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Merger time scale of galaxies
Yipeng Jing
Shanghai Astronomical Observatory
Collaborators:
Chunyan Jiang, A. Faltenbacher, W.P.
Lin, C. Li (astroph/0707.2628; ApJ)
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
Double nuclei in star forming galaxies
• 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)
• 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)
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
Using ln(m_h/m_sub) for Coulomb logarithm (c.f.
Navarro et al. (1995)
Predicted friction timescale
repeated
Navarro et
al. 1995
But our
conclusion
is different
from theirs
Measured merger timescale
Measured to predicted
Mass ratio of satellite to host
Using 1/2ln[1+(m_h/m_sub)**2] for Coulomb logarithm
Measured to predicted
Mass ratio of satellite to host
Using ln(1+m_h/m_sub) for the Coulomb logarithm
Measured to predicted
Mass ratio of satellite to host
Measured to predicted
Circularity parameter
f(ε)
f(ε)=1.48* ε**0.27 Using ln(1+m_h/m_sub)
Circularity parameter
Fitting formula
Merger timescale for all mergers
Distribution of the circularity
No dependence on the mass ratio
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%