A study was completed of the atomic and molecular components of the ISM in NGC 6946. The distribution of molecular clouds was determined from a fully sampled CO map of the inner disk using the 14-meter telescope of the FCRAO. The distribution of atomic gas was derived from VLA observations at 40" resolution in the D configuration. When comparing the global CO and HI properties with other components of the galaxy, it was found that the azimuthally averaged radial distributions of CO, H-alpha, radio continuum and blue light all exhibit similar roughly exponential falloffs, while the azimuthally averaged HI surface densities vary by only a factor of 2 out to R = 16 kpc. This indicates that while the H-alpha/CO ratio is approximately constant with radius, the CO/HI ratio decreases by a factor of 30 from the center of the galaxy to R = 10 kpc

Tacconi-Garman, Linda J.

Young, Judith S.

English

NASA Technical Reports Server

N87- 24 335
The Azimuthal and Radial Distributions of HI and H2 in NGC 6946
Linda J. Tacconi-Garman and Judith S. Young
Five College Radio Astronomy Observatory
University of Massachusetts
Amherst, MA
Abstract. We have completed a study of the atomic and molecular components of
the ISM in NGC 6946. The distribution of molecular clouds has been determined
from a fully sampled CO map of the inner disk (R < 8 kpc) using the 14-meter
telescope of the FCRAO (HPBW = 45"). The distribution of atomic gas was derived
from VLA observations at 40" resolution in the D configuration. When comparing
the global CO and HI properties with other components of the galaxy, we find
that the azimuthally averaged radial distributions of CO, Ha, radio continuum
and blue light all exhibit similar roughly exponential falloffs, while the
azimuthally averaged HI surface densities vary by only a factor of 2 out to R =
16 kpc. This indicates that while the H_/CO ratio is approximately constant
with radius, the CO/HI ratio decreases by a factor of 30 from the center of the
galaxy to R = I0 kpc.
INTRODUCTION
To better understand the cycling of the interstellar medium and the star
formation efficiency in galaxies, we have undertaken a multi-wavelength study of
the ISM of the face-on Scd galaxy, NGC 6946. This galaxy has been well-studied
by a number of authors. Photometric data from Ables (1971) and Elmegreen and
Elmegreen (1984) indicate the exponential nature of the blue and I band
luminosity profiles. H: studies by DeGioia-Eastwood et al. (1984) have been
used to infer the massive star formation rate. The H_ribution has been
determined at a resolution of 2' by Rogstad, Shostak and Rots (1973). Radio
continuum emission has been observed at many wavelengths (cf. Klein et al. 1982;
van der Kruit et al. 1977). Observations of the CO distribution have previously
been determined--_ris and Lo 1978; Rickard and Palmer 1981; Young and Scoville
1982), and most recently, an interferometric study of CO in the central i' of
this galaxy (Ball et al. 1985) has uncovered a molecular bar ~45" long. In this
paper we present t_e results of a complete 12C0 and HI study at moderate
resolution.
DATA AND RESULTS
We have made 12C0 observations at more than 100 positions in NGC 6946 with
the 14-meter antenna of the Five College Radio Astronomy Observatory, and have
fully sampled the inner 6' of this galaxy. Figure 1 shows a sample of the CO
spectra attained at radii < 2.'25, all plotted on the same scale. The CO
intensities peak in the center of the galaxy, and then fall off fairly smoothly
with radius, although azimuthal variations of factors of 3 are observed. We
observe 2 smaller CO peaks to the northeast and northwest of the galaxy in the
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 491
F_ZCF..DII'-IG P_,G_ I3L_NK NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19870014902 2020-03-20T11:08:04+00:00Z
L. J. TACCONI-GARMAN AND J. S. YOUNG
