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Observational Constraints on the Molecular Gas Content in Nearby Starburst Dwarf Galaxies

Using star formation histories derived from optically resolved stellar populations in nineteen nearby starburst dwarf galaxies observed with the Hubble Space Telescope, we measure the stellar mass surface densities of stars newly formed in the bursts. By assuming a star formation efficiency (SFE), we then calculate the inferred gas surface densities present at the onset of the starbursts. Assuming a SFE of 1%, as is often assumed in normal star-forming galaxies, and assuming that the gas was purely atomic, translates to very high HI surface densities (~10^2-10^3 Msun pc^-2), which are much higher than have been observed in dwarf galaxies. This implies either higher values of SFE in these dwarf starburst galaxies or the presence of significant amounts of H_2 in dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with observations of more massive starbursts associated with merging galaxies), still results in HI surface densities higher than observed in 10 galaxies. Thus, these observations appear to require that a significant fraction of the gas in these dwarf starbursts galaxies was in the molecular form at the onset of the bursts. Our results imply molecular gas column densities in the range 10^19-10^21 cm^-2 for the sample. In those galaxies where CO observations have been made, these densities correspond to values of the CO-H_2 conversion factor (X_CO) in the range>3-80x10^20 cm^-2 (K km s^-1)^-1, or up to 40x greater than Galactic X_CO values.


INTRODUCTION
Star formation rates (SFRs) are known to correlate with the surface density of cold gas. This correlation, commonly referred to as the Kennicutt-Schmidt law, was seen using star formation (SF) and gas tracers integrated on galaxy scales (Schmidt 1959;Kennicutt , 1998b. The direct correlation of SF and molecular gas has been seen on sub-galactic spatial scales as well, using maps of atomic and molecular gas (e.g., Martin & Kennicutt 2001;Wong & Blitz 2002;Bigiel et al. 2008;Rahman et al. 2012). However, the variables and conditions that govern when and how much gas is converted into stars remains unclear (e.g., Tan 2000;Martin & Kennicutt 2001;Wong & Blitz 2002;Leroy et al. 2008, and references therein). In addition, the role atomic Hi gas plays in driving the SFR is also poorly understood, particularly in dwarf and low surface brightness galaxies which lack significant CO detections that trace molecular gas (e.g., Taylor et al. 1998;O'Neil et al. 2003;Leroy et al. 2005, and references therein).
While most studies approach this analysis based on the current SFRs and gas surface densities, studies of op-tically resolved stellar populations can provide a longer temporal baseline by probing the SF as a function of time (i.e., SFR(t)). Using Hubble Space Telescope (HST ) observations, McQuinn et al. (2010a,b) reconstructed the star formation histories (SFHs) of 19 nearby, starburst dwarf galaxies. These temporally resolved SFHs enable a calculation of the total stellar mass of stars newly formed in these systems over the duration of the burst events (i.e., a few 100 Myr). As the stellar mass was formed from the gas reservoirs of the galaxies, this calculation provides a way to estimate the gas surface densities present when the star forming episode began.
One way to quantify the amount of gas that is converted into stars is through the star formation efficiency (SFE) parameter, which estimates the fraction of gas that is converted into stars in a given area during a fiducial time period. The SFE defined in this way is dimensionless and based on the direct connection between gas and the stars formed from the gas. Models of SF, based on different SF laws and SF thresholds, assume different SFEs in their calculations, or conversely, predict SFEs given a set of inputs (e.g., Tan 2000;Parmentier & Fritze 2009;Côté et al. 2011). Measurements of the SFE are problematic, as the measurements of the gas being converted into stars and of the stars produced from that gas cannot be made simultaneously. In practice, observations of SFEs usually assume typical observed gas surface densities or gas surface densities in the vicinity of recent star formation.
