A simple model to explain the high gas content of galaxy UGC 8802

Ruixiang Chang

（ Shanghai Astronomical Observatory ）

Collaborators: Jinliang Hou

Shiyin Shen

• 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.

UGC 8802 is an interesting target:

• z=0.0411• M*=2*1010M ⊙

• HI mass: 2.1*1010M⊙

CO observations: • H2 mass: 1.45*109M⊙

Long-slit spectroscopy: • SFR• D4000• Metallicity• .......

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

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.

Our motivations:• to construct a simple model and test if the continuous ga

s-infall model can be viable for UGC 8802• to further understand the origin and evolution of cold gas

in UGC 8802

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 expon

ential in the present day.

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):

22 2/)()(),( pttin erAtrf

free parameter: tp(r)

ΣSFR=Σmol/tSF

Σmol/Σatom=(Ph/Ph,0)γ

tp(r)/Gyr=1.5r/rd+4.0Solid lines: SF law from Leroy et al (2008)Dash lines: K-S SF law,

if tp

ΣSFR

[O/H]

• 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 drive

r for SF in outer parts of galaxies.

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.