Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A107 | |
Number of page(s) | 8 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201014166 | |
Published online | 11 October 2010 |
Molecular cloud formation and the star formation efficiency in M 33
Molecule and star formation in M 33
J. Braine1 - P. Gratier1 - C. Kramer2 - K. F. Schuster3 - F. Tabatabaei4 - E. Gardan1
1 - Laboratoire d'Astrophysique de Bordeaux, Université de Bordeaux, OASU, CNRS/INSU, 33271 Floirac, France
2 -
IRAM, Avenida Divina Pastora, 7, Nucleo Central, 18012 Granada, Spain
3 -
IRAM, 300 Rue de la piscine, 38406 St Martin d'Hères, France
4 -
Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received 31 January 2010 / Accepted 2 July 2010
Abstract
Does star formation proceed in the same way in large spirals such
as the Milky Way and in smaller chemically younger galaxies? Earlier
work suggests a more rapid transformation of H2 into stars in these objects but (1) a doubt remains about the validity of the H2
mass estimates and (2) there is currently no explanation for why star
formation should be more efficient. M 33, a local group spiral
with a mass 10%
and a metallicity half that of the Galaxy, represents a first step
towards the metal poor Dwarf Galaxies. We have searched for molecular
clouds in the outer disk of M 33 and present here a set of
detections of both 12CO and 13CO, including the only detections (for both lines) beyond the R25
radius in a subsolar metallicity galaxy. The spatial resolution enables
mass estimates for the clouds and thus a measure of the
ratio, which in turn enables a more reliable calculation of the H2 mass. Our estimate for the outer disk of M 33 is
with an estimated uncertainty of a factor
2. While the 12/13CO line ratios do not provide a reliable measure of
,
the values we find are slightly greater than Galactic and corroborate a somewhat higher
value. Comparing the CO observations with other tracers of the
interstellar medium, no reliable means of predicting where CO would be
detected was identified. In particular, CO detections were often not
directly on local HI or FIR or H
peaks, although generally in regions with FIR emission and high HI
column density. The results presented here provide support for the
quicker transformation of H2 into stars in M 33 than in large local universe spirals.
Key words: galaxies: groups: individual: M 33 - Local Group - galaxies: evolution - galaxies: ISM - ISM: clouds - stars: formation
1 Introduction
Several recent papers (Gardan et al. 2007; Gratier et al. 2010b; Leroy et al. 2006) have suggested that the rate of transformation of molecular gas (H2) into stars is higher in small chemically young galaxies - those with low metallicities.
The latter articles devoted much attention to whether the conversion factor from CO to ,
,
usually expressed in H2 molecules
,
could be severely underestimated. In this article, we present sensitive
measurements of the CO emission from the outer disk of M 33,
allowing us to estimate Virial masses for isolated molecular clouds far
from the center as well as some 13CO
line measurements, providing a further check on the physical conditions
in the Giant Molecular Clouds (GMCs) in M 33. A further goal is
naturally to understand the mechanisms of molecular cloud formation and
the outer disk provides a means to explore physical conditions
unexplored by earlier work which only presented molecular cloud data in
the inner disk where the stellar mass surface density dominates that of
the gas.
In particular, we present data on a series of mid to outer disk
clouds in M 33, including an interarm GMC with no measured
associated star formation (H
or Far-IR emission) and a GMC beyond the R25 radius. Because the stellar population in M 33 is rather young, the R25 radius (30.8' Paturel et al. 2003),
defined in B band, corresponds to an extremely low stellar surface mass
density, well below that of the gas at the same radius. In M 33,
the radius at which stellar and gaseous surface densities are equal is
about 0.5 R25 whereas in large spirals the stars dominate until about the R25 radius.