SFE's have been studied in a variety of galaxy types and environments including, in order of increasing SFRs, dwarf galaxies (Bigiel et al. 2008), normal disk galaxies (Kennicutt 1998b;Bigiel et al. 2008;Rahman et al. 2012), luminous infrared galaxies (Young et al. 1986;Solomon & Sage 1988;Sanders et al. 1991), and massive starburst galaxies (Kennicutt 1998b). The SFE values range from ∼ 1% to values approaching − ∼ 100% in massive starbursts (Kennicutt 1998b), with the majority of normal star forming galaxies (i.e., dwarfs and spirals) having lower SFEs in the ∼ 1 − 10% range. SFEs higher than 10% have been reported in more massive starburst galaxies (e.g., Young et al. 1986;Solomon & Sage 1988;Kennicutt 1998b;Sanders et al. 1991;Bigiel et al. 2008), possibly as a result of higher gas densities according to the Kennicutt-Schmidt Law. However, on small physical scales SFEs have also been argued to be roughly invariant in any supersonically turbulent medium (Krumholz & McKee 2005).
In this study, we use SFHs derived from resolved stellar populations in 19 nearby, Hi dominated, starburst dwarf galaxies (McQuinn et al. 2010a,b) in order to investigate the gas content in these galaxies prior to their bursts. For the derived stellar mass surface densities of the stars formed in the bursts, we assume different SFEs to infer the gas surface densities present at the onset of the starbursts. Further, we investigate the implications these gas surface densities have for the amount molecular gas that may have been present in these systems. The paper is organized as follows. §2 summarizes the observations, data processing, and SFH reconstruction technique. §3 calculates the surface density both of the SFR and of the stellar mass. §4 applies different SFEs to infer the gas surface densities present when the starbursts began and §5 estimates CO−H 2 conversion factors. §6 summarizes our conclusions.

SFHS
In Table 1 we summarize various physical properties of the 19 nearby starburst dwarf galaxies that comprise our sample. This sample was previously studied by McQuinn et al. (2010a,b, and references therein) using archival HST V and I band observations from either the Advanced Camera for Surveys or Wide Field Planetary Camera 2 instruments. The galaxies all lie close enough for the HST observations to resolve the stellar populations into individual stars (i.e., D 5 Mpc). A full description of the observations and data processing can be found in McQuinn et al. (2010a). For this paper we use the McQuinn et al. (2010a) photometry, which was measured by DOLPHOT or HSTphot (Dolphin 2000) from the HST pipeline processed and cleaned images. In Figure 1, we present an example of the photometric quality in the color magnitude diagram (CMD) for ESO 154−023, whose data reach our minimum required photometric depth. The SFHs (i.e., the rate of star formation across time SF R(t, Z)) were reconstructed by McQuinn et al. (2010a) by fitting the observed CMDs (Dolphin 2002). This well established technique uses stellar evolutionary models (Marigo & Girardi 2007) to model the observed CMD as a linear combination of different age and metallicity stellar populations for an assumed initial mass func- tion (IMF). As described in McQuinn et al. (2010a), the SFHs were reconstructed assuming a binary fraction of 0.35 and a Salpeter IMF (Salpeter 1955), and allowing the models to fit for distance and extinction. The metallicity was constrained to increase with time, except in cases where the photometry reached a full magnitude below the red clump. For a full discussion of the SFHs, the time binning used, and uncertainties we refer the reader to McQuinn et al. (2010a). We scaled down the SFRs by a factor of 1.59 (Bruzual & Charlot 2003) to transform the results to a Kroupa IMF (Kroupa 2001), thus enabling a direct comparison with other studies in the literature on SFR and SFR densities (e.g., Bigiel et al. 2008).
In Figure 2, we present an example of the resulting SFH of ESO 154−023 over the last 3 Gyr. For this starburst system, the recent SFRs are elevated over the average value during the most recent ∼ 450 ± 50 Myr (McQuinn et al. 2010b). This pattern of sustained elevated SFRs at recent times is seen across the starburst sample. For the current analysis, we focus on measuring the SFR surface density over the duration of the starbursts, and on calculating the stellar mass surface density for the stars formed during the same time period.

