1. Merger time scale of galaxies Yipeng Jing Shanghai Astronomical Observatory Collaborators: Chunyan Jiang, A. Faltenbacher, W.P. Lin, C. Li (astroph/0707.2628; ApJ)

2. Problems related to merging of galaxies Observations: –One knows that there are many mergers –The count of close pairs is used to measure the merger rate, but the merger time scale has to be assumed –but one does not know quantitatively how many various mergers (major, minor, gaseous, dry etc) have happened or are happening

3. Double nuclei in star forming galaxies

4. Theories: –The Chandrasekhar formula is widely used, with the parameters based on N-body test of Navarro et al. (1995) –But high-resolution N-body simulations seem indicate that too many mergers are predicted with the above prescription (Kang et al. 2005; Springel et al. 2001)

7. Theories: –The Chandrasekhar formula is widely used, with the parameters based on N-body test of Navarro et al. (1995) –But high-resolution N-body simulations seem indicate that too many mergers are predicted with the above prescription (Kang et al. 2005; Springel et al. 2001)

8. Motivations of the current work Mergers are critical for forming starbursts, elliptical Gs, and maybe AGNs; We are working on a method to determine mergers of various galaxies in observations (and theory as well); But it is important to know the merger timescale

10. Using ln(m_h/m_sub) for Coulomb logarithm (c.f. Navarro et al. (1995) Predicted friction timescale Measured merger timescale repeated Navarro et al. 1995 But our conclusion is different from theirs

11. Measured to predicted Mass ratio of satellite to host

12. Using 1/2ln[1+(m_h/m_sub)**2] for Coulomb logarithm

13. Measured to predicted Mass ratio of satellite to host

14. Using ln(1+m_h/m_sub) for the Coulomb logarithm

15. Measured to predicted Mass ratio of satellite to host

16. Measured to predicted Circularity parameter

17. f( ε ) f(ε)=1.48* ε**0.27 Using ln(1+m_h/m_sub)

18. Fitting formula Merger timescale for all mergers

20. Distribution of the circularity

21. No dependence on the mass ratio

22. Conclusions The Chandrasekhar formula, as is usually used in SAM, under(over)-estimates merger time for minor(major) mergers ; If the Coulomb logarithm is replaced ln(1+lambda), the mass dependence is accounted for correctly; but still underestimates The dependence on the circularity is much weaker than the 0.78 power; We have presented a fitting formula and circularity distribution which are sufficient for modeling the merger rate; the scatters are 40%