PhD Thesis (PDF) - Department of Astronomy - University of Virginia

Pram ≈ 1.6 × 10 −11 dyne cm −2 . The ram pressure would be lower if NGC 1603 were
at a larger radius than its projected radius (i.e., in front of or behind the center of
the NGC 1600 group). Roughly speaking, the condition for ram pressure to strip the
gas is that Pram > Pgas, which appears plausible in the outer 4 ′′ –6 ′′ of NGC 1603.
Since the tail points to the west towards the higher density gas near NGC 1600, the
ambient gas density may have been higher in the past, making ram pressure stripping
easier than it is at its present position.
The NGC 1603 tail has a softer spectrum (Fig. 3.7, right) than the surrounding
gas. Hardness ratios from counts in the same pie-shaped annuli used to construct
Figure 3.8 confirm that the tail is softer in H21 by ≈2.6 σ. Thus, the jump in
emission due to the tail is dominated by a jump in soft emission, consistent with ram
pressure stripping of cooler galactic gas by hotter group gas.
NGC 1601 also has a velocity redshifted from NGC 1600 by ∼ 309 km s −1 , and its
projected distance from NGC 1600 is only ∼28.5 kpc. Since the emission associated
with NGC 1601 is fainter, its estimated thermal pressure is smaller than in NGC 1603.
This, combined with its slightly higher velocity, suggests that ram pressure stripping
should be stronger in NGC 1601 compared to NGC 1603. Although there is a hint of
a tail toward the east of NGC 1601 in Figure 3.7, it has only a 1.3 σ significance when
the surface brightness in a hemispherical annulus east of NGC 1601 is compared with
a similar annulus west of NGC 1601. There are a number of possible explanations of
why we do not see a significant tail around NGC 1601, while we see one for NGC 1603.
First, NGC 1601 might have a small transverse velocity; the tail would then be
projected onto NGC 1601 along our line of sight. In addition, the tail could be too
faint, the projected velocity could be smaller than we estimated for NGC 1603, or
projection effects could mean that NGC 1601 is actually in a lower density region in
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