A search was conducted for J = 1 yields 0 CO emission from 22 galaxies, detecting half, as part of a survey to study star formation in small to medium size galaxies. Although substantial variation was found in the star formation efficiencies of the sample galaxies, there is no apparent systematic trend with galaxy size

Bally, John

Thronson, Harley A., Jr.

English

NASA Technical Reports Server

N87- 24303
CARBON MONOXIDE EMISSION FROM SMALL GALAXIES
Harley A. Thronson, Jr.
Wyoming Infrared Observatory
John Bally
AT&T Bell Laboratories
ABSTRACT. We have searched for J = I ÷ 0 CO emission from 22 galaxies, detecting
half, as part of a survey to study star formation in small- to medium-size galaxies.
Although we find substantial variation in the star formation efficiencies of the
sample galaxies, there is no apparent systematic trend with galaxy size.
i. INTRODUCTION AND BACKGROUND
A correlation between far-infrared and CO emission from galaxies was discovered
by Telesco and Harper (1980) and discussed recently and more fully by Rickard and
Harvey (1984) and Young _t _. (1986). Dust reprocesses starlight into far-
infrared radiation. Massive young stars, which are signposts of ongoing star
formation are usually surrounded by warm (30 to 60 K) dust which re-emits much of
the radiation into the 60 and i00 pm bands surveyed by the IRAS satellite. Thus,
the far-infrared flux should be a tracer of the global rate of star formation in
a galaxy. To first order, the CO emission is a tracer of the principle reservoir
of mass from which the stars are produced. Given that the far-infrared emission
arises from dust heated by young stars, LTR/L^^ is proportional to the star
formation efficiency, defined as the rate-_f _ar formation per unit H2 mass.
We are conducting a survey of the J = I + 0 i2ci60 emission from a sample of
galaxies of modest size in order to estimate the amount of material available to
form stars. Dwarfs are the most abundant type of galaxy in the universe. They
appear to be structurally simple and without the complex gas dynamics, such as
spiral shocks, which can complicate the interpretation of processes in large
star-forming galaxies. On the average, dwarf galaxies are closer than the giants
and may therefore be studied in greater detail.
2. OBSERVATIONS
We have used the AT&T Bell Laboratories 7-m telescope and the National Radio
Astronomy Observatory 12-m telescope to search for CO emission from 22 galaxies
to date. Our sample is deliberately heterogeneous, reflecting the variety of
small galaxies. Our selection criteria are: (I) modest angular size so that the
observed CO line strength is a good measure of the total line flux; (2) detection
in the IRAS Point Source Catalog, so that the total far-infrared luminosity is
known; (3) far-infrared luminosities less than about 10i°L@. Our results are
presented in Table i and Figures I and 2.
3. ANALYSIS
In Figure i we present a plot of far-infrared luminosity versus CO luminosity,
along with three lines showing the Lco/LIR relations found by Young e_t _. to fit
data for giant galaxies of different average dust temperature. These relations
CarolZLonsda_Persson(E_O
S_r FormaHonin Galaxies 267
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H. A. THRONSON JR. AND J. BALLY
TABLE 1
J : 1 ÷ 0 CO Observations of Galaxies of Modest Size
Name Type Distance ITRdV ---S 4_D2S 2 _ __R MH M 2.58F6o + F1oo
__ _ ( K - km/s) (K-km/s-Mpc) _ 7 M_ (Jy)
CPG 330 [B] SBcII 3.1 -0.01 -+0.15 -0.1 -+1.8 x 101 359 7.2 x 107 8.8 x 10 <3.0 x 106 12.5