direction of two major optical spiral arms. Using the standard conversion of CO
intensities to H2 surface densities (Young and Scoville 1982) we find the
H2 density in the central 45" to be 3.3x1022 atoms cm-2 with the lowest observed
values of <6x1020 atoms cm-2 in the disk of this galaxy.
CO IN
NGC 6946
I._.L J..J..
- . -
0.1
Vhw Ikmlt )
1"1"1-'1"_
Figure 1. A
sample of the
CO spectra
attained at
radii <2'.25
all plotted on
the same
scale. The
circles indi-
cate the posi-
tions where
the obser-
vations were
made. These
data represent
~1/2 of the
complete CO
map of NGC
6946. The box
in the lower
Ieft hand
corner indi-
cates the
scale of all
the spectra.
The 21-cm line data were obtained with the VLA in the D configuration,
where the resolution of the synthesized beam was 40"x37". We observe HI
emission at greater than the 3_ level out to a radius of 30 kpc, with HI spiral
structure apparent in the outer disk, well beyond where the optical arms are
seen (see Figure 2). The central surface densit, of HI is 2.5x1020 H cm-2 with
extreme values over the entire HI disk of 2.1 !0 to 2x1021 H cm-2. There are
two noticeable HI peaks located ~4' to the northwest and northeast of the center
which are ~2'.5 x 2' .5. The peak to the northeast contains about 2.4x108 Mo of
HI in an area of 19 kpc 2, and is roughly coincident with the largest, high
surface brightness spiral arm in the optical disk. The stronger peak to the
northwest of the center contains about 3.8x108 Mo of neutral gas in a region of
28 kpc 2, and is also coincident with an o_tical spiral arm. The HI surface
densities in these regions are ~14 Mo pc-'. We also observe several resolvable
492
HI ANDH2 DISTRIBUTIONS IN NGC 6946
HI depressions larger than 45" (2.2 kpc) across in this galaxy to the north and
southwest of the center, as well as in the central region itself.
60 ° I0'
60005 '
Z
0
I- 60000 '
<[
Z
.--I
(..)
I,I
59 ° 55'
59050 '
H I I NTEGRATED INTENSITIES
._ ...: ..
• • • o• _ _°_ °t_ • •
Figure 2. A con-
tour represen-
tation of the
total HI
integrated flux
di stri buti on
superposed on an
enlargement of
the Pal omar Sky
Survey print.
The contour
levels range from
0.2 to 3.0 Jy km
s -I and are
separated by 0.4
Jy km s -1 inter-
vals. Regions of
HI depletion are
represented by
tic marks on the
contours. The HI
synthesized beam
is shown in the
bottom left hand
corner of the
figure.
RIGHT ASCENSION
COMPARISONS OF HI AND H2
In Figure 3 we show the H2/HI ratio as a function of the molecular and
atomic surface densities for 89 positions at R < 3' in the galaxy. We have made
linear fits to the data for each of the plots, and find that the H2/HI ratio is
correlated with H2 surface density and anti-correlated with HI surface density.
These plots indicate that an increase in the H2/HI ratio is due to an increase
in the H2 density as well as a slight decrease in the HI density in the inner
disk of NGC 6946. Thus, fluctuations in the ISM of this galaxy are dominated by
the molecular component rather than the atomic gas.
We have computed the azimuthally averaged radial distributions of HI and
H2 in NGC 6946, and have compared them with the blue luminosity (Ables 1971),
He (DeGioia-Eastwood et al. 1984), I band (Elmegreen and Elmegreen 1984) and
radio continuum (van der Kruit et al. 1977) profiles in Figure 4. The radial
distributions of CO, H_, radio contTnuum, blue and I band all exhibit similar
roughly exponential falloffs (scale lengths = 4-6 kpc), while the azimuthally
averaged HI surface densities vary by only a factor of 2 out to R = 16 kpc.
493
L. J. TACCONI-GARMAN AND J. S. YOUNG
Thus the atomic gas distribution is the only component of NGC 6946 whose shape
differs from the rest. This indicates that while the Ha/CO ratio is
approximately constant with radius, the CO/HI ratio decreases by a factor of 30
from the center of the galaxy to R = 10 kpc.
2
o
oc
z"
0
-'"'I ';' 'I i''' I"
- Slope=-1.79+--0.22