It has long been known that the amount of CO emission per unit star formation or per unit H2 is lower, sometimes much lower, in galaxies with subsolar metallicity (e.g. Rubio et al. 1991). In the solar neighborhood, the conversion factor
is of order
with a strong increase from the center to the outer disk (e.g. Sodroski et al. 1995). This radial variation is not only true for the Galaxy but also for other spirals where it has been studied (Braine et al. 1997b,a). The amount of CO emission per H2
mass varies not only with metallicity but also with the radiation
field, which can act to raise the CO emission by warming the gas. If
the metallicity is low the effect is opposite because the lower dust
content and lesser degree of self-shielding by molecules other than H2 reduce the size of the CO-emitting regions with respect to the H2
cloud size, such that the CO emission principally traces the central
regions of molecular clouds. Because the CO lines are optically thick,
moderate changes in metallicity and radiation field with respect to
Galactic conditions (solar vicinity or molecular ring) can be expected
to only generate moderate changes in the
ratio.
The standard techniques to estimate the H2 mass, and thus
ratio, are through the dust emission and an assumed dust-to-gas mass
ratio, the ``virial'' mass from the cloud size and line width, and from
optically thin tracers such as 13CO. We discuss the latter two methods in Sects. 4.1 and 4.2. The resulting
is then used to estimate the star formation efficiency (SFE), defined as the ratio of the star formation rate per unit H2, in M 33.
M 33 has a metallicity a factor 2 below solar (Rosolowsky & Simon 2008; Magrini et al. 2009), a moderate UV radiation field, and is a spiral galaxy despite having a mass 10 times
lower than the Galaxy. It thus represents a first step towards the
still smaller, lower metallicity, irregular galaxies in the Local Group
and beyond. Based on the above, we assume an Oxygen abundance of
.
The distance to M 33 is assumed to be 840 kpc and we adopt an inclination angle of 56
and a position angle of 22.5
as in Gardan et al. (2007).
Table 1: Positions observed in CO.
Table 2: CO(1-0) detections.
Table 3: CO(2-1) detections.
Table 4: Results of Gaussian fits to the detected clouds in CO(1-0).
Table 5: 13CO observations.
2 Observations
The observations presented here are a follow-up to the Gardan et al. (2007) mapping of a large part of M 33. All data were taken with the 30 m telescope run by the Institut de RadioAstronomie Millimétrique (IRAM) on Pico Veleta near Granada, Spain. Observing runs were in April, August, and Nov. of 2006.
The 12CO and 13CO
and
transitions were observed with the ``AB'' receivers. These receivers
are dual polarization and the beam-splitter enables simultaneous
observation of the two transitions.
Data were taken under good conditions, with system temperatures (
)
of 300-400 K except for the 13CO(1-0) where the average system temperature was
200 K.
As a result, data reduction was simple: bad channels were eliminated, a
zero-order baseline (constant value) was subtracted, and spectra were
averaged for each position.
The positions observed are shown as yellow and black circles in Fig. 1 for the 12CO and red or blue circles for the 13CO. The size illustrates the IRAM CO(1-0) half-power beamsize.
Generally, both the
and
transitions were observed simultaneously.
The data are presented using the main beam temperature scale. The main beam efficiencies are estimated to be
0.72, 0.74, 0.53, and 0.54 for respectively the lines at 115, 110, 230, and 220 GHz (see http://www.iram.es/IRAMES/mainWiki/Iram30mEfficiencies) and forward efficiencies 0.95, 0.95, 0.91, and 0.91.
Table 1 provides the positions at which we observed 12CO.
Tables 2 and 3 give the 12CO
and
fluxes at each detected position and
the rms noise level at 1 MHz resolution for the undetected positions. The results of fitting Gaussian line profiles
to the spectra, in order to robustly measure line widths and velocities, can be found in Table 4.
Table 5 gives the 13CO fluxes and 12/13CO line ratios for both transitions.
