1

GALAXY SPIRAL ARMS,DISK DISTURBANCES AND

STATISTICS Part I: NGC3081 to build background for

NGC4622. Co-authors for Parts I and II: G. Byrd (Univ. of Alabama, Tuscaloosa),

T. Freeman (Bevill State Comm. Coll. Fayette,AL),

R. Buta (Univ. of Alabama, Tuscaloosa)

Support NASA STScI and NSF RUI

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NGC 3081

Galaxy type (R1R2’)SAB(r,nr)0/aaccording to Ron Buta

quintessential resonance ring galaxy

results of ground based and HST WFPC2studies

analysis of individual point sources in innerring

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NGC 3081 Observational Studies

HST study Buta, Byrd, & T. Freeman 2001

Surface photometry & kinematics Buta &Purcell 1998

Goals:

Star Formation Dynamics

Bar Strength

Disk Surface Density

[1]. North is top. UL is B band. Bar=30”≈ 5.4 kpc. URis a deep B. LL is B-I. LR is B with disk subtracted.

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HST blue image.

HST image processed to reveal individualassociations.

Images with possilbe ILR, CR,and OLR marked.

CR most important here.

Note gap between ``r” ring andR1 with CR in middle.

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Nodes, Doppler shifts, CR. Disk must turn CCW.

Stars formed near CR age in CCW sense.

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B and I band colors around ``r” ring.

B higher than I implies blue, young stars have formed.

Blue stars 0 to 90o and 180 to 270o

Like previous diagram => CCW orbit.

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Analytic Solutionfor the Inner

Ring. 37’’ outeredge is well inside

CR) at 52”. Vo=221 km/s, 18 kpc equal to 100”, and

a perturbation pot amp. -qVo2 where q=0.025.

Pointy ends match our images and our simulations.Inside of the ring is more circular as observed [1].

Points are at equal intervals of t. At smaller radii, theangular displacement is larger as observed.

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Fourier m=2componentposition anglesfrom majoraxes around``r” ring.

Note I, V, Bgoing CCWfrom major axis

IVB is a CCWcolor-agesequence

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Simulation (and analytic) ringshape like our HST image.

The gas cloud disk + inert highdispersion stellar disk and halo221 km/s=Vo. Cloud disk surfacemass density 1/15 to produce Vo.

Clumps of gas clouds(``associations”) form.

Simulation ring life is several 109

yrs < ~400.106 yr.

Too low a halo _ a chaotic non-ring disk;

too high _ ring but noassociations.

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Obtaining Disk Parameters

For flat rotation curve.

More general formulas are possible.

Ratio a/b inner ring, r, = (1-sqrt(2q))/(1-2 sqrt(q))

Ftan/Frad =2q

Sur. Density = (q/(I 2/I0))x(Vo2/(2" G ro)) =

(q/(I 2/I0))x(100% Mestel disk)

Halo/Disk = 100% disk/ 3081 disk surface density =f = (I 2/I0)/q

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Conclusions: ``r” and ``R” rings arestar formation laboratories.

IVB color age sequence.M/L disk is not constant with radius.

May actually rise.

Radius“, kpc I2/IO Msun/pc2Halo/disk40 or 7.20.6812.819.450 or 9.000.5911.816.970 or 12.60.2519.47.14

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GALAXY SPIRAL ARMS, DISKDISTURBANCES AND

STATISTICS:

PART II: THE STRANGE CASE OFNGC4622, AN EXCEPTION TO THERULE

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Do spiral arms lead or trailoutward relative to orbital motion?

Lindblad (1941) said they lead. Thesketch shows thin arms & arrow fordisk orbital motion.

Hubble (1943) said they trailed. =>

G. deVaucouleurs (1958) found all totrail in a small sample of highlyinclined spirals.

Trailing accepted as the rule today.

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Byrd et al 1989pointed outNGC4622’s two-way arms.

It must haveleading arm(s)!

