Initial results are presented of a program to map the centers of galaxies in the mid-infrared using the NASA-MSFC 20 pixel bolometer array. Maps at 10.8 micrometer of the galaxies NGC 5236 (M82), NGC 1808, NGC 4536, and NGC 4527 reveal complex emitting regions ranging in size from 500 pc to 2 kpc. The infrared spatial distributions generally resemble those in the visible and radio. In all cases a large fraction of the IRAS 12 micrometer flux originates in spatial structures prominent in the maps

Decher, R.

Leggett, S. G.

Ramsey, B. D.

Telesco, C. M.

Wolstencroft, R. D.

English

NASA Technical Reports Server

N87" 24336
THE INFRARED MORPHOLOGY OF GALACTIC CENTERS
C. M. Telesco, R. Decher, and B. D. Ramsey
Space Science Laboratory, NASA Marshall Space Flight Center
R. D. Wolstencroft and S. G. Leggett
Royal Observatory, Edinburgh
ABSTRACT. We present initial results of a program to map the centers of
galaxies in the mid-infrared using the NASA-MSFC 20-pixel bolometer array.
at 10.8 um of the galaxies NGC 5236 (M83), NGC 1808, NGC 4536, and NGC 4527
reveal complex emitting regions ranging in size from 500 pc to 2 kpc. The
infrared spatial distributions generally resemble those in the visible and
radio. In all cases a large fraction of the IRAS 12 um flux originates in
spatial structures prominent in the maps.
Maps
i. INTRODUCTION
The central regions of many galaxies emit intense radiation at I > 5 _m
usually attributed to warm dust. Complexes of young stars and, in some cases,
non-thermal sources appear to play a key role in heating the dust, but our
understanding of this phenomenon is severely limited by our not knowing how the
emission is spatially distributed in the centers of a large sample of
galaxies. We must be able to relate their infrared spatial structure to
fundamental galactic properties determined throughout the spectrum.
Although numerous galaxies have been observed at I > I0 _m by IRAS and from
the ground, these survey data are either of low spatial resolution, as in the
case of IRAS, or obtained with a single small focal-plane aperture (typically 6"
in diameter) centered on a visually prominent position. Multi-aperture
observations (e.g., Rieke 1976) have established scale sizes for central
infrared-emitting regions in a few galaxies, while mapping (e.g., Telesco and
Gatley 1984) has defined the details of the infrared spatial distributions in
several others. However, single-beam infrared mapping is very time consuming,
and the list of mapped galaxies is still short; only nine were mapped prior to
the present study. To address this problem, we at NASA Marshall Space Flight
Center have developed a 20-pixel bolometer spatial array for use at 10-30 um.
This array has substantially increased our mapping speed, as illustrated by the
fact that the total time required to map the four galaxies discussed here was
less than half an observing night.
2. OBSERVATIONS AND ANALYSIS
The maps were made at 10.8 um (AI = 5.3 _m) and were obtained at the NASA
Infrared Telescope Facility in 1985 November (NGC 1808) and 1986 March (M83, NGC
4536, and NGC 4527). The 20 array pixels were arranged in a 5 x 4 (RA x Dec)
configuration with each square pixel having dimensions (FWHM) 4']3 4']3.x The
pixel center-to-center separation was 415. All maps are presented here at the
same scale, with the pixel size shown in Figure 2. Except for the lower 5" of
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 497
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C. M. TELESCO ET AL.
the map of NGC 1808, the separation between adjacent observed positions was one-
half pixel. The lowest contours drawn in each map correspond to a signal-to-
noise ratio _ 2, and the mapped regions are enclosed by dashed lines. In these
maps, the cross designates the visually prominant position used for guiding at
the telescope. We have also found that the 2 _m peaks, which correspond to the
true nuclei as defined by the peaks in the mass distribution of old stars, are
coincident with the visible nuclei in M83, NGC 4536, and NGC 4527.
The four galaxies are bright IRAS far-infrared sources also detected by
IRAS at 12 and 25 _m. The quoted flux densities are for 10.8 _m, but before
comparison of our and IRAS fluxes, our flux densities are converted to
equivalent 12 _m values using the 12 and 25 _m spectral slope, which is also
used to color-correct the 12 _m IRAS data. Presented flux uncertainties do not
include the ± 10% inherent in the absolute flux calibration.
3. RESULTS
Our 10.8 _m maps are fully reduced, but our analysis of these data is in
progress. These results are therefore presented without significant
interpretation. Preliminary comparison of the central infrared sources with
available radio and visual images shows that the shapes are similar. The study