3 Molecular clouds in the outer disk of M 33
In this article we present CO and even 13CO detections beyond the R25 radius of M 33. These are the first such detections for a sub-solar metallicity galaxy. Only a few detections have been made in the outer parts of galaxies: in the Milky Way, using the rotation curve to estimate the distance (e.g. Digel et al. 1994; Brand et al. 1987), in NGC 4414 (Braine & Herpin 2004), and in NGC 6946 (Braine et al. 2007). One of the issues is whether large quantities of molecular gas could have escaped detection in the outer disk (Pfenniger et al. 1994). In very nearby galaxies such as M33, molecular clouds can be spatially resolved, allowing the use of the Virial theorem and isotopic lines to (roughly) assess their masses and resemblance to inner disk clouds. This is particularly interesting for small, chemically young, galaxies like M 33.
After the mapping of the northern part of M 33 by Gardan et al. (2007),
we wanted to integrate more deeply at positions where we thought CO
might be found, with the goal of testing whether we could predict from
other means if molecular gas were present. As molecular clouds form out
of the atomic gas, which dominates in the outer disk, one of the
criteria was the presence of high HI column density. Dust emission is
frequently linked to CO emission but in the outer parts of M 33
the FIR and MIR emission was often too weak to be detected by Spitzer
(or earlier satellites), despite the substantial HI column densities. A
series of positions was observed to a lower noise level, and in CO
(1-0), rather than in the CO(2-1) mapping by Gardan et al. (2007); the coordinates are given in Table 1 and they are shown in
Fig. 1 as circles. Figures 2-4 present the CO(1-0) and HI spectra at these positions on the 70 m
image with HI column density contours. The HI data is at 17''
resolution from a mosaic of VLA C and D configuration fields described
in Gratier et al. (2010a). These outer positions were chosen because either star formation was present
from the H
image (Greenawalt 1998; Hoopes & Walterbos 2000) or high HI antenna temperatures (either narrow lines or
high total column densities) were found in the
Deul & van der Hulst (1987) data. The FIR data was not available at the time of the observations yet the spectra show that the
CO detections are not always at the HI or FIR/H
maxima although they tend to be part of larger HI structures with
H
emission. At higher sensitivity, the FIR emission would probably be detected as well. Here we show the
70
m data but the 160
m images do not show more extended emission.
Figures 2-4
show the sources labelled M 33 18, 20, 22, 23, 24, 26, and 27 -
all of which are north of Dec 31:05. Only deep NIR images detect the
stars at these distances from the center.
Figure 5 shows the CO(1-0) and CO(2-1) spectra of the outer disk clouds M33_2, M33_3, M33_5, and the CO(1-0) spectra of the two detected positions in M33_20. Figure 6 shows the 8 clouds for which 13CO emission was detected as well. Among the 13CO detections is cloud M33_18, beyond the R25 radius (see Fig. 3 as well). This is the first such detection and 13CO is detected in both transitions. It is also of interest that the line widths decrease significantly from the inner regions (bottom of figure) to the outermost parts (top). The narrow lines show that we are only observing a single cloud, allowing us to try to use the Virial theorem in the next section to estimate the cloud masses. Cloud 4 (the Gardan et al. 2007, ``Lonely Cloud'') is very well-detected but is in an interarm region with little or no detected star formation despite the strong CO emission. Clouds 4 and 18 have been observed with the Plateau de Bure Interferometer and will be discussed at length in a dedicated article.
While there is a general link between CO emission and HI and FIR (e.g. 70 m) emission, the CO emission of clouds 4 and 18 appears surprising even with the benefit of hindsight. The 70
m emission, for example,
is only very marginally detected at the positions of these strong CO detections.
However, something must provoke the condensation of atomic gas clouds to form H2. In the case of cloud 18, the HI spectra are complicated so perhaps the H2 is the result of merging HI clouds (Heitsch et al. 2005; Brouillet et al. 1992; Hennebelle & Pérault 1999; Ballesteros-Paredes et al. 1999).
In the other molecular clouds in the outer regions of M 33, this
seems less likely because the HI spectra are not wider or more
complicated than in many places where CO is not found.