STELLAR MASS SURFACE DENSITIES OF NEWLY FORMED STARS
SF intensity is frequently parameterized by calculating the SFR surface density, Σ SFR ≡ SFR area −1 (M ⊙ yr −1 kpc −2 ) (e.g., Meurer et al. 1997;Lanzetta et al. 2002;Kennicutt et al. 2005). This calculation of Σ SFR depends upon the timescale over which one measures the SFRs. For example, the SFR calculated from Hα flux (e.g., Kennicutt 1998a; James et al. 2004) is the SFR averaged over the past few Myr, whereas the SFR calculated from UV emission (Meurer et al. 1997) is an average over the past few hundred Myr. If the SFR remains constant over the two time periods, the averages are equivalent, but variations in the SFR over time can make the different values difficult to compare. The Figure 2. The SFH of ESO 154−023 during the last 3 Gyr. The SFR is calculated assuming a Kroupa IMF. The SFR is higher over the past 500 Mr, preceded by a period (∼ 2.5 Gyr) of quiescence when the SFR was a factor of ∼ 5 lower. This pattern of sustained elevated SFR at recent times is seen across the entire starburst sample. In this study, we measure the Σ SFR and calculate the stellar mass surface density during this period of elevated SF. The bin size along the horizontal axis was chosen based on temporal resolution achieved from SFHs reconstructed from synthetic CMDs of similar completeness. The same binning is used for each galaxy in the sample. The error bars include both systematic uncertainties and statistical uncertainties estimated from Monte Carlo simulations. The uncertainties are typical of the SFH of the sample.
optically resolved stellar populations and temporally resolved SFHs allow us to select the timescale over which to calculate Σ SFR . We chose to calculate Σ SFR over the duration of a burst (see Figure 2). The durations and SFRs averaged over these timescales, listed in Table 1 There are a number of possible choices for normalizing the SFR to a fiducial area. In nearby systems, both James et al. (2004) and Kennicutt et al. (2005) use the area of the currently active star forming regions as traced by Hα emission. The Σ SFR calculated in this way follows a Gaussian distribution for nearby galaxies ranging from 10 −4 to 10 2 M ⊙ yr −1 kpc −2 , with massive starburst galaxies generally falling in an extended tail of the distribution above 10 −1 M ⊙ yr −1 kpc −2 . Alternatively, the area has been defined as the half-light radius of a galaxy in the UV (i.e., radius ∼ 0.1−10 kpc; Meurer et al. 1997), or as the optical disk of a galaxy measured to the Bband 25 mag arcsec −2 isophotal radius (i.e., radius ∼ kpc scales ;Schmidt 1959;Kennicutt 1998b). In a comparative study using Hi, UV, and optical data, Wyder et al. (2009) use the minimum area for which all wavelengths showed a detection and the Σ SFR was calculated using an average SFR based on UV surface brightness over the adopted area.
Over the duration of a starburst, in many dwarf galaxies, SF has been shown to be widely distributed across the disks of the galaxies, rather than solely concentrated at the very center (McQuinn et al. in prep.). Therefore, for this study, we use the optical extent of the galaxies included in the HST observational fields of view. The Σ SFR calculated from this area represents a measurement of the SF activity in the optical disk over the life of a starburst. These values could potentially be compared with SFR surface densities calculated for high redshift bursting dwarf galaxies, should observations of this type become available. Note that for our analysis of Σ SFR , this choice in area represents a conservative assumption. In addition, if the gas is assumed to be clumped (as expected), then this will result in even higher values of precursor gas surface density.
