 1986.
A solution to this problem may be found in the similarity
of the 12/25 micron color of the MI01 disk to that of the hot
cirrus found within our own galaxy (Boulanger, Baud and van
Albada 1985; Weiland et al. 1986). The warm Galactic cirrus has
been attributed to small (<10 Angstrom) grains, possibly
polyaromatic hydrocarbons (Puget, Leger and Boulanger 1985)
transiently heated by the absorption of single UV photons. In
this process the maximum temperature reached by a grain is
independent of the intensity of the radiation field; the
intensity of emergent spectrum, but not its shape, depends on
the input intensity. We suggest that the existence of large
amounts of 12 and 25 micron emission in the disk of MI01 is due
to the existence of such small, stochastically heated grains.
The poorer spatial resolution of the 60 and 100 micron maps
makes it harder to separate the disk component from the HII
regions. The 60/100 micron color of the disk is considerably
hotter than that seen toward typical Galactic cirrus and may
partially be due to unresolved HII regions. However, not all of
the infrared emission can originate in HII regions. On the
basis of a catalog of 21 cm continuum observations of HII
regions in MI01 and the integrated intensity of the galaxy at
2.8 cm (Klein and Emerson 1981), Viallefond (1986) has
estimated that about 100 mJy of the 21 cm emission from MI01 is
thermal. From this one calculate that the infrared excess (IRE)
of this disk exceeds 16, far higher than for typical HII regions
with values of 5 (Myers et al. 1986). Such a high IRE suggests
that predominantly non-ionizing photons, from A or later type
stars and not from OB stars, dominate the heating of the disk.
References
Aaronson, M. Mould, J. and Huchra, J. 1980, Ap. J., 237, 655.
Boulanger, F., Baud, B. and van Albada, T.
1985, Astr. Ap.,144,L9.
Code, A.D. and Welch, G. A. 1982, A__p__ J.,256,1.
Elmegreen, D. M. 1980, Ap. J. (Suppl), 43,37.
Helou, G., Soifer, B.T. and Rowan-Robinson, M. 1985,
J., 298, L7.
Klein, U. and Emerson, D.T. 1981, Astron. and Astrop_, 29, 94.
3OO
THE GIANT SPIRAL MI01
I IIHo2Mo0 S IqHoIMo0 s I q_fo0_o00_
\ V" \ \ --_ \
ItFFF I_'FF FF F FF F F FF F F IFEF IFFF FFF
," i, I/ I ,,tl I'_-. f ,_ _ . ' .... .i , , . _ _! F
_F ; !, "",C' "" ",;;'/".',';, ,, ;..'t.'V
It" I_, 'hi 'V "."+ ''J'. • .. ' "\I
, r i i. ¢, ,) _,, _ q ¢. P i ,"'_" z _ 5_" 40 0i_
i_ Ix t it I.. "_v ," ./-. # _i |.'_i,'._.,.rI 13n59'"00°
,. I_. LI . . • I Ii _11 I II lllti" t
F • • ,e "I ' lit % |) bt • ++ *1-_
I*% I " m *% .'1 _ . * . It I I _ II II |
I • , " ' I '}. _" .. ,,,, [\ +1 -- I I +, ,,I I_
\ u I i/r'_ "'_ • . " " • .. _l I Il P I . • i i ¢"
't I 'l _' ' _ ' _ I .fl, . j,, i, . i_ /
