Transcript M81/M82/NGC3077

M81/M82 Interacting Galaxies
(M81/M82交互作用星系之研究)
BATC學術研討會(山東威海)
Wei-Hsin Sun (孫維新)
Institute of Astronomy, NCU, Taiwan
(台灣中央大學天文研究所)
M81/M82/NGC3077 – A Famous
Interacting Galaxy Group
BATC Field of View
Full-size BATC image of M81/M82 region
Images taken in 16 filters –
the existence of continuum emission
Continuum emission in 4 filters –
north-west side of M81
Arp Loop Region —
BATC observations suggest:
Continuum emission exist all over the
intergalactic region.
These emission overlap with the HI
observation very well.
These continuum emission are from
stars, since the emission appear in
every continuum filters.
Questions:
How did the stars get there?
— Were the stars stripped off from
galaxies by the tidal interaction?
— Did they form from the gas stripped
off the galactic disks?
Any evidence for the “Tidal Interaction”?
— Dynamical History of this group
Possibilities:
If the stars were stripped off the disks,
then the color of the stars should be
similar to the edge of the galaxies.
If the stars have formed in situ in the
stripped off gas, then the color of the
stars will be determined by the IMF of
the newly formed stars, not necessarily
correlated with the disks.
SEDs from 7 locations
Preliminary suggetsions:
From the SEDs taken at 7 locations in
this area, we noticed that the SEDs of
the edge of M82 is similar to the SEDs of
continuum emission on both sides of M81.
This suggests that part or all of the
galactic disk of M82 was seriously
disrupted during the previous encounter.
Previous works related to this issue:
Sofue (1992) observed a peculiar
rotational curve of M82 which suggests
that M82 might be a “bare” bulge and
the disk is missing!
This strengthens the possibility that
most, if not all, of the continuum
emission may have been the relics of
M82 disk.
Was there really a close encounter?
— A Dynamical History Analysis
The distances of M81 and M82:
M81 — 3.63 ± 0.34 Mpc
(Freedman 1994, observing Cepheids in M81
using HST)
M82 — 3.9 ± 0.3 Mpc
(Sakai and Madore 1999, measuring the tip
of RGB stars using HST)
Was there really a close encounter?
— A Dynamical History Analysis
The heliocentric radial velocities and
distances to M81 and M82:
M81 — -35 km/sec (3.63 ± 0.34 Mpc)
(CBR value: 48 km/sec)
M82 — 202 km/sec (3.9 ± 0.3 Mpc)
(CBR value: 296 km/sec)
M81 is closer and approaching, while M82
is farther away and receding;
Implications from the distances and
velocities:
The two galaxies have indeed passed
each other not long ago.
The line-of-sight separation is 0.27 Mpc.
Calculated using the velocity difference
gives a time scale of approximately 1x109
years, consistent with the enhanced star
formation epoch.
Conclusions:
M81/M82 have passed each other several
hundred million years ago.
The tidal interaction between these two
galaxies disrupted M82 disk seriously, since
M82 is smaller and hence more vulnerable.
The stripped-off stars are scattered in the
intergalactic region, coinciding well with the
HI distribution.