The optical areas of the galaxies were calculated based on the major diameter measured from the extent of the B-band 25 mag arcsec −2 isophotal radius and the ellipticity of the galaxy (e.g., D 25 ; Karachentsev et al. 2004, see Table 1). These D 25 measurements were corrected for inclination angle and Galactic extinction following the prescription by de Vaucouleurs et al. (1976). Included in Table 1 are the deprojected D 25 areas covered by the HST fields of view and the equivalent areas in kpc 2 for the sample, accounting for distance to the system. In eleven galaxies, the observational field of view covered the extent of the galaxy out to the D 25 limits. In the remaining cases, the angular area of the observations did not extend to this surface brightness limit. For these galaxies, we normalized the SFR by the area covered by the HST observations corrected for inclination. We discuss the impact of these smaller fields of view on our results below.
In Figure 3, we present a histogram of our measured Σ SFR ranging from 10 −3.5 to 10 −1.5 M ⊙ yr −1 kpc −2 . Values for individual galaxies are listed in Table 2. The range of values in this starburst sample is lower than the 0.1 M ⊙ yr −1 kpc −2 threshold set for starbursts in more massive galaxies based on Hα derived SFRs (Kennicutt et al. 2005). Σ SFR is generally higher in more massive disk galaxies (Kennicutt 1998b;Bigiel et al. 2008). However, Σ SFR values from different studies are difficult to compare as the adopted areas and the methods of measuring the SFRs often vary.
The total stellar mass formed during the bursts can be calculated directly from the SFHs by integrating the SFRs(t) over the duration of the bursts. Applying the same de-projected physical areas in Table 1, we calculate the stellar mass surface density of stars formed during the burst (Σ * ,burst ), ranging from 0.2 to 13 M ⊙ pc −2 . Values for individual galaxies are listed in Table 2. For the galaxies where the observational field of view did not reach the isophotal radius of 25 mag arcsec −2 , our analysis applies to only the central regions of the systems. The difference between this central value of the stellar mass surface density of the newly formed stars and the value for the entire optical disk is difficult to estimate as the SFRs in the outer regions of galaxies do not scale linearly with area. This may be an important difference in the three galaxies where the HST field of view covers less than half of the optical disk (see Table 1), but will not impact our results significantly in the remaining systems. Note also that the stellar mass values are dependent on the form of the IMF assumed in deriving the SFHs. If we adopted a Salpeter IMF (Salpeter 1955), these stellar mass values would be larger by a factor of 1.59.

DENSITIES
Having measured Σ * ,burst , we can infer the hydrogen gas surface density (hereafter Σ gas ) present at the onset Figure 3. The histogram shows the range of Σ SFR found in the sample. The values are lower than typical starburst SFR surface densities of more massive, H 2 rich galaxies found in the literature (e.g., Kennicutt 1998b; Gao & Solomon 2004), both because the sample consists of lower mass galaxies that are Hi dominated and because we use the larger area of the optical disk as our baseline area. These Σ SFR are similar to measured values for spiral disks galaxies when averaged over sub-kpc scales (Bigiel et al. 2008). Thus, while the sample is comprised of dwarf galaxies, the bursting SF makes their SFR surface densities comparable to the higher end of more massive spiral disk galaxies. of the starbursts, for an assumed SFE and an assumed primordial Helium abundance of 1.33 (Cyburt et al. 2008;Komatsu et al. 2011). This simple model assumes that the initial hydrogen gas mass is a multiple of the stellar mass formed in the burst in the stars (i.e., Σ gas ≡ (Σ * ,burst /1.33)/SFE). The SFEs in dwarf galaxies are typically between 1 − 3%, comparable to the SF in outer disks of spiral galaxies. However, while the starburst dwarf galaxies in our sample have low masses and low metallicities typical of dwarf galaxies, their Σ SFR are comparable to or higher than the values of Σ SFR for typical spiral galaxies which show SFEs between 5 − 10% (Bigiel et al. 2008;Rahman et al. 2012).