HST photo showsarms in disk &young blueassociations

Outer pair windsout CW

Inner arm CCW

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SimulatingNGC4622’s Arms

Byrd, Freeman, and Howard (BFH 1993) found anew, plunging, disk plane passage by a smallperturber produced a two-way pattern.

Assuming a flat rotation curve, two trailing outerarms and a single inner leading arm could beproduced.

Predicted that NGC4622’s disk turns counter-clockwise (CCW on the sky). Wanted to check this.

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Checking predicted disk rotation Need

NGC4622’svelocity field anddisk orientation.

Ground-based HAlpha emissionvelocity field forNGC4622. =>

Red = away

Blue = toward

Ref. Scott 1996MS thesis

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NGC4622 Disk orientation The line of nodes is

the intersection ofthe disk with the skyplane.

The line is obtainedfrom the elongationof NGC4622’s outerisophotes ~NNEtoward SSW.

Need to know whichedge of disk is near

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Obtaining Orientation: HST V-I image Use GdV method.

Darker bluer. Whiterredder.

Note blue spiral armassociations along outerarms.

Central regions, nuclearbulge.

Near side of NGC4622disk shows sharper dustsilhouettes with reddercolor than far side.

East side is nearer.

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Boy, were wesurprised!

Why is the result sosurprising?

East edge is nearer.=>

North line of nodesrecedes.

The disk thus orbitsCW on the sky,opposite prediction.

Inner single armtrails & the outer pairleads!

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How can the outer two leading arms andinner trailing arm be triggered?

As in BFH, we simulate asmall, planar plungingperturber .

However , we must use anew unusual rising,steepening rotation curvefrom the observations.

We fit the observed curve vwith a formula giving v~ 100 km/s 16 to 30arcsec

v~ 0.08r^2 km/s at 45 to 60arcsec.

Simulationvs HSTimage

Highcontrastimage.m=0 disksubtracted=>

Simulationplotfollows.

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Simulationdisk390x10^6 yrafter a lowmass plungingperturberentered thenleft along diskplane.

Outer armslead out CWlike diskorbital motion.

Inner armtrailssurrounded byoval.

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Two CR Disk Resonances

A decliningorbital angularrate through firstCR.

A rising orbitalangular ratethrough secondCR.

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m=1 armresonance

colorsequences

Inner set of perturbed orbits insidefirst CR of turning m=1 (one sided)disturbance. Orbits have faster CWangular rate than smaller CR.

Another set outside first CR. Orbitshave slower CW angular rate than CR.

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Position angle vs r ofm=1 intensity peaks.

I---, V…, & B solid

180o jump at ~21” (first CR) as expectedfor one set of m=1perturbed orbits insideCR & one outside.

Expected reversal of I,V, B color peaksequence CW insideand CCW outside 21”.

==> CW orbitalmotion & trailingsingle inner arm.

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Simulationdemonstrating m=1

CR resonance Flat rotation curve

region.

Rotating m=1perturbation acts ondisk of particles. Littleself-gravity producesbest match.

Empty region is CRresonance radius.

Trailing arm and ringinside and outside CR.

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Position angle vs r of m=2IVB peaks

m=2 IVB orderswitches at CR= 21”& CR2= 36” whereorbital angular rateequals pattern speed.Similar to m=1.

Confirms CW orbit& leading outer pairof arms!

Will use CR,CR2 torevise rotation curve.

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Another 2 Way Galaxy Found

R. Buta and graduatestudent R. Grouchy inrecent observationshave identifiedanother two-waygalaxy, ESO297-27

It has a single innerarm and two and threefold oppositelywinding outer arms.

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Summary Our conclusion from HST images that the two

outer NGC4622 arms lead was simply notacceptable to some. One prominent astronomerstated, ``You’re the backward astronomers whofound a backward galaxy.”

Here we have substantiated our conclusion. Withthe observed flat then rising rotation curve a plunging passage of a small galaxy could have

triggered the inner trailing arm & outer leading pair.

NGC4622’s disk orientation & m=1, 2 IVB sequencesversus radius are consistent with an inner trailing + anouter leading pair of density waves.

36 Now we know the origin of crop circle creators!