of detailed correspondance (or lack thereof) of structure at various wavelengths
must await the accurate relative positioning of our maps with respect to radio
and visual images, a procedure now underway. Note that quoted infrared
luminosities correspond to the range 10-300 _m and have been estimated by
assuming a reasonable extrapolation beyond i00 _m (see Telesco and Harper
1980). We have also assumed that the energy distribution at each position in a
map resembles that observed from the whole region.
3.1 NGC 5236 (M83)
The total 10.8 _m flux
density in our map of NGC 5236
(Figure I) is 3.3 ± 0.2 Jy,
which accounts for 80% of the
IRAS 12 _m flux. The diameter
of the lowest contour is 500 =
pc at 6 Mpc, and the total
O
source luminosity is ~I x w
1010L e. Each of the two o
10.8 _m peaks emits ~I x
109Le in a region 130 pc in
size. It is noteworthy that
no prominent infrared source
occurs at the visual and
2 _m nucleus. The infrared
map bears a striking
resemblance to the 21 cm map
presented by Condon et al.
(1982). The visual properties
of the region spanned by our
map are dominated by normal
10
-5
I I I I I
I I I I I
10 5 0 -5 -10
A RA (arc $ec)
Figure I. NGC 5236 (SBc) at 10.8 _m.
The contour levels (mJy/pixel) are: I00,
150, 200, 250, 300, 350, 400, and 450.
498
INFRARED MORPHOLOGY OF GALACTIC CENTERS
HII regions (Pastorlza 1975), which, along with the infrared energy
distribution, implies that the complicated and extended infrared emission is
powered by young stars. A dust lane in the galactic bar enters the field near
the bright infrared sources to the northwest. Another dust lane enters the
field near the tongue-like feature in the southeast. We speculate that gas flow
along the bar may _e feeding the star formation which is occurring at rates (OBA
stars) of 3 M® yr- throughout the mapped region and 0.4 M e yr at the peaks.
3.2 NGC 1808
The 10.8 _m flux
density within the lowest
contour of our map of NGC
1808 (Figure 2) is 2.5
0.i Jy, which accounts for 5
69% of the flux in the IRAS
12 _m band. The remaining
IRAS flux can be accounted
for by lower level emission 0
in the mapped region. The
luminosity within the
lowest contour, which has a
maximum dimension of I kpc _ -5
at II Mpc, is 2 x IoIOL e.
v
The peak luminosity is 5 x
109L® from a region 230 pc
in size. NGC 1808 appears _-10
to contain a weak Seyfert
nucleus embedded in a
bright emlsslon-llne region
ionized by young stars -15
(Veron-Cetty and Veron
1985). Our map implies
that the Seyfert nucleus
emits < 25% of the infrared
luminosity. The most intense -20
IR emission originates in a
region 200 pc x 400 pc in size
which, if the lower visual
and infrared contours coincide,
is centered on the Seyfert
nucleus.
I I I I
1
I
L
I
10.8 pm
PIXEL
-- I I I I I
10 5 0 -5 -10
A RA (arc see)
Figure 2. NGC 1808 (Sbc pec.) at 10.8 _m.
The contour levels (mJy/plxel) are: 200
300, 400, 500, and 600.
3.3 NGC 4536 and NGC 4527
The 10.8 _m flux density within the lowest contour of our map of NGC 4536
(Figure 3) is 0.43 ± 0.03 Jy. From the entire mapped region, we detect 0.70 ±
0.09 Jy, which accounts for 57% of the IRAS 12 _m flux. The total infrared
luminosity within the lowest contour, which has a long dimension of 1.4 kpc at
22 Mpc, is 1.4 x 1010L s The peak luminosity is 5 x I09L e from a region 460 pc
in size and originates within i" of the visual peak. The shape of the infrared
source in NGC 4536 closely resembles that of the radio continuum emission
(Condon et al. 1982).
499
C.M.TELESCOETAL.
I I
5 -
"u 0-
¢..)I.U
_] -5 -
-10 -
I
IO
I I I
.... .J
i
-10
I I I
5 0 -5
A RA (arc sec)
Figure 3. NGC 4536 (Sc) at 10.8 _m. The con-
tour levels (mJy/plxel) are: 50, I00, and 150.
The 10.8 _ flux density
from NGC 4527 (Figure 4) is
0.51 _ 0.04 Jy within the
lowest contour, which accounts
for 54% of the IRAS 12 _m
flux. The lowest infrared
contour has a long dimension
of 2 kpc at 21 Mpc. The
luminosity within the lowest
contour is 3 x IoIOL®, with 7
x 109Le originating in a 430
pc region centered on each of
the two brightest peaks.
Although the complicated
emission region is roughly
centered on the visual peak,
no obvious infrared peak
occurs there.
Figure 4. NGC 4527 (Sb) at _ 0
10.8 _m. Contour levels
(mJy/plxel) are: 50, 75,
Q
i00, and 12 5. _ -5
-10
I I I I
©
I I I I I
10 5 0 -5 -10
A RA (arc sec)
REFERENCES
Condon, J. J., Condon, M. A., Gisler, G., and Puschell, J. J. 1982, Ap. J., 252,
102.
Pastoriza, M. G. 1975, Astrophys. Sp. Scl., 33, 173.
Rieke, G. H., 1976, Ap. J. (Letters), 206, LI5.
Telesco, C. M., and Gatley, I. 1984, Ap. J., 284, 557.
Telesco, C. M., and Harper, D. A. 1980, Ap. J., 235, 392.
Veron-Cetty, M.-P., and Veron, P. 1985, Astron. Astrophys., 145, 425.
500


The infrared morphology of galactic centers

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