Based on the previous results for both dwarf and spiral galaxies, we might expect SFEs for our sample to lie in the range of 1 − 10%. Starting with the lower limit SFE value of 1%, typical of the lower SFRs typically found in the outer regions of spirals and in dwarf galaxies (Kennicutt 1998b;Bigiel et al. 2008), the inferred Σ gas ranges from 25 − 900 M ⊙ pc −2 . However, if we assume that the ISMs in these dwarf galaxies are Hi dominated, as suggested by observations (e.g., Leroy et al. 2008), this derived range of gas surface densities includes densities far greater than any Hi gas surface densities observed to date. The typical value of the maximum Σ HI observed in dwarf galaxies is limited to the range of ∼ 5 − 10 M ⊙ pc −2 (Swaters et al. 2002) with a sharp saturation at ∼ 10 M ⊙ pc −2 (Wong & Blitz 2002;Bigiel et al. 2008) when measured on sub-kpc scales. These numbers will be lower if averaged over larger physical scales. The implication is that the SFE must be higher than 1%, or that not all of the hydrogen is in the atomic phase (or both). Because the highest stellar mass surface densities of newly formed stars derived assuming a SFE of 1% are roughly 100 times larger than the highest observed HI surface mass densities, solving this problem would require SFEs of order 100%, which are very unlikely.
Atomic hydrogen gas surface densities higher than any observed would be possible if the triggering event of the starburst provided additional gas to the systems (Ekta & Chengalur 2010), temporarily increasing Σ gas far above what is typically observed in Hi. As none of the galaxies in the sample show evidence of an obvious merger, gas infall would be the most likely source of the additional mass. However, Hi maps, available for roughly half of the sample (Begum et al. 2008;Bigiel et al. 2008;Warren et al. in prep), do not show highly disturbed Hi morphologies, as would be expected from a gas infall event. We will go forward assuming a higher SFE of 10% and see what that implies about the gas surface densities and the molecular gas fractions. Figure 4 shows the relationship between the observed SFR surface density and the inferred gas surface density with an assumed SFE of 10%. The points deviate from a straight line because systems with similar average SFR densities can have different stellar mass surface densities of recently formed stars depending on the duration of each burst. While 5 galaxies host "fossil" bursts, the remaining 14 host on-going starburst activity. Thus, many of the gas surface density values will increase as the bursts mature. Figure 4 demonstrates that even with a higher SFE of 10%, half the sample have inferred Σ gas that exceed any observed atomic hydrogen gas surface densities to date (Σ HI ∼ 10 M ⊙ pc −2 ; Swaters et al. 2002;Wong & Blitz 2002;Bigiel et al. 2008). Conversely, if we assume that Σ gas is 10 M ⊙ pc −2 based on Hi observations, we infer a minimum SFE of 3% and a maximum SFE far exceeding 100%. Thus, it appears that the calculated stellar mass surface densities of newly formed stars imply that the typical dwarf galaxy has a significant reservoir of molecular hydrogen.
Assuming the initial gas mass is some combination of atomic and molecular gas, the values of the inferred Σ gas seen in Figure 4 suggest that a significant fraction of the gas is in molecular form in 10 of the galaxies. If we adopt both a SFE of 10% and a Σ HI = 10 M ⊙ pc −2 , then the inferred molecular gas surface density (Σ H2 ) ranges from ∼ 1 − 80 M ⊙ pc −2 or an equivalent average H 2 column density of 10 19 − 10 21 cm −2 ; values for the individual galaxies are listed in Table 2. This range would be higher if a lower SFE were assumed (this is likely the case for the systems with log Σ SFR < −3). Our estimated values of Σ H2 are averages for the optical disk; regions of even higher molecular gas densities would be expected in starforming regions.
