| Summary: | 博士 === 國立臺灣師範大學 === 地球科學系 === 99 === In order to understand the galaxies evolution, we study how galaxies loss their fuel, molecular gas, through galaxy activities. Starburst galaxies are good candidates to study the molecular gas consumption in galaxies. They are currently undergoing intense star formation in the central region of the galaxies, and their strong star formation produced galactic-scale outflow and ejected large amount of molecular gas. However, the accurate measurement of losing molecular gas are very rare because it is difficult to directly observe molecular outflows or superbubbles
due to their diffuse/extended nature and the poor instrumental sensitivities.
Single dish data, although do not have missing flux problem, the poor resolution is not able to resolve the detail structure of molecular outflows or superbubbles. Recent interferometric observations, although some molecular outflows have been detected from high-velocity broad-wing components, the directly evidence from images is sill missing to constrain their properties. So far, only a few molecular outflows and superbubbles in galaxies have been observed with high accuracy. One famous example is the kpc-scale molecular outflow detected from the typical starburst galaxy M82. The detail properties, such as size, velocity, mass, can be measured directly from the interferometer data. However, to known the common features of how molecular gas are losing from galaxies,
we need more samples. We includes two more samples from nearby edge-on starburst galaxies as our case studies,
NGC 2146 and NGC 3628, which have similar characteristics with M82.
We observed the CO(1-0) line on these two galaxies by using the Nobeyama Millimeter Array (NMA). Our results successfully detected the molecular outflows and superbubbles. Molecular outflows and superbubbles can be clearly seen above or below the galactic plane with size of sub-kpc to kpc scale. The expanding velocity is 35-200 km s^{-1}, their mass is 10^{7-8} M_{\sun}, and the kinetic energy is 10^{53-55} erg. The molecular outflow rate in NGC 2146 is 17-34 M_{\sun} yr^{-1}, that in NGC 3628 is 4-7 M_{\sun} yr^{-1}.
In order to know whether and how galaxies losing their molecular gas through outflows and superbubbles, we include the Chandra X-ray Observatory (CXO) soft X-ray archive data to see how plasma gas affect the molecular outflows. The distribution between soft X-ray data and molecular outflows or superbubbles have good spatial correlation. We can see the strong soft X-ray emission either locates within the molecular outflows or superbubble, or distributes in the same area with molecular outflows or superbubbles. Besides, the thermal pressure of plasma gas are 1-2 order larger than that of molecular outflows. This indicates the molecular outflows are pushing by plasma gas due to the thermal expansion.
Since we have accurate measurement on losing molecular gas, we can precisely study the effect of molecular gas consumption on galaxies evolution. For this purpose, we include our NMA 3 mm continuum data, which is a good tracer of massive star forming region at radio waveband. The results show that the SFR in starburst region in NGC 2146 is 16 M_{\sun} yr^{-1}, and that in NGC 3628 is 2 M_{\sun} yr{-1}. This indicates that galaxies loss their molecular gas through outflows or superbubbles faster than galaxies forming stars. The molecular gas mass loss rate is about 1-3 times larger than the SFR in starburst region. This provides a direct evidence that molecular outflows or superbubbles can quench star formation.
After comparing several parameters derived from molecular outflows among our galaxies and M82, we found that they are in three different evolutionary stages of starburst activities. NGC 3628 is the youngest one, NGC 2146 is the middle-age, and M82 is the oldest one. In NGC 3628, the starburst activity is enhanced, the timescale that molecular outflow returns to the galactic disk is one order of magnitude shorter than the the molecular gas consumption timescale, suggesting that the returned gas will fuel the coming star formation. In NGC 2146, the starburst activity is also enhanced, but the molecular outflow gas fallback timescale is a little bit smaller than the gas consumption timescale, suggesting the gas could extend the starburst timescale, but not too much. In M82, the starburst activity is decreasing, the molecular outflow gas fallback timescale is about one order of magnitude longer than the molecular gas consumption timescale, suggesting the gas do not help to extend the starburst timescale. The three cases indicate the molecular outflow extends starburst timescale in the early and middle stage, but quenches the star formation in the late stage of starburst activity.
Keywords: Galaxy, Molecular gas, Starburst, Outflow, Superbubble, Evolution, Star formation, Astronomy
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