1. A simple model to explain the high gas content of galaxy UGC 8802 Ruixiang Chang Shanghai Astronomical Observatory Collaborators: Jinliang Hou Shiyin Shen

2. Understanding the content and distribution of cold gas in galaxies is an important step to understand the formation and evolution of galaxies. The disk galaxy UGC 8802 is an interesting target.

3. Moran et al.(2010, ApJ, 720,1126)

4. UGC 8802 is an interesting target: z=0.0411 M * =2*10 10 M HI mass: 2.1*10 10 M CO observations: H 2 mass: 1.45*10 9 M Long-slit spectroscopy: SFR D4000 Metallicity....... Moran et al.(2010, ApJ, 720,1126)

5. The origin and the fate of the gas in UGC 8802: Scenarios that the gas was acquired in a recent merging event are disfavored because of the regular kinematics of the disk. Further investigations are required to answer this question.

6. Our motivations: to construct a simple model and test if the continuous gas-infall model can be viable for UGC 8802 to further understand the origin and evolution of cold gas in UGC 8802

7. Main assumptions of our model: The disk is sheet-like and composed by a set of independent rings. The disk origins and grows by continuous gas infall. The radial profile of stellar mass surface density is exponential in the present day.

8. Main ingredients of the model: gas infall rate: A(r) is normalized by the mass distribution along the disk in the present-day. star formation law: a) Kennicutt star formation law b) star formation law taken from Leroy et al. (2008): free parameter: t p (r) Σ SFR =Σ mol /t SF Σ mol /Σ atom =(P h /P h,0 ) γ

9. t p (r)/Gyr=1.5r/r d +4.0 Solid lines: SF law from Leroy et al (2008) Dash lines: K-S SF law, if t p Σ SFR [O/H]

11. At large radius the availability of HI may be a bottleneck for SF. Even if the stars form directly from H2, molecular clouds must be assemble from HI. In the inner parts of galaxies many physical conditions important to the HI- H2 conversion change while HI density remains approximately fixed, but in the outer parts HI density varies while other environmental conditions show comparatively little variation. As a result, HI density turns to be an important driver for SF in outer parts of galaxies.

12. Summary: The model adopts late infall-peak time in the outer disk results in high gas surface density in outer disk, which can explain the observed high gas fraction of UGC 8802. Our results show that the continuous gas-infall model is also viable for UGC 8802. This suggests that the cold gas in UGC 8802 may be originated from continue gas- infall from the dark matter halo. The predicted gas fraction is very sensitive to the adopted SF law.