The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18
Draft version October 24, 2018
Preprint typeset using LATEX style emulateapj v. 08/22/09
THE LOW CO CONTENT OF THE EXTREMELY METAL POOR GALAXY I ZW 18
Adam Leroy
, John Cannon
, Fabian Walter
, Alberto Bolatto
, Axel Weiss
Draft version October 24, 2018
ABSTRACT
We present sensitive molecular line observations of the metal-poor blue compact dwarf I Zw 18
obtained with the IRAM Plateau de Bure interferometer. These data constrain the CO J = 1 → 0
luminosity within our 300 pc (FWHM) beam to be LCO < 1×10
5 K km s−1 pc2 (ICO < 1 K km s
−1), an
order of magnitude lower than previous limits. Although I Zw 18 is starbursting, it has a CO luminosity
similar to or less than nearby low-mass irregulars (e.g. NGC 1569, the SMC, and NGC 6822). There is
less CO in I Zw 18 relative to its B-band luminosity, H I mass, or star formation rate than in spiral or
dwarf starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the star formation
rate to our CO upper limit reveals that unless molecular gas forms stars much more efficiently in
I Zw 18 than in our own galaxy, it must have a very low CO-to-H2 ratio, ∼ 10
−2 times the Galactic
value. We detect 3mm continuum emission, presumably due to thermal dust and free-free emission,
towards the radio peak.
Subject headings: galaxies: individual (I Zw 18); galaxies: ISM; galaxies: dwarf, radio lines: ISM
1. INTRODUCTION
With the lowest nebular metallicity in the nearby uni-
verse (12+ logO/H ≈ 7.2, Skillman & Kennicutt 1993),
the blue compact dwarf I Zw 18 plays an important role
in our understanding of galaxy evolution. Vigorous ongo-
ing star formation implies the presence of molecular gas,
but direct evidence has been elusive. Vidal-Madjar et al.
(2000) showed that there is not significant diffuse H2, but
Cannon et al. (2002) found ∼ 103 M⊙ of dust organized
in clumps with sizes 50 – 100 pc. Vidal-Madjar et al.
(2000) did not rule out compact, dense molecular clouds,
and Cannon et al. (2002) argued that this dust may in-
dicate the presence of molecular gas.
Observations by Arnault et al. (1988) and
Gondhalekar et al. (1998) failed to detect CO J = 1 → 0
emission, the most commonly used tracer of H2. This
is not surprising. The low dust abundance and intense
radiation fields found in I Zw 18 may have a dramatic
impact on the formation of H2 and structure of molecular
clouds. A large fraction of the H2 may exist in extended
envelopes surrounding relatively compact cold cores. In
these envelopes, H2 self-shields while CO is dissociated
(Maloney & Black 1988). The result may be that in
such galaxies [CII] or FIR emission trace H2 better than
CO (Madden et al. 1997; Israel 1997a; Pak et al. 1998).
Further, H2 may simply be underabundant, as there is a
lack of grains on which to form while photodissociation
is enhanced by an intense UV field. Indeed, Bell et al.
(2006) found that at Z = Z⊙/100, a molecular cloud
may take as long as a Gyr to reach chemical equilibrium.
A low CO content in I Zw 18 is then expected, and a
stringent upper limit would lend observational support to
predictions for molecular cloud structure at low metallic-
1 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117,
Heidelberg, Germany; email: leroy@mpia-hd.mpg.de
2 Astronomy Department, Wesleyan University, Middletown, CT
06459, cannon@astro.wesleyan.edu
3 Radio Astronomy Lab, UC Berkeley, 601 Campbell Hall,
Berkeley, CA, 94720
4 MPIfR, Auf dem Hügel 69, 53121, Bonn, Germany
ity. However, while the existing upper limits are sensitive
in an absolute sense, they do not even show I Zw 18 to
have a lower normalized CO content than a spiral galaxy
(e.g. less CO per B-band luminosity). The low luminos-
ity (MB ≈ −14.7, Gil de Paz et al. 2003) and large dis-
tance (d=14 Mpc, Izotov & Thuan 2004) of this system
require very sensitive observations to set a meaningful
upper limit.
In this letter we present observations, obtained with
the IRAM Plateau de Bure Interferometer (PdBI)5, that
constrain the CO luminosity, LCO, to be equal to or less
than that of nearby CO-poor (non-starbursting) dwarf
irregulars.
