Gerhardt R.\ Meurer, PASA, 14 (1), 77.
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HI properties
In Fig. 1 the optical (I band) and HI morphologies of NGC 2915 are compared. The HI is very extended relative to the optical and has a barred spiral morphology. The optical counterpart corresponds well in size and position angle to the bar (and does not extend beyond it to our detection limits). The velocity field is dominated by the rotating disk, but also shows the oval distortion due to the bar: the kinematic major and minor axes are not aligned. The disk is quiescent beyond the optical extent, having a velocity dispersion of 
km 
, which is common for spiral galaxies. Near the center of the galaxy the velocity dispersion reaches 
km , and the HI line is often split. The broad and split line profiles are associated with massive star forming regions and expanding bubbles that are prominent in the H
image of Marlowe et al. (1995). Thus star formation is energizing the neutral interstellar medium in the center of NGC 2915.
Figure: NGC 2915 in the I band (left) from an 800s integration using the Anglo-Australian Telescope, and in total HI emission (right) from our ATCA observations. North is up, east is to the left, and the scalebars are 5' = 7.7 kpc long.
The rotation velocity 
curve of NGC 2915 is shown in Fig. 2. The raw measurements were corrected for pressure support (asymmetric drift) to obtain circular velocities 
, which are also shown. The optical portion of NGC 2915 is wholly contained in the rising portion of the rotation curve. This is where rotational shear is minimized, hence where molecular clouds have the longest lifetime and star formation should be most efficient. The rotation curve levels off shortly beyond the Holmberg radius 
, remains approximately flat out to about 10 kpc, and then rises again out to the last measured point (R = 15.2 kpc).
Figure 2: Rotation curve of NGC 2915 compared to two mass models. Raw rotation velocities are shown with open symbols, while pressure support corrected circular velocities are shown as closed symbols. The Holmberg radius is indicated with the vertical arrows.
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