Low CO Luminosities in Dwarf Galaxies

Abstract: [Abridged] We present maps of CO 2-1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13 arcsec angular resolution, ~250 pc at our average distance of 4 Mpc, and sample the galaxies by 10-1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarfs outside the Local Group ranging from single lines-of-sight, stacked over IR-bright regions of embedded star formation, and stacked over the entire galaxy. We detect 5 dwarfs in CO with total luminosities of L_CO = 3-28 1e6 Kkmspc2. The other 11 dwarfs remain undetected in CO even in the stacked data and have L_CO < 0.4-8 1e6 Kkmspc2. We combine our sample of dwarfs with a large literature sample of spirals to study scaling relations of L_CO with M_B and metallicity. We find that dwarfs with metallicities of Z ~ 1/2-1/10 Z_sun have L_CO about 1e2-1e4x smaller than spirals and that their L_CO per unit L_B is 10-100x smaller. A comparison with tracers of star formation (FUV and 24 micron) shows that L_CO per unit SFR is 10-100x smaller in dwarfs. One possible interpretation is that dwarfs form stars much more efficiently, however we argue that the low L_CO/SFR ratio is due to significant changes of the CO-to-H2 conversion factor, alpha_CO, in low metallicity environments. Assuming a constant H2 depletion time of 1.8 Gyr (as found for nearby spirals) implies alpha_CO values for dwarfs with Z ~ 1/2-1/10 Z_sun that are more than 10x higher than those found in solar metallicity spirals. This significant increase of alpha_CO at low metallicity is consistent with previous studies, in particular those which model dust emission to constrain H2 masses. Even though it is difficult to parameterize the metallicity dependence of alpha_CO, our results suggest that CO is increasingly difficult to detect at lower metallicities.

Analyzing CO Luminosity in Dwarf Galaxies: Implications and Observations

The paper by Schruba et al. presents an extensive study on the carbon monoxide (CO) luminosities in dwarf galaxies, utilizing data from the IRAM HERACLES survey. The study focuses on 16 nearby dwarf galaxies, offering detailed CO $J=2-1$ emission maps that cover entire star-forming disks. The angular resolution stands at 13", equating to approximately 250 pc at the average galaxy distance of 4 Mpc. A variety of stacking techniques are employed to detect CO emissions, ranging from individual lines-of-sight to entire galaxies.

Key Findings

• CO Detection: Out of the 16 dwarf galaxies studied, only 5 showed detectable CO emissions. The CO luminosity for these galaxies ranged from $3 - 28 \times 10^6$ K km s$^{-1}$ pc$^2$, while the remaining 11 galaxies were undetected even in stacked images.

• CO Luminosity vs. Galaxy Properties: Dwarf galaxies were found to be significantly underluminous in CO relative to spiral galaxies, with dwarf galaxies having CO luminosities $2-4$ orders of magnitude lower than their spiral counterparts. In terms of B-band luminosity, dwarf galaxies showed $L_{\rm CO}$ per unit $L_{\rm B}$ being $1-2$ orders of magnitude lower.

• Star Formation Correlation: Dwarf galaxies also exhibited $1-2$ orders of magnitude lower $L_{\rm CO}$ per unit star formation rate (SFR), emphasizing a fundamental difference in star formation efficiency or molecular cooking in these environments compared to typical spiral galaxies.

• CO-to-H$_2$ Conversion Factor: The researchers argue that by assuming a constant H$2$ depletion time of $\tau{\rm dep} = 1.8$ Gyr, the CO-to-H$2$ conversion factor, $\alpha{\rm CO}$, is significantly higher in dwarf galaxies than spirals, especially those with $Z \approx 1/2 - 1/10~Z_\odot$. This finding is consistent with models that point to variations of $\alpha_{\rm CO}$ owing to poor metal content affecting the CO visibility in dwarf environments.

Implications

The findings have profound implications for understanding star-forming environments at lower metallicity contexts, especially in the early universe. The paper highlights that with decreasing metallicity, the detectability of CO diminishes, due to a substantial increase in the conversion factor $\alpha_{\rm CO}$. This observation indicates that CO becomes less efficient as a tracer for molecular hydrogen in metal-poor environments, which holds significant consequences for observations of distant galaxies with presumably sub-solar metallicity.

Future Directions

This study strongly suggests a need for revised models that accurately account for changes in $\alpha_{\rm CO$ across different environments, particularly for early universe star-forming galaxies. Additionally, an exploration into other tracers apart from CO, such as dust modeling, may provide complementary insights and refine molecular gas estimations. As theoretical models evolve to reflect star formation and molecule formation conditions across varied metallicity environments, large-scale observatories equipped with enhanced sensitivity and resolution will better probe molecular gas in distant, metal-poor galaxies.

The research conducted by Schruba et al. serves an important role in redefining our understanding of molecular gas and its intricate dynamics in dwarf galaxies—an aspect critical to unveiling galactic evolution and star formation processes at different metallicity levels.