2. OBSERVATIONS
I Zw 18 was observed with the IRAM Plateau de
Bure Interferometer on 17, 21, and 27 April and 13
May 2004 for a total of 11 hours. The phase calibrators
were 0836+710 (Fν(115GHz) ≈ 1.1 Jy), and 0954+556
(Fν(115GHz) ≈ 0.35 Jy). One or more calibrators with
known fluxes were also observed during each track. The
data were reduced at the IRAM facility in Grenoble us-
ing the GILDAS software package; maps were prepared
using AIPS. The final CO J = 1 → 0 data cube has beam
size 5.59′′ × 3.42′′, and a velocity (frequency) resolution
of 6.5 km s−1 (2.5 MHz). The velocity coverage stretches
from vLSR ≈ 50 to 1450 km s
−1. The data have an RMS
noise of 3.77 mJy beam−1 (18 mK; 1 Jy beam−1 = 4.8
K). The 44′′ (FWHM) primary beam completely covers
the galaxy. Based on variation of the relative fluxes of
the calibrators, we estimate the gain uncertainty to be
< 15%.
3. RESULTS
3.1. Upper Limit on CO Emission
To search for significant CO emission, we smooth the
cube to 20 km s−1 velocity resolution, a typical line width
5 Based on observations carried out with the IRAM Plateau
de Bure Interferometer. IRAM is supported by INSU/CNRS
(France), MPG (Germany) and IGN (Spain).”
http://arxiv.org/abs/0704.0862v1
Fig. 1.— CO 1 → 0 spectra of I Zw 18 towards the radio
continuum/Hα peak (left) and the highest significance spectra
(right), which is still too faint to classify as more than marginal.
The locations of both spectra are shown in Figure 2. Dashed hor-
izontal lines show the magnitude of the RMS noise.
for CO at our spatial resolution (e.g., Helfer et al. 2003).
The noise per channel map in this smoothed cube is
σ20 ≈ 0.25 K km s
−1. Over the H I velocity range (710
– 810 km s−1, van Zee et al. 1998), there are no regions
with ICO,20 > 1 K km s
−1 (4σ) within the primary beam.
We pick a slightly conservative upper limit for two rea-
sons. First, if there were CO emission with this intensity
we would be certain of detecting it. Second, the noise in
the cube is slightly non-Gaussian, so that the false posi-
tive rate for ICO,20 > 1 K km s
−1 — estimated from the
negatives and the channel maps outside the H I velocity
range — is ∼ 0.2%, very close to that of a 3σ deviate.
For d = 14 Mpc, the synthesized beam has a FWHM
of 300 pc and an area of 1.0 × 105 pc2. Our intensity
limit, ICO < 1 K km s
−1, therefore translates to a CO
luminosity limit of LCO < 1× 10
5 K km s−1 pc2.
There is a marginal signal toward the southern knot
of Hα emission (9h34m02s.4, 55◦14′23′′.0). This emission
has the largest |ICO,20| found over the H I velocity range,
corresponding to LCO ∼ 8×10
4 K km s−1 pc2, just below
our limit. This same line of sight also shows |ICO| > 2σ
over three consecutive channels, a feature seen along only
one other line of sight (in negative) over the H I velocity
range. The marginal signal is suggestively located in the
southeast of I Zw 18, where Cannon et al. (2002) identi-
fied several potential sites of molecular gas from regions
of relatively high extinction. While tantalizing, the sig-
nal is not strong enough to be categorized as a detection.
Figure 1 shows CO spectra towards the Hα/radio contin-
uum peak (Cannon et al. 2002, 2005; Hunt et al. 2005a,
see Figure 2) and this marginal signal.
3.2. Continuum Emission
We average the data over all channels and produce a
continuum map with noise σ115GHz = 0.35 mJy beam
The highest value in the map is I115GHz = 1.06±0.35mJy
beam−1 at α2000 = 9
h34m02s.1, δ2000 = +55
◦ 14′ 27′′.0.
This is within a fraction of a beam of the 1.4 GHz peak
identified by Cannon et al. (2005, α2000 = 9
h34m02s.1,
δ2000 = +55
◦ 14′ 28′′.06) and Hunt et al. (2005a, α2000 =
9h34m02s, δ2000 = +55
◦ 14′ 29′′.06). Figure 2 shows the
radio continuum peak and 115 GHz continuum contours
plotted over Hα emission from I Zw 18 (Cannon et al.
