1. The extremely high gas content of galaxy UGC8802 Chang Rui-xiang Hou Jin-liang Shen Shi-yin

2. Understanding the content and distribution of cold gas in galaxies is an important step to understand the formation and evolution of galaxies. The GALEX Arecibo SDSS Survey (GASS): to gather high quality HI-line spectra using the Arecibo radio telescope for ~1000 galaxies selected from SDSS and GALEX: M * >10 10 M ⊙ 0.025 < z < 0.05

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

4. UGC8802 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. Milky WayM31UGC8802 Stellar mass (10 10 M ⊙ ) 5.05.9~8.72 Disk scale-length (kpc) 2.34.85.8 HI mass (10 10 M ⊙ ) 0.40.52.1 H 2 mass (10 10 M ⊙ ) 0.110.02~0.040.14 Gas fraction 0.090.080.5 Total SFR (M ⊙ yr -1 ) 1~50.4~1.03.7 Abundance gradient (dex kpc -1 ) -0.04~-0.07-0.18~-0.027-0.04

6. The origin and the fate of the gas in UGC8802: 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. Our motivations: to construct a simple model and test if the continuous gas-infall model can be viable for UGC8802 to further understand the origin and evolution of cold gas in UGC8802

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: modified K-S star formation law: gas outflow rate: free parameters:

9. Chang et al.(2010,ApJ,722,380) showed that later infall- peak time or lower SFE predicts bluer color, higher gas fraction, lower metallicity and higher SFR density. The predicted SFR surface density are more sensitive to the SFE. Our best-fitting model: It is adopted that the infall-peak time increases while the SFE decreases from the inner disk to the outer parts

11. The predicted evolution of gas surface density at three different Regions, 1r d (dot line), 3r d (solid line), and 5r d (dash line). 1r d 3r d 5r d

12. The model adopts late infall-peak time and low SFE in the outer disk results in high gas surface density in outer disk, which can explain the observed high gas fraction of UGC8802. Our results show that the continuous gas-infall model is also viable for UGC8802. This suggests that the cold gas in UGC8802 may be originated from continue gas-infall from the dark matter halo.

13. next steps: H 2 mass: new star formation law