![]() |
Figure 1:
70 |
Open with DEXTER |
![]() |
Figure 2:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of sources M33_20, M33_24, and M33_27
on 70 |
Open with DEXTER |
![]() |
Figure 3:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of sources M33_22, M33_23, and M33_26 on 70 |
Open with DEXTER |
4 Determining the H
mass
4.1 Via the Virial theorem applied to individual clouds
Historically, cloud masses have been estimated assuming that molecular
clouds are bound under their own gravity. Galactic GMCs show a relationship between size and linewidth (Larson 1981)
which is difficult to explain in other ways.
The masses calculated are usually referred to as ``Virial masses'',
twice as large as marginally gravitationally bound clouds. In addition
to GMCs, diffuse clouds are also observed in the Galaxy (Polk et al. 1988)
and these are probably unbound. The scales we observe in M 33
correspond
to GMCs, justifying the use of the Virial method. Since the only force
opposing
gravity in the standard calculations is the internal motion, measured
by the line width, magnetic support could increase masses further if
clouds really were stable long-lived ``Virialized'' objects. Since the
idea of virialized clouds came out, it has become clear that GMCs are
not long-lived entities but rather have ``lifetimes'' of order 107 years (e.g. Kawamura et al. 2009).
However, the ``Virial'' masses have generally been found to be
consistent with other means of measuring GMC masses, such as dust
measurements (Sodroski et al. 1995) or gamma-ray observations (see e.g. Combes 1991). Using the expression from Dickman et al. (1986),

where






The Virial masses given are upper limits in the sense that we have
assumed that the clouds are as big as the beam, 85 pc in diameter,
whereas this is likely an overestimate of the size of a single cloud
(e.g. Solomon et al. 1987). Furthermore, multiple clouds would broaden the CO lines and again cause an overestimate of the mass. However, the
value may be overestimated and is used for consistency with Braine et al. (1997a).
These authors also found, through basic radiative transfer calculations
and for modestly subsolar metallicites, that a low radiation field (as
for the clouds in the outer disk of M 33) could counteract the
deeper CO photo-dissociation expected with decreasing metallicity, such
that the CO-emitting part of a cloud was not necessarily smaller in an
outer disk environment.
Nonetheless, the virial masses given here are higher (possibly due to
the large assumed size) in most cases than galactic GMCs of similar
line width (using
,
Eq. (6a)
from Solomon et al. 1987). Attributing an uncertainty is difficult; the sources of error are chiefly the size, the
parameter, and the hypothesis of ``Virial'' equilibrium (gravity alone). The size and
likely lead to an uncertainty of a factor 2 at most. The presence of
CO-bright isolated clouds in the outer disk is evidence that they are
robust, and thus gravitationally bound, structures, comforting the
Virial hypothesis.
If the clouds we identify were simply due to the fluctuating turbulent
field then we would expect to find many weak lines for a single strong
one (if indeed strong lines could be formed this way) and the Gardan et al. (2007) observations suggest this is not the case. The relatively strong 13CO emission shows that the 12CO
line is highly optically thick, also arguing for a non-transient
nature.
The spectra of some clouds, particularly in the inner disk, are
complicated and single-Gaussian fits lead to significant uncertainties
in the linewidth and thus Virial mass. These are a minority, however,
so we estimate an uncertainty
of at most a factor 2 on the value given below.
With the exception of the two bright inner disk clouds, M33_11 and M33_12,
which are consistent with a galactic
factor,
and the very uncertain sources, the virial masses are consistent with a
factor (the average for clouds 4, 5, 7, 9, 10, and 18).
![]() |
Figure 4:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of source M33_18 and major offsets
on 70 |
Open with DEXTER |
![]() |
Figure 5:
Spectra of the 4 clouds where only 12CO emission was detected. The
|
Open with DEXTER |
![]() |
Figure 6:
Spectra of the 8 clouds where 13CO emission was detected. The
|
Open with DEXTER |
4.2 From isotopic lines and ratios
Another approach to estimating cloud masses is to measure isotopic lines assumed to be optically thin. In principle, this allows one to ``count'' the molecules of the optically thin species. In practice, this is quite difficult but isotopic line ratios provide a means to determine whether the GMCs are intrinsically very different from Galactic clouds. Eight of the clouds have been observed in 13CO and all of them were detected in the 13CO(1-0) line and all but one in 13CO(2-1).