Haro 2 IN] Im(p) 20.5 0.94 -+0.22 5.0 +-1.2 x 103 1435: 4.4 x 109 4.6 x 108 1.7 x 108 17
Haro 3 [N] Sin? 13.9 0.96 + 0.28 2.3 ± 0.7 x 103 930: 2 x 109 5.4 x 108 7.8 x 107 17
DDO 154 [B] - 10 0.19 ± 0.10 2.4 + 1.2 x 102 375 <1.5 x 108 2.5 x 109 <4.1 x 107 < 2.5
IC 10 [B] I 3.0 1.01 + 0.10 1.1 + 0.1 x 102 -330 1.9 x 108 -- 9.4 x 106 35
Mk 86 [B] ScIIIp 7.0 0.05 +-0.27 0.3 ± 1.7 x 102 449 4.2 x 108 2.4 x 108 <3.2 x 107 14
Mk 527 [B] 47.4 0.40 ± 0.12 1.1 ± 0.3 x 104 3554 2.5 x 1010 8.4 x 108 9.4 x 108 18.5
NGC 185 [B] dE3p 0.7 0.35 -+0.15 2.2 + 0.9 195 9.5 x 105 4 x 105: 1.9 x 105 3.2
NGC 205 [B] SO/E5p 0.7 -0.02 ± 0.25 -0.1 -+1.5 230 1.4 x 106 4 x 105: <2.5 x 105 4.8
NGC 1569 [B] SmIV 3.1 -0.24 ± 0.19 -2.8 ± 2.3 x 101 -83 9.8 x 108 1.7 x 108 <1.5 x 106 170
NGC 2814 [B] 23.2 -0.03 ± 0.17 -0.2 -+1.1 x 103 1740 1.9 x 109 -- <1.7 x 108 5.9
NGC 2976 [B] SdIII 3.1 1.2 + 0.2 1.5 ± 0.2 x 102 13 3.2 x 108 -- 1.3 x 107 ' 56
NGC 3274 [B] SIV 6.4 0.1 ± 0.14 5.0 ± 7.2 x 101 540 1.3 x 108 7 x 108 <i.7 x 107 5.3
NGC 3738 [B] SdIII 5.4 -0.02 ± 0.06 -0.6 ± 2.1 x 101 225: 1.3 x 108 3 x 108 <3.1 x 106 7.4
NGC 4449 [N] SmIV 5.4 >2.5 ± 0.5 >9.2 + 1.8 x 102 244 3.2 x 109 2 x 109 >3.2 x 107 180
NGC 4605 [B] ScIII 4: 1.3 -+0.3 2.6 ± 0.6 x 102 141 5.8 x 108 -- 2.2 x 107 60
NGC 5506 [B] 24.2 0.31 ± 0.28 2.3 -+2.1 x 103 1815 1.1 x 1010 1.2 x 109 <5.5 x 108 31
NGC 7800 IN] Im 25.8 -0.38 ± 0.37 -3.2 ± 3.0 x 103 1950 2.4 x 109 3.4 x 108 <9.5 x 107 6.0
I Zw 33 [B] Stop? 34.6 1.55 +-0.19 2.3 ± 0.3 x 104 2502 6.0 x 1010 1.7 x 109 2.0 x 109 84
I Zw 89 [B] Sb 31.1 1.96 ± 0.26 2.4 -+0.3 x 104 2329 8.4 x 109 2.4 x 109 2.0 x 109 14
II Zw 40 [N] 10.1 0.62 ± 0.29 7.9 -+3.7 x 102 800 1.4 x 109 4.5 x 108 2.7 x 107 23
Ill Zw 102 [B] Sp 21.7 3.0 + 0.24 1.8 -+0.1 x 104 1626 1.2 x 1010 2.3 x 109 1.5 x 109 42
Telescope: [B] Bell Telephone Laboratory 7m; [N] NRAO 12m
2 *
1. was calculated from M (M) = 1 07 x 106 D2(Mpc)fT_dv [Bell 7m] and MH (M8) = 4.25 x 105 D (Mpc)fTRdV [NRAO 12m].
H2 8 "
MH2 Observations with the signal-to-noise ratio greater than 2 were treated as detections. Upper limits for MH2are 2c.
2. [2.58F6o + FIoQ] = LIR(L8)/6 x 105D2(Mpc) and is proportional to the integrated far-infrared flux.
extend reasonably well to include the objects of low luminosity in our sample.
The center line in the figure corresponds to LIR/Lco = 25, which is about the
average value for our sample.
Luminosity-luminosity plots such as Figure 1 demonstrate that both CO and
infrared emission scale with the size of the galaxy: larger galaxies, on average,
have more CO and infrared emission than do smaller galaxies. Although the range
of far-infrared luminosities (presumably roughly proportional to mass of young
stars) reported in the literature vary over almost 7 orders of magnitude, the
range of star formation efficiencies is less than 2 orders of magnitude. Moreover,
some of the more efficient star-forming galaxies are dwarfs. We suggest that
concentration upon infrared-luminous galaxies in a study of star formation may
be misplaced effort.
Such plots alone, however, cannot be used to infer a good correlation between
CO and infrared emission since, in converting the observed fluxes to luminosities,
both species are multiplied by a common factor: the distance (D) squared. If the
range in the quantity D 2 is comparable to or greater than the range of fluxes,
then even an uncorrelated set of CO and infrared fluxes will produce a correlation
in a plot of luminosity! The situation is made worse by the presence of errors
268
CO FROM SMALL GALAXIES
