The structure, kinematics and evolution of the magellanic clouds

Abstract

The Large and Small Magellanic Clouds (LMC & SMC), along with the Magellanic Bridge and the Stream, comprise the Magellanic system. The presence of the Stream and the Bridge connecting the two Clouds suggest that these two galaxies might be inter- acting as a pair for a couple of Gyr. The Bridge in particular indicates that they have had a close encounter in the recent past. This system moves in the gravitational potential of the Milky Way (MW). The L&SMC form the nearest interacting binary to our Galaxy and the proximity allows us to understand the details of such interactions. Such interacting systems are numerous in the universe and this pair serves as the test bed to understand such systems. It is expected that the structure, kinematics, and evolution of the Clouds and the Galaxy are modified by their interactions. The details of these interactions have been studied over a time scale. It was long believed that the Clouds orbit our Galaxy and that the bursts of star formation episodes as well as the structural changes seen in both the Clouds are probably due to their perigalactic passage and tidal effects. On the other hand, the recent estimates of the proper motion of the Clouds find that the Magellanic System is probably passing close to the MW for the first time (Besla et al. 2007). Hence the inter-action of the Clouds with the Galaxy is restricted to a recent past of a few hundred Myr. As we have a reliable estimate of the proper motion of the Clouds, we understand their orbits, velocity and the direction of motion. In the light of these new estimates, we study the effect of the interaction on the Clouds in the recent past. We study the recent star formation history, in the last 500 Myr, in the two Clouds which could reveal the effect due to their interaction with the Galaxy as well as their mutual interaction, at similar time scales. We also study the Atomic Neutral Hydrogen (H I) gas kinematics, which could identify signatures of the ongoing interaction between the three galaxies. The Magellanic System is introduced with a history of the Clouds and related features, and the back ground and motivation of this thesis study are outlined, in Chapter 1.

We made use of optical photometric data for estimating the recent star formation his- tory. We used the photometric catalogues from the third phase of the Optical Gravitational Lensing Experiment (OGLE III) and the Magellanic Cloud Photometric Survey (MCPS) for this purpose. To analyse the gas kinematics we used the radio data from three differ- ent surveys - the combined data set of Australian Telescope Compact Array (ATCA) and Parkes telescopes, the all sky survey data sets from the Parkes Galactic All Sky Survey (GASS) and the Leiden Argentine Bonn Survey (LAB). The details of the data sets, var- ious corrections applied to the data, including the correction for the transverse motion of the Clouds, as well as the methods adopted for the analysis are discussed in Chapter 2.

The star formation episodes of the Clouds are influenced by the LMC-SMC-MW in- teraction. The estimation of star formation history is proven to be a useful tool to decode the evolutionary history of the Clouds. We traced the age of the last star formation event (LSFE) in the inner L&SMC using the photometric data in V and I passbands from the OGLE III and the MCPS catalogues. The LSFE is estimated from the main sequence turn-off point in the colour magnitude diagram (CMD) of a sub region. After correcting for extinction, the turn-off magnitude is converted to age, which represents the LSFE in a region. The spatial distribution of the age of the LSFE shows that the star formation has shrunk to within the central regions in the last 100 Myr in both the galaxies. The location as well as the age of LSFE is found to correlate well with those of the star clusters in both the Clouds. The SMC map shows two separate concentrations of young star formation, one near the center and the other near the wing. We detected peaks of star formation at 0 - 10 Myr and 90 - 100 Myr in the LMC, and 0 - 10 Myr and 50 - 60 Myr in the SMC. The quenching of star formation in the LMC is found to be asymmetric with respect to the optical center such that most of the young star forming regions are located to the north and east. On deprojecting the data onto the LMC plane, the recent star formation appears to be stretched in the northeast direction and the H I gas is found to be distributed pref- erentially in the north. We found that the centroid is shifted to the north during the time interval 200 - 40 Myr, whereas it is found to have shifted to the northeast in the last 40 Myr. In the SMC, we detected a shift in the centroid of the population younger than 500 Myr and as young as 40 Myr in the direction of the LMC. The analysis of the recent star formation history of the L&SMC are presented in Chapters 3 and 4 respectively. Kinematics of a galaxy is basically the study of its velocity field using either gaseous or stellar tracers. The observed radial/line of sight velocity field can be resolved to com- ponents of internal rotation, transverse motion, systemic motion, precession and nutation. We analysed the gas kinematics of the L&SMC using H I as tracer. The value of proper motion do play a crucial role in the kinematic analysis of a galaxy. The recent proper motion estimates of Piatek et al. 2008, Kallivayalil et al. 2006b, Kallivayalil et al. 2006a and Kallivayalil et al. 2013 are very different from the older values and the estimation of the H I velocity field is affected by this change, especially in the outer disk. Hence it is necessary to compute the H I velocity field of the Clouds using the new proper motion values. This study is a re-estimate of H I kinematics of the Clouds using the recent proper motion estimates. It has been recently realised that including the component of preces- sion and nutation of the disk can change the estimated kinematics. Various features in the gas, which are connected to the Magellanic Bridge, the Leading Arm & the Magellanic Stream have been identified in the literature, but their formation and the details of the gas flow in these features are not clearly understood. The H I kinematics of the LMC disk is analysed considering the two recent and accurate proper motion estimates (Piatek et al. 2008 and Kallivayalil et al. 2013). The value of Position Angle (PA) of Line of Nodes (LON) of the LMC, estimated using ATCA/Parkes data (126◦±23◦