Assuming optically thin 13CO(1-0) emission and Local Thermodynamic
Equilibrium at a temperature
,
the remarkably
narrow range in 12/13CO line ratios (Table 5) yields
values within a factor 2
of
(see formulae 14.40 and 14.41 in Rohlfs & Wilson 2004)
for realistic excitation temperatures (assumed 10-30 K). This is true for both
the 13CO(J=1-0) and J=2-1 transitions.
The calculations assumed a 13CO abundance with respect to H2 of
- lower 13CO abundances will yield a higher
value. Of course, a fraction of the 13CO is probably not optically thin and the
departure from LTE will be much greater in 13CO than 12CO
due to the inefficient radiative excitation. Furthermore, the fraction of the GMC
where 13CO is dissociated will be much greater than for 12CO.
The combination of these ingredients likely explains the much lower
value obtained in this way. Low 12/13CO line ratios lead to high
ratios.
The 13CO integrated intensities and 12/13CO line ratios for each transition are given in Table 5. The ratios are of order 10 for the J=2-1 transition and slightly higher for the J=1-0. These values are typical of (large) galaxies at large scales (Rickard & Blitz 1985; Sage & Isbell 1991; Paglione et al. 2001). Values for individual clouds in the Milky Way are usually lower, in the range 3-5 (Digel et al. 1994; Polk et al. 1988) but may not include all of the low column density material with significant 12CO but little 13CO emission. Since M 33 has a metallicity which is subsolar by a factor 2, the 12/13CO line ratio could be expected to be higher, as the optically thick 12CO will be less affected than the 13CO by the lower abundance.
The 13CO spectra shown in Fig. 6 are multiplied by 10 and can be seen to lay
well over the 12CO spectra. The line ratios observed in M 33 are thus
in good agreement with the picture in which molecular clouds in M 33 are similar to
their Galactic counterparts but with a lower metallicity and a
higher by about a factor two than in the Milky Way disk.
5 Conclusions
We present the first detections of both 12CO and 13CO in the extreme outer disk (
R > R25) of a subsolar metallicity galaxy.
Several clouds have been detected in our search for molecular gas in
the outer disk, enabling us to try to identify the environments in
which detectable quantities of H2 are found in the dim outer disks of spirals.
The HI antenna temperature and column density and the level of star formation
as traced byFIR or H
emission are indicators of the likelihood of detecting CO but quite
imperfect because two of the strongest outer disk clouds would not have
been found that way.
In and of itself, the unreliability of this link is interesting.
As we are close to spatially resolving clouds, the ``Virial'' method is used to estimate molecular cloud masses and thus the
ratio. The inner disk clouds measured are compatible with a Galactic
factor. Our estimate of
in the outer disk of M 33 is
with an uncertainty of at most a factor 2. The 12/13CO line ratios are compatible with a
factor somewhat greater than in the Milky Way molecular ring, such as the
estimated by the Virial method presented here.
Adopting this conversion factor results in a lower star formation efficiency (
)
than previously found but the SFE remains higher than the SFE in large local universe spirals (e.g. Kennicutt 1998).
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All Tables
Table 1: Positions observed in CO.
Table 2: CO(1-0) detections.
Table 3: CO(2-1) detections.
Table 4: Results of Gaussian fits to the detected clouds in CO(1-0).
Table 5: 13CO observations.
All Figures
![]() |
Figure 1:
70 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of sources M33_20, M33_24, and M33_27
on 70 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of sources M33_22, M33_23, and M33_26 on 70 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
CO(1-0) spectra (yellow) and HI spectra (red dotted) of source M33_18 and major offsets
on 70 |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Spectra of the 4 clouds where only 12CO emission was detected. The
|
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Spectra of the 8 clouds where 13CO emission was detected. The
|
Open with DEXTER | |
In the text |
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