1. Our Changing View of the Galaxy NGC 2915 Ed Elson Department of Astronomy, UCT Supervised by: Prof. R. C. Kraan-Korteweg Prof W. J. G. de Blok 3 rd Annual SKA Postgraduate Bursar Conference, December 2008

2. Introducing NGC 2915 Classified as a blue compact dwarf galaxy. Classified as a blue compact dwarf galaxy. Distance ~ 3.78 Mpc (TRGB, Karachentsev et al, 2004) Distance ~ 3.78 Mpc (TRGB, Karachentsev et al, 2004) 2 distinct stellar pops: Compact blue core, more diffuse red outer disk. 2 distinct stellar pops: Compact blue core, more diffuse red outer disk. Huge HI spiral disk ~22 h B (Meurer et al, 1996). Huge HI spiral disk ~22 h B (Meurer et al, 1996). Very DM dominated, M tot /L B >65 (Meurer et al, 1996). Very DM dominated, M tot /L B >65 (Meurer et al, 1996).

3. V band (optical) image ~ 1.1 kpc

4. V band image with V band contours overlaid

5. 3.6 micron (IR) image with V band contours overlaid

6. 3.6 micron (IR) image with 3.6 micron contours overlaid ~ 2.2 kpc

7. HI image with IR contours overlaid ~20 kpc

8. HI Data NGC 2915 forms part of the THINGS south survey (carried out on ATCA). NGC 2915 forms part of the THINGS south survey (carried out on ATCA). THINGS aims to map the HI distribution of a representative sample of nearby THINGS aims to map the HI distribution of a representative sample of nearbygalaxies. Spatial res~0.3 kpc Spatial res~0.3 kpc at D=3.78 Mpc, vel res~3.2 km/s. Independent IR (SINGS) Independent IR (SINGS) and UV (GALEX) data

10. Data products Integrated HI column density map Velocity field

11. Previous results Fit a tilted ring model to the velocity field of the galaxy. Fit a tilted ring model to the velocity field of the galaxy. V los =V sys +V R sin(i) cos(θ) V los =V sys +V R sin(i) cos(θ) –V los =line of sight vel –V sys =systemic vel –V R =circular rotation speed of HI –i=ring inclination –Θ=orientation of kinematic major axis Tilted ring model of NGC 5055. Battaglia et al., 2006

12. Tilted ring model parameters

13. Evidence for an anomalous component Inner HI overdensities enveloping young, hot stars Inner HI overdensities enveloping young, hot stars Spit line profiles Spit line profiles Wiggles in iso-velocity contours Wiggles in iso-velocity contours Sudden changes in incl and systemic vel in tilted ring model Sudden changes in incl and systemic vel in tilted ring model Velocity spikes in position-velocity slices Velocity spikes in position-velocity slices Gas expelled outwards, i.e. expanding gas annulus seen ~ edge on Gas expelled outwards, i.e. expanding gas annulus seen ~ edge on Departures from circular motion Departures from circular motion Kinematically decoupled inner component Kinematically decoupled inner component Fast rotating anomalous component Fast rotating anomalous component

14. Modelling of anomalous component Model anomalous as a fast rotating, expanding, highly inclined HI annulus. Model anomalous as a fast rotating, expanding, highly inclined HI annulus. Radius~1.6 kpc, Radius~1.6 kpc, ~1 kpc thick Inclination=80° Inclination=80° V exp =40 km/s V exp =40 km/s V rot =60 km/s V rot =60 km/s

15. Model 1: underlying disk

16. Model 2: Underlying disk + rotating ring

17. Model 3: Underlying disk + rotating, expanding ring

18. Model 4: underlying disk + un-inclined rotating, expanding ring

19. More data-model comparisons Position-velocity slice at 290°

20. More data-model comparisons Position-velocity slice at 211°

21. More data-model comparisons Position-velocity slice at 205° through HI overdensity

22. More data-model comparisons Position-velocity slice at 151°

23. More data-model comparisons Position-velocity slice at 40° through HI overdensity

24. Conclusions and future work HI data exhibit evidence of an anomalous gas component at inner radii. HI data exhibit evidence of an anomalous gas component at inner radii. A tilted, rotating, expanding inner ring model is consistent with these data. A tilted, rotating, expanding inner ring model is consistent with these data. Origin of anomalous component? Gas infall? Interaction? Origin of anomalous component? Gas infall? Interaction? Analyse stellar populations to derive star formation histories. Analyse stellar populations to derive star formation histories. Build up a well-rounded picture of NGC 2915. Build up a well-rounded picture of NGC 2915.