Our inferred Σ H2 can be compared with known measurements of Σ H2 in more massive disk galaxies with similar Σ SFR . For example, Σ H2 has been measured to range from ∼ 3 − 70 M ⊙ pc −2 in the inner regions of lower metallicity spiral galaxies (Bigiel et al. 2008;Leroy et al. 2011). Somewhat higher H 2 column density estimates of up to ∼ 10 22 cm −2 (i.e., 100 M ⊙ pc −2 ) have been found in the SMC from infrared dust measurements on 200 pc scales ). In the central regions of 14 normal spiral galaxies, Rahman et al. (2012) have measured Σ H2 to range from 20 − 1000 M ⊙ pc −2 , however . Assuming a SFE of 10%, the inferred hydrogen gas surface densities present at the onset of the bursts range from 2 − 90 M ⊙ pc −2 , after correcting for the abundance of Helium. The galaxies plotted in red have ongoing bursts, while those plotted in black are fossil bursts. Uncertainties in the Σ SFR are smaller than the size of the points, except where drawn in the Figure. We adopted one value of SFE, but the SFE is likely to be lower for the systems with log Σ SFR < −3, based on measurements in galaxies with similar SFR surface densities (Bigiel et al. 2008). Therefore, the gas surface densities in these systems are likely to be higher. For the galaxies with log Σ SFR −2.9, even with assuming a SFE of 10%, the inferred Σgas is higher than the observed range in maximum values of Σ HI = 5 − 10 M ⊙ pc −2 observed in dwarf galaxies (Swaters et al. 2002), with a sharp saturation at ∼ 9 M ⊙ pc −2 (Bigiel et al. 2008)). Thus, we conclude that a significant fraction of the gas is in molecular form and remains unobserved in most of these galaxies. Further, we estimate the X CO factor to range from 3 to 80 ×10 20 cm −2 (K km s −2 ) −1 based on the inferred molecular gas surface densities and CO observations, where available, in the literature. giant molecular clouds in the Milky Way have a mean hydrogen molecular gas surface density of 42 M ⊙ pc − 2, with variations up to 80 − 120 M ⊙ pc −2 (Heyer et al. 2009). Based on these comparisons, the estimated values of Σ H2 (ranging from ∼ 1 − 80 M ⊙ pc −2 ) and the corresponding fraction of molecular to total gas are in agreement with molecular gas surface densities and with molecular to atomic ratios measured in galaxies with similar Σ SFR . In addition, the typical timescale to deplete the molecular gas by star formation in a disk galaxy is ∼ 2 Gyr Rahman et al. 2012). This depletion timescale is up to 4× longer than the duration of the starburst events indicating that typical disk galaxies will contain sufficient reservoirs of molecular gas over the timescales of a starburst to fuel and sustain the elevated levels of SF.

LIMITS ON THE CO-H 2 CONVERSION RATIO
While the atomic Hi component of galaxies is easily measured using 21 cm observations, the molecular H 2 component is generally measured indirectly by measuring emission from the CO molecule and then inferring a ratio of the CO to H 2 molecules (i.e., the CO-H 2 conversion factor, X CO ). Estimating H 2 masses from CO emission has proven more difficult in dwarf galaxies for two reasons. First, there are well known difficulties in detecting molecular gas in low metallicity environments (e.g., Bolatto et al. 2008, and references therein). Since few dwarf galaxies have CO emission detections, conversions from CO emission to H 2 masses are often only upper limits. Second, X CO is uncertain for low metallicity systems. Historically, X CO was based on measurements at higher metallicities and application to lower metallicity systems was uncertain. Indeed, the conversion factor has been measured to be a strong function of metallicity below 12+log(O/H) ∼ 8.4 − 8.2 , the upper end of the metallicity range of our sample. Thus, even in cases with CO emission detections, conversion to an H 2 mass is uncertain for low metallicity dwarfs.