2002). There is only one other region with |I115GHz| >
3σ115GHz within the primary beam and the star-forming
extent of I Zw 18 occupies ≈ 10 % of the primary beam.
Therefore, we estimate the chance of a false positive co-
incident with the galaxy to be only ∼ 10%.
4. DISCUSSION
Here we discuss the implications of our CO upper limit
and continuum detection. We adopt the following prop-
erties for I Zw 18, all scaled to d = 14 Mpc: MB =
−14.7 (Gil de Paz et al. 2003), MHI = 1.4 × 10
(van Zee et al. 1998), Hα luminosity log10 Hα = 39.9 erg
s−1 (Cannon et al. 2002; Gil de Paz et al. 2003), 1.4 GHz
flux F1.4 = 1.79 mJy (Cannon et al. 2005).
4.1. Point Source Luminosity
Our upper limit along each line of sight, LCO <
1 × 105 K km s−1 pc2, matches the luminosity of
a fairly massive Galactic giant molecular cloud (Blitz
1993). For a Galactic CO-to-H2 conversion factor, 2 ×
1020 cm−2 (K km s−1)−1, the corresponding molecular
gas mass is MMol ≈ 4.4× 10
5 M⊙, similar to the mass of
the Orion-Monoceros complex (e.g. Wilson et al. 2005).
4.2. Comparison With More Luminous Galaxies
In galaxies detected by CO surveys, the CO content
per unit B-band luminosity is fairly constant. Figure 3
shows the CO luminosity normalized by B-band lumi-
nosity, LCO/LB, as a function of absolute B-band mag-
nitude (LB is extinction corrected). LCO/LB is nearly
constant over two orders of magnitude in LB, though
with substantial scatter (much of it due to the extrapo-
lation from a single pointing to LCO).
Based on these data and assuming that LCO is not
a function of the metallicity of the galaxy, we may ex-
trapolate to an expected CO luminosity for I Zw 18.
For MB,IZw18 ≈ −14.7 the CO luminosity correspond-
ing to the median value of LCO/LB (dashed line) in
Figure 3 is LCO,IZw18 ≈ 1.7 × 10
6 K km s−1 pc2.
The Hα, 1.4 GHz, and H I luminosities lead to simi-
lar predictions. Young et al. (1996) found MH2/LHα ≈
10L⊙/M⊙ for Sd–Irr galaxies, which implies LCO,IZw18 ∼
4 × 106 K km s−1 pc2. Murgia et al. (2005) measured
FCO/F1.4 ≈ 10 Jy km s
−1 (mJy)−1 for spirals, that would
imply LCO,IZw18 ∼ 10
7 K km s−1. For Sd/Sm galax-
ies, MH2/MHI ≈ 0.2 (Young & Scoville 1991), leading
to LCO,IZw18 ∼ 5 × 10
6 K km s−1 pc2. Both MH2/LHα
and MH2/MHI tend to be even higher in earlier-type
spirals.
Therefore, surveys would predict LCO,IZw18 & 2 ×
106 K km s−1 pc2, very close to the previously established
upper limits of 2− 3 × 106 K km s−1pc2 (Arnault et al.
1988; Gondhalekar et al. 1998). With the present obser-
vations, we constrain LCO < 1 × 10
5 K km s−1pc2 and
thus clearly rule out LCO ∼ 10
6 K km s−1 pc2. This
may be seen in Figure 3; even if I Zw 18 has the highest
possible CO content, it will still have a lower LCO/LB
than 97% of the survey galaxies.
4.3. Comparison With Nearby Metal-Poor Dwarfs
The subset of irregular galaxies detected by CO surveys
tend to be CO-rich and actively star-forming, resembling
scaled-down versions of spiral galaxies (Young et al.
1995, 1996; Leroy et al. 2005). Such galaxies may not
be representative of all dwarfs. Because they are nearby,
several of the closest dwarf irregulars have been detected
Fig. 2.— V -band and Hα (right, Cannon et al. 2002) images of I Zw 18. Overlays on the left image show the size of the synthesized
beam and the locations of the spectra shown in Figure 1. Contours on the right image show continuum emission in increments of 0.5σ
significance and the location of the radio continuum peak. The primary beam is larger than the area shown. Both optical maps are on
linear stretches. V -band data obtained from the MAST Archive, originally observed for GO program 9400, PI: T. Thuan).
despite very small LCO. With their low masses and
metallicities, they may represent good points of compar-
ison for I Zw 18. Table 1 and Figure 3 show CO lumi-
nosities and LCO/LB for four nearby dwarfs: NGC 1569,
the Small Magellanic Cloud (SMC), NGC 6822, and
IC 10. The SMC, NGC 1569, and NGC 6822 have
LCO ∼ 10
5 K km s−1 pc2, close to our upper limit, and
occupy a region of LCO/LB-LB parameter space similar
to I Zw 18. All four of these galaxies have active star for-
mation but very low CO content relative to their other
properties.
