1. Counterrotating core in the LMC: Accretion and/or Merger ? Annapurni Subramaniam Indian Institute of Astrophysics, Bangalore, INDIA (Evidence of a counterrotating core in the LMC: Annapurni Subramaniam & Tushar P Prabhu ApJ Letters, 625, L47 -- 20 May 2005 Issue)

2. Introduction:  The Large Magellanic Cloud (LMC) has been studied kinematically and photometrically very well.  Though the radial velocities were found to be quite chaotic at certain locations, a reasonably well fit to a rotating disk model could be obtained (stars and gas).  The rotation curve of the LMC out to a distance of 8 kpc using carbon stars was estimated by van der Marel et al. (2002). They also estimated the kinematical line of nodes (LON) of the LMC.  The rotation curve had one peculiarity, the presence of negative velocity near the center. This result was not given any importance due to poor statistics.  Two kinematically different components were found among the CH stars (Hartwick & Cowley 1988) and carbon stars (Graff et al. 2000).

3. Graff et al. (2000) van der Marel et al. (2002)

4.  HI velocity studies --- two kinematic components, (McGee & Milton 1966, Rohlfs et al. 1986 and Luks & Rohlfs 1992).  Double peaked HI velocities --- due to HI clouds located in two layers.  The radial velocity curve of HI --- reversal of slope near the center (Rohlfs et al. 1986).  All the above point to the possibility of a kinematically distinct component in the inner LMC. Luks & Rohlfs (1992) Rohlfs et al. (1986)

5.  Zhao et al. (2003) estimated and studied the radial velocities of 1347 stars in an attempt to detect the kinematically distinct component in the inner LMC. They assigned a probability of less than 1% for its presence. Here we use a different technique to analyse the above data and search for evidence of a second kinematic component. Zhao et al. (2003)

6. How does one search for a kinematically different component?  Previously used methods concentrated on the profile and the width of the line-of-sight velocity distribution --- might point to the existence of a second component, but does not reveal the details.  The kinematics of far away galaxies are studied using the radial velocity profile (position-velocity (P-V) diagram) along its major and minor axes.  In a barred galaxy, the linear part of the curve near the center would correspond to the bar. Towards the end of the bar, the disk starts dominating, which is indicated by a flattening of the curve.  If a kinematically different component exists in the inner region, (within 2 kpc), one would expect to see a deviation from a straight line profile.  Thus the radial velocity profiles along various position angles (PA) are used to search for the kinematically different component.  So far, no model has been proposed to explain the different kinematics shown by the gas and stars.  We propose a two-disk model for the inner LMC.

7. LMC- optical LMC- IR LMC-UV

8. The data  We used the following stellar velocity data:  Velocity of 1347 stars (Zhao et al. 2003). This the result of three observing campaigns between 1999 – 2002, using the 2dF instrument at the AAT. The bar region is more or less covered by these data and it is homogeneous. The stars belong to a heterogeneous population.  Stellar velocities of red super giants ( 158 stars, Massey & Olsen 2003)  Carbon stars (501 stars, Kunkel et al. 1997)  Red giants (371 stars, Cole et al. 2005)  H I data : Luks & Rohlfs 1992  Molecular cloud data: Mizuno et al. (2001)

9. Evidence of counterrotation  Initially we used the LON as estimated by van der Marel (2002) using carbon stars (~130°).  Stars located along this PA are selected. Their radial velocity as a function of radial distance is plotted. This was suggestive of a V-shaped profile, instead of a straight line profile.  The average velocity in radial bins were obtained. The profile was found to show the V-shaped deviation very clearly. The rotation curve as estimated from carbon stars is also plotted. We confirm their suggestion of counterrotation. We obtained the P-V diagrams for PAs from 0° - 180°, to find the LON of the counterrotation. It was found to be maximum at 120° - 130°.  This LON is same as that of the main stellar disk of the LMC. Thus we find the evidence of counterrotation in the central region of the main stellar disc.  HI velocity were also analysed in the same way. HI also shows evidence of counterrotation in the inner LMC.

11. Kinematic model for the inner LMC  The analysis using the P-V diagrams also showed that the stars located immediately outside the counterrotating region were found not to have the same LON. This is indicated in the velocity profile along the minor axis (PA= 40°). The profile has a gradient when no gradient is expected.  Such a gradient can be obtained if the LON of those stars are between 160° - 170°. This value of LON is similar to that of HI gas, indicating that some stars have the same LON as that of the gas.  The above facts point to the existence of a disk which is mostly of HI and some stars, with LON different from the main disk of the LMC.  Thus we propose a two-disk model for the inner LMC, the two disks being: the main stellar disk (but counterrotating) and the gas dominated disk.