) is found to be similar to the recent estimate of the PA using stellar tracers. The effect of precession and nutation in the estimation of PA is found to be significant. The mean H I disk is found to be disturbed within 1.o0 radius and beyond 2.o9 radius. Most of the H I gas in the LMC (∼ 87.9% of the data points) is located in the disk. We detected 12.1% of the data points as kinematic outliers. Significant part of the type 1 as well as the slow type 2 H I gas are found to be similar to Arm E. We identified features similar to the well known Arm S, Arm W, Arm B and a new stream, Outer Arm, as part of the faster type 2 outlier component. The GASS data analysis brings out the details of the

Magellanic Bridge and its connection to the LMC disk. We found that the arm B is con- nected to the Magellanic Bridge and the velocities suggest a possible gas infall through

arm B, if the gas is in the plane of the LMC. We detected possible outflows from the western disk of the LMC and the southwest and southern parts of the Magellanic Bridge, likely to be due to ram pressure stripping. The analysis and results of the H I kinematics of the LMC is presented in Chapter 5. In the case of the SMC, the estimation of the nutation (di/dt) of the SMC H I disk is done for the first time, using the mean H I velocity field. The SMC is found to undergo fast tumbling motion with a di/dt value −416.39 ± 19◦

/Gyr. The H I kinematics of the SMC is analysed in detail for the first time covering almost the entire gaseous disk of the SMC, applying the recent high precision proper motion estimates (Piatek et al. 2008 and Kallivayalil et al. 2013). The rotation curve of the SMC is estimated to a larger radial extend using GASS data and the flat part of the rotation curve is detected for the first time. The turn over radius is found to be 4.1 kpc and the deprojected rotational velocity is 64 km s−1

. The dynamical mass estimated within the outermost data point at 5.7 kpc

is 5.4 × 109 M

, which is more than twice of the previous estimate (Stanimirovic et al. ́ 2004). This could be an indicator of the presence of a dark matter halo in the SMC. The PA of LON is estimated to be 67◦ .2 ± 9 ◦ .2, which is different from the stellar PA, located in the southeast quadrant, as suggested by previous studies (Stanimirovic et al. ́ 2004). We identified a few tidal features in the SMC disk as kinematical outliers. The gaseous feature which connects the SMC to the Magellanic Bridge is identified as a slow type 2 outlier. The inner SMC seems to be unperturbed, while the outer regions are strongly disturbed possibly by the tidal interaction with the LMC. The analysis and results are presented in Chapter 6. In this thesis study we find that the star formation in the last 100 Myr is propagating radially inward in both the Clouds. The recent star formation in the LMC is dictated by the perigalactic passage. The lopsidedness in the gas distribution, and the northeast extension of the young stellar as well as cluster populations are the combined effect of the gravitational interaction of the MW and the movement of the LMC in the MW halo. The recent star formation in the SMC is due to the combined gravitational effect of the LMC and the perigalactic passage. Our study finds the PA of LON of the gaseous disk of the LMC is not mis-aligned with that of the stellar disk. In the case of the SMC, the PA estimate of the gaseous disk is flipped to another quadrant compared to the stellar disk estimate. The precession and nutation motion of the disk of the Clouds are found to be significant in understanding the kinematics. The disk of the SMC tumbles in a significantly high rate, which is probably due to its interaction with the LMC.

The valuable signatures of on going interactions of the LMC-SMC-MW system is de- tected and analysed in both the aspects of star formation history as well as gas kinematics.

The star formation episodes and the kinematics of the LMC disk are strongly influenced by its hydrodynamic and tidal interaction with the MW. There are possible on going gas accretions and outflows in the LMC which are likely to be connected to the Magellanic Bridge, the Leading Arm and the Magellanic Stream. The star formation in the SMC is dictated by a combined gravitational effect of the MW and the LMC, while the gas distribution bears the signature of the LMC-SMC tidal interactions. These results and conclusions are summarised in Chapter 7. The Clouds of Magellan are unquestionably the test beds to understand the nuances of galaxy interactions at our door step.