We test whether our observations would have detected
CO in NGC 1569, the SMC, and IC 10 at the plausible
lower limit of 10 Mpc (from H0 = 72 km s
−1) or our
adopted distance of 14 Mpc. We convolve the integrated
intensity maps to resolutions of 210 and 300 pc and mea-
sure the peak integrated intensity. The results appear in
columns 4 and 5 of Table 1. The PdBI observations of
NGC 1569 resolve out most of the flux, so we also apply
this test to a distribution with the size and luminosity
derived by Greve et al. (1996) from single dish observa-
tions. Our observations would detect an analog to IC 10
but not the SMC, with NGC 1569 an intermediate case.
With a factor of ∼ 3 better sensitivity (requiring ∼ 10
times more observing time) we would expect to detect
all three nearby galaxies. However, achieving such sen-
sitivity with present instrumentation will be quite chal-
lenging. ALMA will likely be necessary to place stronger
constraints on CO in galaxies like I Zw 18.
IC 10 may be the nearest blue compact dwarf
(Richer et al. 2001), so it may be telling that we would
detect it at the distance of I Zw 18. The blue compact
galaxies that have been detected in CO have LCO/LB
similar to IC 10 (Gondhalekar et al. 1998, the diamonds
in Figure 3). Most searches for CO towards BCDs have
yielded nondetections, so those detected may not be rep-
resentative, but I Zw 18 is clearly not among the “CO-
rich” portion of the BCD population.
4.4. Interpretation of the Continuum
We measure continuum intensity of F115GHz = 1.06±
0.35 mJy towards the radio continuum peak. The
continuum is detected along only one line of sight,
so we refer to it here as a point source and com-
pare it to integrated values for I Zw 18. F115GHz is
expected to be the product of mainly two types of
emission: thermal free-free emission and thermal dust
emission. At long wavelengths, the integrated ther-
mal free-free emission is F1.4GHz(free− free) ≈ 0.52 –
0.75 mJy (Cannon et al. 2005; Hunt et al. 2005a), imply-
ing F115GHz(free− free) = 0.36 – 0.51 mJy at 115 GHz
(Fν ∝ ν
−0.1). The Hα flux predicts a similar value,
F115GHz(free− free) = 0.34 mJy (Cannon et al. 2005,
Equation 1). Hunt et al. (2005b) placed an upper limit
of Fν(850) < 2.5 mJy on dust continuum emission at
850µm; this is consistent with the ∼ 5 × 103 M⊙ esti-
mated by Cannon et al. (2002) given almost any reason-
able dust properties. Extrapolating this to 2.6 mm as-
suming a pure blackbody spectrum, the shallowest plau-
sible SED, constrains thermal emission from dust to be
< 0.25 mJy at 115 GHz. Based on these data, we would
predict F115GHz . 0.75 mJy. Thus our measured F115GHz
is consistent with, but somewhat higher than, the ther-
mal free-free plus dust emission expected based on opti-
cal, centimeter, and submillimeter data.
4.5. Relation to Star Formation
I Zw 18 has a star formation rate ∼ 0.06 – 0.1 M⊙
yr−1, based on Hα and cm radio continuum measure-
ments (Cannon et al. 2002; Kennicutt 1998a; Hunt et al.
2005a). Our continuum flux suggests a slightly higher
value ≈ 0.15 – 0.2 M⊙ yr
−1 (following Hunt et al. 2005a;
Condon 1992), with the exact value depending on the
contribution from thermal dust emission. For any value
in this range, the star formation rate per CO luminosity,
SFR/LCO is much higher in I Zw 18 than in spirals. For
Fig. 3.— CO luminosity normalized by absolute blue mag-
nitude for galaxies with Hubble Type Sb or later (black cir-
cles, Young et al. 1995; Elfhag et al. 1996; Böker et al. 2003;
Leroy et al. 2005). We also plot nearby dwarfs from Ta-
ble 1 (crosses) and blue compact galaxies compiled by
Gondhalekar et al. (1998, , diamonds). The shaded regions shows
our upper limit for I Zw 18, with the range inMB for distances from
10 to 20 Mpc. The dashed line and light shaded region show the
median value and 1σ scatter in LCO/LB for spirals and dwarf star-
bursts. Methodology: We extrapolate from ICO in central pointings
to LCO assuming the CO to have an exponential profile with scale
length 0.1 d25 (Young et al. 1995), including only galaxies where
the central pointing measures > 20% of LCO. We adopt B mag-
nitudes (corrected for internal and Galactic extinction), distances
(Tully-Fisher when available, otherwise Virgocentric-flow corrected
Hubble flow), and radii from LEDA (Paturel et al. 2003).