We can constrain the values of X CO for a few systems in our sample by combining our inferred values of Σ H2 with confirmed CO detections (or limits on the CO emission) from the literature. While most galaxies in our sample have very low metallicities (i.e., 12+log (O/H) 8.0; see Table 1 for individual values) where CO is completely non-detected (e.g., Taylor et al. 1998), a few of the more luminous galaxies in our sample with somewhat higher metallicities (i.e., 12+ log(O/H) ∼ 8.1 − 8.4) do have CO detections (e.g., NGC 1569, Young et al. (1984); NGC 4449 and NGC 4214, Tacconi & Young (1985); NGC 5253, Turner et al. (1997); NGC 6822, ). In Table 2, we list confirmed CO detections from the literature for five galaxies and CO upper limits for an additional three galaxies. Using these CO observations and our estimates of N H2 in the galaxies with inferred molecular gas content, we calculate X CO to range from (> 3 − 80)×10 20 cm −2 /(K km s −1 ). The lower end of this range is ∼ 1.5× greater than Galactic measurements while the upper end is 40× greater. These estimated values of X CO are in agreement with measured values of ∼ 20 × 10 20 and ∼ 45 × 10 20 cm −2 /(K km s −1 ) found for two dwarf galaxies in the Local Group from infrared dust emission ) and the lower limit of 10× Galactic X CO values placed on a low metallicity (12+[O/H]∼ 7.67) Local Group dwarf galaxy DDO 154 (Komugi et al. 2011).
Since nearby, metal poor, dwarf galaxies are often considered the prototypes of more massive galaxies at high redshift, it is also interesting to compare our estimated X CO values to measurements in low metallicity systems at high redshift. In a sample of star forming galaxies in the mass range 10 10 − 10 11 M ⊙ , one to two orders of magnitude more massive than any of the galaxies in our sample, Genzel et al. (2012) found the CO luminosity to molecular gas mass conversion factors to be ∼ 2.5 − 14× Galactic values in a metallicity range of 12+[O/H] ∼ 8.1 − 8.4, with higher values expected in lower mass galaxies. For comparison, for the lower mass galaxies in our sample which span the range in metallicities from 12+[O/H]∼ 7.4 − 8.4 (see Table 1), our estimated X CO factors include values up to 40× greater than Galactic X CO values.

CONCLUSIONS
We have used optical imaging of resolved stellar populations obtained from the HST data archive to measure the SFRs and stellar mass surface densities of stars formed during starburst events in a sample of 19 nearby starburst dwarf galaxies. The SFR surface densities of these dwarf galaxies are comparable to the higher end of SFR surface densities of more massive spiral galaxies while still having low metallicities typical of dwarf systems.
By assuming different SFEs, the stellar mass surface densities were used to infer gas surface densities present at the onset of the bursts. Using a SFE of 1% and assuming Σ gas ∼ Σ HI , all inferred gas surface densities are greater than the observed range of Hi gas surface densities in dwarf galaxies of 10 M ⊙ pc −2 (Swaters et al. 2002;Wong & Blitz 2002;Bigiel et al. 2008). Using a higher SFE of 10%, half of the inferred gas surface densities are still higher than the observed range of Σ HI by factors of 9.
The simplest explanation appears to be that a significant fraction of the gas is in molecular form but remains unobserved in most of these galaxies due to their low metallicities. Indeed, the elevated levels of recent SF are an unambiguous sign that our sample of galaxies have hosted significant reservoirs of cold molecular gas. In this low metallicity regime, the H 2 masses can be significant, even for very low CO luminosities. Thus, the lack of CO detections in our sample is not inconsistent with their having significant reservoirs of molecular gas. Coupled with limits on CO emission from the literature, the inferred molecular gas surface densities suggest X CO factors of up to 40× greater than Galactic measurements in these low metallicity and low mass galaxies.

ACKNOWLEDGMENTS
Support for this work was provided by NASA through a ROSES grant (No. NNX10AD57G). E. D. S. is grateful for partial support from the University of Minnesota. The authors thank the anonymous referee for helpful and constructive comments. K. B. W. M. gratefully acknowledges Matthew, Cole, and Carling for their support. This research made use of NASA's Astrophysical Data System and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.