1 LEADING PAIRS AND TRAILING SINGLE SPIRAL ARMS: l HST OBSERVATIONS AND SIMULATIONS OF NGC4622 l G. Byrd (Univ. of Alabama), l T. Freeman (Bevill State Coll.), l R. Buta (Univ. of Alabama) l NASA STScI GO 8707 and NSF RUI

2 Summary l Our conclusion from HST images that the two outer NGC4622 arms lead was simply not acceptable to some. One prominent astronomer stated, ``You’re the backward astronomers who found a backward galaxy.” l Here we substantiate our conclusion, finding that with the observed flat then rising rotation curve l a plunging passage of a small galaxy could have triggered the inner trailing arm & outer leading pair. l NGC4622’s disk orientation & m=1, 2 IVB sequences versus radius are consistent with an inner trailing + an outer leading pair of density waves.

3 Do spiral arms lead or trail outward relative to orbital motion? l Lindblad (1941) said they lead. The sketch shows thin arms & arrow for disk orbital motion. l Hubble (1943) said they trailed. => l G. deVaucouleurs (1958) found all to trail in a small sample of highly inclined spirals. l Trailing accepted as the rule today.

4 l Byrd et al 1989 pointed out NGC4622’s two- way arms. l It must have leading arm(s)! l Our HST photo shows arms in disk & young blue associations l Outer pair winds out clockwise, CW l Inner arm CCW

5 Simulating NGC4622’s Arms l Byrd, Freeman, and Howard (BFH 1993) found a new, plunging, disk plane passage by a small perturber produced a two-way pattern. l Assuming a flat rotation curve, two trailing outer arms and a single inner leading arm could be produced. l Predicted that NGC4622’s disk turns counter- clockwise (CCW on the sky). Wanted to check this.

6 Checking predicted disk rotation l Need NGC4622’s velocity field and disk orientation. l Ground-based H Alpha emission velocity field for NGC4622. => l Red = away l Blue = toward l Ref. Scott 1996 MS thesis

7 NGC4622 Disk orientation l The line of nodes is the intersection of the disk with the sky plane. l The line is obtained from the elongation of NGC4622’s outer isophotes ~NNE toward SSW. l Need to know which edge of disk is near

8 Obtaining Orientation: HST V-I image l Use GdV method. l Darker bluer. Whiter redder. l Note blue spiral arm associations along outer arms. l Central regions, nuclear bulge. l Near side of NGC4622 disk shows sharper dust silhouettes with redder color than far side. l East side is nearer.

9 Boy, were we surprised! l Why is the result so surprising? l East edge is nearer. => l North line of nodes recedes. l The disk thus orbits CW on the sky, opposite prediction. l Inner single arm trails & the outer pair leads!

10 How can the outer two leading arms and inner trailing arm be triggered? l As in BFH, we simulate a small, planar plunging perturber. l However, we must use a new unusual rising, steepening rotation curve from the observations. l We fit the observed curve v with a formula giving l v~ 100 km/s 16 to 30arcsec l v~ 0.08r^2 km/s at 45 to 60 arcsec.

Simulation vs HST image High contrast image. m=0 disk subtracted => Simulation plot follows.

12 l Simulation disk 390x10^6 yr after a low mass plunging perturber entered then left along disk plane. l Outer arms lead out CW like disk orbital motion. l Inner arm trails surrounded by oval.

13 Disk resonances & arms l Rotation curve has a declining then rising orbital angular rate curve. l Arm pattern speed can have TWO corotation (CR, CR2) resonances.

14 m=1 arm resonance color sequences l Inner arm resembles set of perturbed orbits inside CR of turning m=1 (one sided) disturbance. Orbits have faster CW angular rate than CR. l Inner ring resembles set outside CR. Orbits have slower CW angular rate than CR. l

15 Verifying single arm color sequence predictions l Position angle vs r of m=1 intensity peaks. l I---, V…, & B solid l 180 degree jump at ~21” (CR) as expected for one set of m=1 perturbed orbits inside CR & one outside. l See expected reversal of I, V, B color peak sequence CW inside and CCW outside 21”. l Confirms CW orbital motion & trailing single inner arm.

16 Simulation demonstrating m=1 CR resonance l Flat rotation curve region. l Rotating m=1 perturbation acts on disk of particles. Little self-gravity produces best match. l Empty region is CR resonance radius. l Trailing arm and ring inside and outside CR.

17 Position angle vs r of m=2 IVB peaks l m=2 IVB order switches at CR= 21” & CR2= 36” where orbital angular rate equals pattern speed. Similar to m=1. l Confirms CW orbit & leading outer pair of arms! l Will use CR,CR2 to revise rotation curve.

18 Another 2 Way Galaxy Found l R. Buta and graduate student R. Grouchy in recent observations have identified another two-way galaxy, ESO l It has a single inner arm and two and three fold oppositly winding outer arms.