the morphology and pattern speed of spiral structurednelson/doc/dnelson.cefalu.poster.pdf ·...
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The Morphology and Pattern Speedof Spiral Structure
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3. Measuring the Pattern Speed of Simulated Spiral Galaxies
4. Main Conclusions and Future Work
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2. Veri�cation of the TW Method with a Barred Galaxy
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1. Model Construction and Simulation
We consider the spiral structure arising from the gravitational in�uence of massive perturbers, for instance giant molecular clouds, corotating in a stellar disk. Within a single rotation period our simulations develop a prominent, large-scale response in an initially smooth disk.
We explore the spectrum of spiral morphologies generated by this mechanism by varying the critical wavelength parameter (λcrit). We make quantitative correlations between spiral structure and host galaxy and halo properties, and construct a catalog of arm morphologies to compare to spatially resolved observations of nearby galaxies. We measure radial variation of the spiral pattern speed using a modi�ed Tremaine-Weinberg method, and �nd that the pattern speed both decreases with radius and appears to track the circular velocity of the disk.
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• The proposed mechanism (collective swing amplification of a large number of perturbers co-rotating within the disk) can recreate a broad range of spiral morphologies.• Measurements made with the observationally based Tremaine-Weinberg method are consistent with the true feature pattern speeds, though with large scatter.• The spiral pattern speeds of our model spiral galaxies decrease with radius, consistent with recent observational work (e.g. Merri�eld+ 2006, Meidt+ 2009, Speights+ 2011).• Furthermore, the pattern speed closely tracks the disk circular velocity, a natural consequence for a local response arising around a collection of co-orbiting perturbers.• Our results are fundamentally inconsistent with the quasi-steady density wave theory of Lin and Shu (1964), and rather imply that spiral structure is not steady in time.
2. Chosen to have varying λcrit, the maximum wavelength stable against axisymmetric instability, and the natural length scale in the swing ampli�er solution:
Dylan Nelson ([email protected]), Elena D’Onghia, Lars Hernquist (Harvard University, Center for Astrophysics)
Meidt et al. (2009)M101
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Speights et al. (2011)NGC1365
1. We construct a suite of stable, live exponential stellar disks embedded in rigid dark matter halos (following Hernquist 1993).
Projected surface density after ~2 orbital periods shown for each model.
LC-2λcrit/h = 1.1
Models locally stable (Q>1) against axisymmetric instability at all radii.
3. N-body forces evolved using the hierarchical Barnes-Hut tree code implemented in GADGET-3.
λ crit =4π2GΣ
κ2
Tracking peaks in the surface overdensity pro�le to cal-culate the bar normal angle. The time derivative of this angle then provides a direct measurement of Ωp(r).
1. Adopt techniques used observationally to measure the structural properties of disk galaxies: modi�ed Tremaine-Weinberg (TW) method for measuring spiral pattern speeds.
Contours of the m=2 Fourier mode power reveal a coherent bar structure out to ~one disk scale length rotat-ing with a frequency of ~90/m km/s/kpc.
2. Verify the methods using time-domain information available in the simulations, and compare directly with observations.
Observational methods (colored regions) based on the TW method are consistent with direct simulation measurements of the pattern speed, though they exhibit signi�cant scatter with time (shown as vertical errors).
Five methods to measure Ωp(r) for the bar simulation.
Gravitational perturbers visible - 1,000 GMCs(constant mass fraction) in prograde rotation.
1. Best �t powerlaw models using the Radial Tremaine-Weinberg method (green) in excellent agreement with direct measurements (points).
LC-4λcrit/h = 1.4
LC-6λcrit/h = 1.7
LC-8λcrit/h = 2.0
2. Pattern speed decreases with radius and closely follows the circular velocity of the disk (solid orange line).
3. Radial behavior in broad agremeent with recent observational work, and inconsistent with constant Ωp.
Additional visualization and movies available at www.cfa.harvard.edu/~dnelson/
M101 (NASA/ESA)NGC7217 (Hubble)
Global Constant Ωp
Piecewise Ωp(r) Model Fit
Radially Binned Constant Ωp
Ωc