text: SparsePak Survey of Nearby Galaxy Mergers 17 Table 4. List of systems with available dynamical models. The source of the model, time since pericenter, time left to coalescence ( t), pericentric separation ∆ (R ), and fraction of shocked H emission (f ), are shown. All of these systems except UGC 12914 have been modeled with equal mass galaxies. Mass peri α shocked ratio of the model of UGC 12914 is 3. Vollmer et al. (2012) and Struck & Smith (2003) do no provide the time left to coalescence in their models. The table is sorted by shock fraction. system name source time (Myr) t (Myr) R (kpc) f ∆ peri shocked NGC 5257/8 Privon et al. (2013) 230.0 1200.0 21.0 0.15 NGC 4676 Mortazavi et al. (2018) 190.0 775.0 18.0 0.26 UGC 12914 Vollmer et al. (2012) 26.0 - 1.2 0.37 Arp 284 Struck & Smith (2003) 170.0 - 6.5 0.39 UGC 07593 this work 27.0 12.0 2.5 0.40 NGC 2623 Privon et al. (2013) 220.0 -80.0 1.0 0.90 1.0 0.9 NGC 2623 NGC 2623 0.8 0.7 0.6 0.5 0.4 UGC 07593 UGC 07 5 9A3rp 284 UGC 12914 0.3 NGC 4676 NGC 4676 0.2 NGC 5257/8 NGC 5257/8 0.1 0 200 400 600 800 1000 12000 5 10 15 20 25 time left to coalescence (Myr) pericentric separation (kpc) Figure 13. Trends of shock fraction in models with available dynamical model. The left panel shows how shocked H emission changes with time until the α galaxies coalesce in the four systems for which dynamical model provides this information. The red dashed line is the time of coalescence. The right panel shows the trend with pericentric separation. ISM on their way out and produce shocks. In this mode shocks are material moves in the same direction in prograde discs. Therefore, produced a few dynamical times after the tidal impulse kicks in. In this mode of shock production, one expects to ﬁnd more shocks This mode is a side eﬀect of the gravitational tidal impulse during in retrograde and polar discs interacting with small pericentric sep- a close passage, so factors that aﬀect the strength of the tidal force aration, and these shocks should become less prominent with time should aﬀect the shock production through this mode. since pericenter. In this mode of shock production, pericentric distance and 7.1 Shock Production and Encounter Parameters initial disc orientation should strongly aﬀect the amount shocks. We know that the tidal response is proportional to the inverse cube of the Our sample in this work was selected based on the visibility of distance to the perturber, so a factor of two in pericentric separation strong tidal features (see §), so it should be biased toward prograde changesthetidalimpulsebyafactorofeight.Inprogradeencounters mergers, and the ﬁrst mode of shock production should be domi- tidally induced inﬂows are even more prominent, because the tidal nant. We predict higher shock fraction in systems that are closer to force pushes or pulls the rotating material in the same direction coalescence, because not only is there more time for the gas inﬂows during the pericentric passage, and the resulting eﬀect of tides is to enhance central starbursts/AGNs and produce shocks through maximized. Therefore, in prograde mergers with small pericentric outﬂows, but also violent relaxation, random orbital crossings, and separation, we expect to ﬁnd stronger shocks produced through the the return of tidally striped material during coalescence may gen- ﬁrst mode, and the amount of shock should increase with time since erate shocks. This trend can be seen in the left panel of Figure 13. pericenter. Notice that in this panel we do not have the data for UGC 12914 In a second mode, sometimes, the geometry of galactic en- and Arp 284. counter allows the ISM of the two gas rich discs to directly collide From the spacial distribution of the H emission, it seems that α with each other. For this to take place, the two discs should have in UGC 12914 shocks are produced through the second mode. The small enough distance at the pericenter, so that the discs and not presence of the continuous gaseous bridge observed on diﬀerent just the dark matter halos cross during the ﬁrst passage. The shocks bands (Condon et al. 1993; Braine et al. 2003; Gao et al. 2003; are produced immediately, and decay as they cool down through ra- Vollmer et al. 2012; Appleton et al. 2015) implies that gas has been diation. Unlike the other mode of shock production, this one is less striped from the two galaxies because of a recent head-on encounter. likely to happen in prograde discs, because the overlapping rotating According to the model presented by Vollmer et al. (2012), the MNRAS 000, 1–21 (2017) αH dekcohs fo noitcarf ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTFigure## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine## ##LTLine##

text: SparsePak

text: Survey

text: of

text: Nearby

text: Galaxy

text: Mergers

text: Table

font: KQJEEK+TeXGyreTermes-Bold

text: List

text: of

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text: MNRAS

text: αH

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