10
IIi[I
NGC 5506 e
NGC 44
• °Ha
2
Zw
6
0 1 2 3 _4/Mp ca) 5log CO LUMINOSITY (K km s
Figure i -- The far-infrared luminosity versus the J = i ÷ 0 CO line
luminosity for the sample of galaxies observed in this program. The three
lines are those found by Young _t a£. for more luminous galaxies.
in the distance of the galaxies, which will tend to increase the dispersion in
the common weighting factor. A distance-independent plot, such as Figure 2, is
a better way to investigate the relationship between infrared and CO emission.
Figure 2 presents the CO surface brightness versus the total infrared flux,
measured by FIR = 2.58S60(Jy) + Sl00(Jy). This plot clearly demonstrates the
range of star fS_mation efficiencies which are encountered in our sample of
smaller galaxies. Objects located in the upper left of the figure are energetic
star-forming galaxies: they have LIR/Lco = 50, and perhaps as high as 5001 These
objects include the energetic galaxles NGC 1569 and NGC 4449, as well as the mega-
maser dwarf IC i0 (Henkel, Wouterloot, and Bally 1986). Although Young, Gallagher,
and Hunter (1984) report a detection of NGC 1569 using the FCRAO telescope, we
failed to detect it at a level 4 times fainter, indicating that the CO emission
from this object comes from an area smaller than the beam of the 14-m FCRAO
telescope. Objects located in the lower right of the diagram are lethargic star
forming galaxies, with LIR/Lc^ < 7. These galaxies have a relatively large
molecular content, but are no_ putting it to use in forming stars at a rapid
rate. Objects in this category include I Zw 89 and III Zw 102.
It is interesting to note that these lethargic galaxies are relatively large
spirals, whereas all of the very energetic galaxies in our sample are irregulars.
Extensive spiral structure is obviously not necessary for a high efficiency of
star formation.
269
H.A. THRONSON JR. AND J. BALLY
>., 150
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- I I I I I
-NGC 1569 IC i0
-- l--o-4
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NGC 4449
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- NGC 4605 -
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NGC 2976
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_ l : _ Haro 3
I Zw 69 _='I_ I I .- ',
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-I
CO SURFACE BRIGHTNESS (K km s )
Figure 2 -- The far-infrared flux versus the J = i ÷ 0 CO line flux.
limits are open circles. Note the wide variation in FIR/Fco.
Upper
A possible explanation for the high star-forming activity in irregular
systems may be a higher efficiency of induced star formation (or, more
significantly, molecular cloud formation) in a system having a thick gas layer.
Within a thin disk, with a small scale-height gas layer, much of the pressure
generated by massive star formation might be released above and below the plane
of the gas layer. Only an annular region within the plane is compressed. Within
an irregular galaxy or a thick disk, this pressure is confined and may therefore
produce compression in all directions.
H. Thronson was partially supported by NASA grant NAG 2-134.
REFERENCES
Henkei, C., W0uterloot, J. G. A., and Bally, J. 1986, A6£_. Ap., 155, 193.
Rickard, L. J, and Harvey, P. M. 1984, ASSOrt. J., 89, 1520.
Telesco, C. M., and Harper, D. A. 1980, AN. 7., 235, 392.
Young, J. S., Gallagher, J. S., and Hunter, D. A. 1984, Ap. 7., 276, 476.
Young, J. S. e£ a£. 1986, Ap. J., 304, 443.
270


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