x
x
× x
,,,,lll,,llJ,,I,,
20 20.5 21 2 I. 5
"l''"l'"'l'"'l''"l'
+
0
0
H2 _ '
0 _/'00
oo_ ' --Sc_, qb
0
pe=1.16-+0.04 -
,,!,,,ll lit Iltl,llJlt, li-
20.5 21 2 I.5 22 22.5
log Surface Density (atoms cm -z)
Figure 3. H2/HI
ratio as a func-
tion of the (a)
atomic and (b)
molecular surface
densities for 89
positions in the
galaxy. A typical
error bar is shown
in the upper right
hand corner of the
plot. Filled
circles on the H2
plot represent
upper limits to
the CO intensity,
and thus the H2
surface density.
We interpret the CO/HI ratio as the efficiency of molecular cloud formation
from the atomic medium, and the Ha/CO ratio as the star formation efficiency
(SFE) from the molecular medium. Therefore, while the SFE is constant with
radius (DeGioia-Eastwood et al. 1984) the cloud formation efficiency decreases
sharply with radius. That is, once molecular clouds are formed, stars are
produced at a rate which is proportional to the mass of the molecular clouds.
The formation of molecular clouds in the outer parts of NGC 6946 may be greatly
reduced due to the decreasing volume density of atomic gas. The optical edges
of galaxies, therefore, probably reflect the edges of the molecular disks where
the formation of molecular clouds from atomic clouds is greatly reduced due to
the increasing HI scale height.
A_cknowledgements. The FCRAO is operated with support from the National
Science Foundation under grant 82-12252 and with the permission of the
Metropolitan District Commission, Commonwealth of Massachusetts. The National
Radio Astonomy Observatory is operated by Associated Universities Inc., under
contract with the National Science Foundation. L.T-G. acknowledges a
Grant-in-Aid of Research from Sigma Xi, the scientific research society, for
partial funding of this work.
494
HI AND//2 DISTRIBUTIONS IN NGC 6946
t_ 22.5
E
u
22.0
E
13,
o
21.5
T
Z 21 .0
O
20.5
i I l i I I i I I
O) Molecular ond Atomic Hydrogen
o I
H2
--o--.-o'_ 1-,_o_
: I : I i I I I i
b) Blue Surfoce Brightness ond
A 20.0 I SFR __fr°mbLHo 3.0 --
_ 2'°I , sF, 2622.O 2.2
_ 23.0 ,.B _
E !
::k
25.0 I.O
26.0 0.6
0 4 8 12 16 20
RADIUS (kpc)
t I I _ I t I _ I i
C) Radio Continuum
2"51. --e-- 49cm -
TE --_ --0-- 2, cm
.o..)>, 2.0 F _
_ 1.5
f ,.o _
0.5 \ '_ ", _
I I i I t I i I '
d) Totol ISM ond I Bond
22.5
--i-- HI ÷ H 2 _
_E --- X _ T Bondu 22.0
o 2,.5
0
_ T T
21.0 i ; ,
zo, _ _ _
o 20.5
J I = I I I i I I
0 4 B I 2 16 20
RADIUS (kpc)
m
f-.
]>
-I
Am
3 ¢n
o C
e_
n 0
m rrl
¢D
I
--t
Z
m
Figure 4. (a) The azimuthally averaged radial distributions of HI and
H2 surface densities. (b) The blue surface brightness radial profile from Ables
(1971) and total SFR's calaculated from Hot observations of DeGioia-Eastwood et
al. (1984). (c) Radial continuum radial distributions of van der Kruit et a]_--.
_77). (d) I band (Elmegreen and Elmegreen 1984) and total ISM (HI + H2-T
radial distributions. At radii greater than 12 kpc, the closed circles in panel
d represent HI + H2 upper limits and the open circles represent the HI lower
limits to the total ISM density.
REFERENCES
Ables, H.D. 1971, Pub. US Naval Obs., Series II, Vol. 20, Part 4.
Ball, R., Sargent, A.I., Scoville, N.Z., Lo, K.Y., and Scott, S.L. 1985, Ap.J.
Lett., 298, L21.
DeGio_stwood, K., Grasdalen, G.L., Strom, S.E., and Strom, K.M. 1984, Ap.J.,
278, 564.
Elmegreen, D.M. and Elmegreen, B.G. 1984, Ap.j. Suppl., 54, 127.
Klein, U., Beck, R., Buczilowski, U.R., and Wielebinski, R. 1982, Astr. Ap.,
108, 176.
Morris, M. and Lo, K.Y. 1978 Ap.J., 223, 803.
Rickard, L.J. and Palmer, P. 1981, Astr. Ap.(Letters), 102, L13.
Rogstad, D.H., Shostak, G.S., and Rots, A.H. 1973, Astr. Ap., 22, 111.
van der Kruit, P.C., Allen, R.J., and Rots, A.H. 1977, Astr. Ap., 55, 421.
Young, J.S. and Scoville, N.Z. 1982, Ap.J., 258, 467.
495


The azimuthal and radial distributions of HI and H2 in NGC 6946

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server