' 'dl i I I ' i I , i'll*i I
' I' I I I i _ | Ill
II Ii ill I iii/%', I %l _-1 '.l A ' l
, , A/-'vyllf_ #,., _v Irl (__t i!,, .\_ 54"30 01_
',,,,'_ _i. 'llJ V [I II/ III C"/_I; i t 4
'"/,,/ (/A b }/c4ltltilt",_, ' _1 I I%--. 'l I "
5,,m o_'_ - ,.ill#, , .\t°).,llll/t/_.#_tlll<>\l'll , , ,
' ,; I_.l ,,' ' t_'lltilffv,-.--CIIr--_llll /lUll " . 'r
_F -, i, V'.'_< .... I /
, ,, ,., w _tltl_]_l]',,llill/ IN/, .. ,,,,
F ..... ,, ,," ,".-" It_J..IU//_X"/AuJ/J _..," , ...._
!1 I, 7 " /_"1 ""-'" C_v// t_f "' I' !)t)l ) c "F
_;:° , ,, :,:,', , ',l w._,'.,.." '"" _i_ s'_ o
_, ','..' " ':,, ' , ,'" ,. ,':'. {;F - ._dlool;loo_
I tJ,' i (I I I I , I ,.' ' I
F I _ P I ._ _ It "I I I , I # j( • I'.F
FFF F FF FFF F F F FFF F P
\ \_\ \ WI \
I qHo 3Moo 5 l 4Ho 2Moo 5 I qHo LMoo5
5q"20 O_
IqHo2Mo0 5 iqitOlMO0 S 1_t_l'O_O0 01_b
FFF FF F FF F F FF F F FIFF IFFF FFF
tllll " ll''_ °'l I )l" ' "1 )% . "i I' %'1 ' "" '
,' ,, v (.,I.' ,'7"," " _'-' '".... ' ,, ,, F
• Y"_ 1',;^",./'.. I.l_,..J "l . (,, /t.' all'if,. (, i3ns9no0 ';:
' , ,/,, ! ,",.'s_t 't .1,'",?' +',i I I +i
I .'_ I Ill" ,-", I I I , "t\ • I I
f "l '_. I:'1 I1[ I _-i!l_l '1
_U°_°B : , ,-'. , t, ; ,::.j v..,.,ot
l I', I'#l_'-- _ . '
I/ II I:l tl i" i,.% It'%t I
II I i,iI I II /\~7 \"-"_--'%"k
I' I ' I.I ..
I - _-t J" i, "_
I I I '11
,:,,,' • ,," _I __ 5_'30 oO
'".,"--:""" ,"_i I_l!<'_',',.",; ;' .llI'i;I,,_'" ] till, +l" ,',, : ,,,i, .., ", ,
F "" I tl I "" 7il• ill)i,,, ,O,Vl:l_f",, ,ll,l, v _,,
," ', I"l,,, 'i'l '"- '' , :u/'., ' ',- /
XF ",, .h U! I i " I i ," ,.(.-.
F i/ t, "7 ""l.- ILl -wi, i"
/FF I: , _l,_' ' )(_l.i ii /' t/' ." ¢/;_.-"_ ill ' I-F |qrloorlo0_;" v!, i.,o, f'-,: _ 51r2o oO
F 11_ P I ._ • II #'* i II
FFF F FF FFF F F FFF FFF F F
I wHo3Moo 5 IqHo2Moo 5 I qHo lMoo5
5U"20 Ob
Figure 2: a) 25 micron map of M1011 b) 25 micron map after
subtraction of an exponential disk (dashed contours are
negative). The HII regions N5_7, N51155, N51161, !I51162 and N5]171
are marked with letters a-e.
301
C.BEICHMANlETAL.
Myers, P. C. Dame, T.M., Thaddeus, P., Cohen, R.S.
Silverberg, R.F., Dwek, E., and
Hauser, M.G. 1986, A ._., in press.
Okamura, S., Kanazawa,T., Kodaira, K. 1976, P.A.S.J.,28,329.
Puget, J.L., Leger, A. and Boulanger, F. 1985,
Astr. _Ap., in press.
Rice et al. 1986, in preparation.
Soifer, B.T. e_ttal___.1984, A__ J., 278, L71.
Solomon, P. Barrett, J., Sanders, D.B. and
de Zafra, R. 1983, Ap.J. _Letters_, 255, L99.
Viallefond, F. 1986, private communication.
Viallefond, F., Goss, W. M. and
Allen, R. J., 1981, Astr. Ap., 104,127.
Weiland, J.L., Blitz, L., Dwek, g., Hauser, M.G.,
Magnani, L. and Rickard, L.J. 1986 preprint.
302


Global properties of the nearby spiral M101

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server