comparison, our upper limit and the molecular “Schmidt
Law” derived by Murgia et al. (2002) predicts a star for-
mation rate . 2 × 10−4 M⊙ yr
−1. Fits by Young et al.
(1996) and Kennicutt (1998b, applied to just the molec-
ular limit) yield similar values. Again, I Zw 18 is similar
to the SMC and NGC 6822, which have star formation
rates of 0.05 M⊙ yr
−1 and 0.04 M⊙ yr
−1 (Wilke et al.
2004; Israel 1997b) and LCO ∼ 10
5 K km s−1 pc2.
4.6. Variations in XCO
Several calibrations of the CO-to-H2 conversion factor,
XCO as a function of metallicity exist in the literature.
The topic has been controversial and these calibrations
range from little or no dependence (e.g. Walter 2003;
Rosolowsky et al. 2003) to very steep dependence (e.g.,
XCO ∝ Z
−2.7 Israel 1997a). Comparing the star for-
mation rate to our CO upper limit, we may rule out
that I Zw 18 has a Galactic XCO unless molecular gas
in I Zw 18 forms stars much more efficiently than in the
Galaxy. Either the ratio of CO-to-H2 is low in I Zw 18
or molecular gas in this galaxy forms stars with an effi-
ciency two orders of magnitude higher than that in spiral
galaxies.
5. CONCLUSIONS
We present new, sensitive observations of the metal-
poor dwarf galaxy I Zw 18 at 3 mm using the Plateau de
Bure Interferometer. These data constrain the integrated
CO J = 1 → 0 intensity to be ICO < 1 K km s
−1 over our
300 pc (FWHM) beam and the luminosity to be LCO <
1× 105 K km s−1 pc2.
I Zw 18 has less CO relative to its B-band luminosity,
H Imass, or SFR than spiral galaxies or dwarf starbursts,
including more metal-rich blue compact galaxies such as
IC 10 (ZIC 10 ∼ Z⊙/4, Lee et al. 2003). Because of its
small size and large distance, these are the first observa-
tions to impose this constraint.
We show that I Zw 18 should be grouped with several
local analogs — NGC 1569, the SMC, NGC 6822 — as
a galaxy with active star formation but a very low CO
content relative to its other properties. In these galax-
ies, observations suggest that the environment affects the
molecular gas and these data suggest that the same is
true in I Zw 18. A simple comparison of star formation
rate to CO content shows that this must be true at a
basic level: either the ratio of CO to H2 is dramatically
low in I Zw 18 or molecular gas in this galaxy forms stars
with an efficiency two orders of magnitude higher than
that in spiral galaxies.
We detect 3mm continuum with F115 GHz = 1.06 ±
0.35 mJy coincident with the radio peak identified by
Cannon et al. (2005) and Hunt et al. (2005a). This flux
is consistent with but somewhat higher than the thermal
free-free plus dust emission one would predict based on
centimeter, submillimeter, and optical measurements.
Finally, we note that improving on this limit with cur-
rent instrumentation will be quite challenging. The order
of magnitude increase in sensitivity from ALMA will be
needed to place stronger constraints on CO in galaxies
like I Zw 18.
We thank Roberto Neri for his help reducing the data.
We acknowledge the usage of the HyperLeda database
(http://leda.univ-lyon1.fr).
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TABLE 1
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Galaxy MB LCO ICO,210
a ICO,300
a Reference
(mag) (K km s−1 pc2) (K km s−1) (K km s−1)
NGC 1569 −16.5 1.2× 105 1.1 0.8 Greve et al. (1996)
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SMC −16 1.5× 105 0.5 0.4 Mizuno et al. (2001, 2006)
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IC 10 −16.5 2.2× 106 3.8 2.2 Leroy et al. (2006)
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