12. The Model  Disk 1 (D1): The main stellar disk, with counterrotation, LON=130°, inclination=35°, systemic velocity = 260 km/s, has a linear rise up to 3°.  Disk 2 (D2): Dominated by HI, LON= 170°, inclination =35°, systemic velocity =260 km/s. It has a linear profile near the center and flattens beyond that. It extends only up to 3° (Most of the HI is found to be located within the central 3° of the LMC).  Both the disks contribute to the radial velocity profile. The observed velocities could be the same as that of the individual disks or an average of the two components, depending upon their distribution.

13. Comparison with the observation It can be seen that: In the inner region, stars from Zhao et al. (2003) mostly follow D1. The carbon stars and the red super giants are mostly in the D2, the red giants follow the average. Therefore even inside the counterrotating region, two-disk model is needed to fit the observed velocities. The stellar data also indicates that in the region just outside the c-core, the stars are disturbed. For HI, both the components are plotted. It is remarkable to see that the model naturally explains the two velocities observed in the same line of sight. It can be seen that most of the HI are located between D2 and the average. The fit the observed data estimates the parameters for the disks: D1 has a slope of 30 km/s/degree up to 3°. D2 has a slope of 60 km/s/deg up to 0.7° and a constant velocity of 42 km/s/deg up to 3°. An error of 15% is expected. We also find that the main disk and D2 have clockwise rotation and the core has counter clockwise rotation.

14. Model Predictions  The two-disk model assumes the presence of two disks with the same inclination and different LONs. This could result in some locations which are in the same line of sight, but physically and kinematically different. We look for the evidence.  The disks can be considered to be physically separated if the separation between them is more than 360 pc and kinematically separated if the velocity difference is more than 20 km/s.  One of the observed features in the LMC is the presence of HI in two layers.The locations of double-peaked HI clouds are given by Rohlfs et al. (1984).  Majority of the double peaked clouds are located within the predicted location.

15.  The microlensing events observed towards the LMC could be due to the self lensing within the LMC. This requires a population differing in their radial velocity, proper motion etc. to be present in the LMC.  In the two-disk model, stars present in the both the disks differ in radial velocity, projected distance and distance modulus and probably reddening also.The stars located in different disks can increase the star-star microlensing in the line of sight thereby increasing the probability of self- lensing within the inner LMC.  The locations of observed ML events are found to be located were the two disks are separated. Thus the two disks in the inner LMC fit most of the criteria required for self-lensing within the LMC.

16.  The observed and the predicted velocity in the LMC is well matched for HI, where the velocity model was derived from the stars. Figure 3, Kim et al. (1998) Model

17. Possible origin of counterrotation  Kinematically peculiar cores are generally thought as fingerprints of the merger history of the host galaxy (Mehlert et al. 1998).  In disk galaxies, it could also arise due to the accretion of gas (Bertola et al. 1992).  Thus the counterrotation could be due to accretion or merger in the early history of the LMC.  The secondary bar (Subramaniam 2004) and the counterrotating region are found to have similar location and PA. Thus the identified counterrotation could be associated with the secondary bar.  The proposed formation scheme is again the accretion of a satellite in retrograde orbit (Friedli 1996), Or due to the instabilities in the primary bar (Friedli & Martinet 1993).  Therefore the origin of the counterrotation could be internal (bar perturbation) or external (merger).

18. Possible origin of the gas disk  Possibility of external origin of the gas explored :  Brüns et al. (2005) presented a survey of the Magellanic system. The LMC and the SMC were found to be associated with large gaseous features.  The HI gas connect the two not only in position but also in velocity. Bruns et al. suggest that some of the gas in the vicinity could be accreted by the Clouds.  The gas in the Bridge have low velocities in the LMC standard of rest frame making the accretion of some of the gas by the LMC very likely.  This infalling gas could form a new disk and initiate star formation.

19. Brüns et al. (2005)

20. Next step  To find whether the counterrotation is due to internal or external effects; kinematics of different population, age of the bar, tidal tails  More stellar velocity data are required to refine the model.  The proper motion estimates of the LMC have been found to give different answers based on similar techniques, but different location. Can the counterrotation explain this? What is the true proper motion of the LMC?  How does one connect gas accretion and the recent star formation in the LMC?