Velocity Coordinate Spectrum (VCS)¶
Overview¶
The Velocity Coordinate Spectrum (VCS) was present in the theoretical framework of Lazarian & Pogosyan 2000, and further developed in Lazarian & Pogosyan 2006. The VCS is complementary to the VCA, but rather than integrating over the spectral dimension in the VCA, the spatial dimensions are integrated over. This results in a 1D power-spectrum whose properties and shape are set by the underlying turbulent velocity and density fields, and the typical velocity dispersion and beam size of the data.
There are two asymptotic regimes of the VCS corresponding to high and low resolution (Lazarian & Pogosyan 2006). The transition between these regimes depends on the spatial resolution (i.e., beam size) of the data, the spectral resolution of the data, and the velocity dispersion. The current VCS implementation in TurbuStat fits a broken linear model to approximate the asymptotic regimes, rather than fitting with the full VCS formalism (Chepurnov et al. 2010, Chepurnov et al. 2015). We assume that the break point lies at the transition point between the regimes and label velocity frequencies smaller than the break as “large-scale” and frequencies larger than the break as “small-scale”.
A summary of the VCS asymptotic regimes is given in Table 3 of Lazarian 2009.
Using¶
The data in this tutorial are available here.
We need to import the VCS class, along with a few other common packages:
>>> from turbustat.statistics import VCS
>>> from astropy.io import fits
And we load in the data cube:
>>> cube = fits.open("Design4_flatrho_0021_00_radmc.fits")[0]
The VCA spectrum is computed using:
>>> vcs = VCS(cube)
>>> vcs.run(verbose=True)
OLS Regression Results
==============================================================================
Dep. Variable: y R-squared: 0.909
Model: OLS Adj. R-squared: 0.908
Method: Least Squares F-statistic: 815.9
Date: Fri, 21 Jul 2017 Prob (F-statistic): 3.55e-127
Time: 16:18:42 Log-Likelihood: -402.80
No. Observations: 249 AIC: 813.6
Df Residuals: 245 BIC: 827.7
Df Model: 3
Covariance Type: nonrobust
==============================================================================
coef std err t P>|t| [0.025 0.975]
------------------------------------------------------------------------------
const -11.4835 0.231 -49.755 0.000 -11.938 -11.029
x1 -9.7426 0.232 -41.915 0.000 -10.200 -9.285
x2 7.4106 2.683 2.762 0.006 2.126 12.696
x3 -0.0399 0.314 -0.127 0.899 -0.658 0.578
==============================================================================
Omnibus: 133.751 Durbin-Watson: 0.042
Prob(Omnibus): 0.000 Jarque-Bera (JB): 1872.647
Skew: -1.766 Prob(JB): 0.00
Kurtosis: 15.962 Cond. No. 44.9
==============================================================================
By default, the VCS spectrum will be fit to a segmented linear model with one break point (see the Power Spectrum tutorial for a more detailed explanation of the model). If no break points are specified, a spline is used to estimate the location of break points, each of which are tried until a valid fit is found.
With the default settings, the fit is horrendous. This is due to the model’s current limitation of fitting with a single break point. For these simulated data, there is no information on spectral frequencies below the thermal line width. This cube was created from an isothermal simulation, and at a temperature of 10 K, the thermal line width of \(\sim 200\) m / s is oversampled by the 40 m /s channels. We can avoid fitting this region by setting frequency constraints:
>>> vcs.run(verbose=True, high_cut=0.17 / u.pix)
OLS Regression Results
==============================================================================
Dep. Variable: y R-squared: 0.988
Model: OLS Adj. R-squared: 0.987
Method: Least Squares F-statistic: 2140.
Date: Fri, 21 Jul 2017 Prob (F-statistic): 2.90e-76
Time: 16:29:33 Log-Likelihood: -37.805
No. Observations: 84 AIC: 83.61
Df Residuals: 80 BIC: 93.33
Df Model: 3
Covariance Type: nonrobust
==============================================================================
coef std err t P>|t| [0.025 0.975]
------------------------------------------------------------------------------
const 1.8245 0.379 4.819 0.000 1.071 2.578
x1 -1.8412 0.219 -8.404 0.000 -2.277 -1.405
x2 -14.8271 0.412 -35.968 0.000 -15.648 -14.007
x3 0.1035 0.167 0.621 0.536 -0.228 0.435
==============================================================================
Omnibus: 10.043 Durbin-Watson: 0.058
Prob(Omnibus): 0.007 Jarque-Bera (JB): 3.501
Skew: -0.116 Prob(JB): 0.174
Kurtosis: 2.027 Cond. No. 21.4
==============================================================================
high_cut is set to ignore scales below \(\sim 240\) m / s, just slightly larger than the thermal line width. To see this more clearly, we can create the same plot above in velocity units:
>>> vcs.run(verbose=True, high_cut=0.17 / u.pix,
... xunit=(u.m / u.s)**-1)
The dotted lines, indicating the fitting extents, are now more easily understood. The lower limit, at about \(4 \times 10^{-3} {\rm m / s}^{-1}\) corresponds to \(1 / \left(4 \times 10^{-3}\right) = 250 {\rm m / s}\).
This is still not an optimal fit. There are large deviations as the single break-point model tries to interpret the smooth transition at large scales. This flattening at large scales could be from the periodic box condition in the simulation: there is effectively a maximum size cut-off at the box size beyond which there is no additional energy. For the next example, assume that this is indeed the case and that we can remove this region from the fit:
>>> vcs.run(verbose=True, high_cut=0.17 / u.pix, low_cut=6e-4 / (u.m / u.s),
... xunit=(u.m / u.s)**-1)
OLS Regression Results
==============================================================================
Dep. Variable: y R-squared: 0.996
Model: OLS Adj. R-squared: 0.996
Method: Least Squares F-statistic: 5443.
Date: Fri, 21 Jul 2017 Prob (F-statistic): 6.70e-81
Time: 17:10:57 Log-Likelihood: 15.889
No. Observations: 72 AIC: -23.78
Df Residuals: 68 BIC: -14.67
Df Model: 3
Covariance Type: nonrobust
==============================================================================
coef std err t P>|t| [0.025 0.975]
------------------------------------------------------------------------------
const -8.8409 0.275 -32.183 0.000 -9.389 -8.293
x1 -9.1948 0.217 -42.371 0.000 -9.628 -8.762
x2 -12.3859 0.488 -25.404 0.000 -13.359 -11.413
x3 -0.0062 0.093 -0.067 0.947 -0.191 0.179
==============================================================================
Omnibus: 6.011 Durbin-Watson: 0.067
Prob(Omnibus): 0.050 Jarque-Bera (JB): 5.617
Skew: -0.476 Prob(JB): 0.0603
Kurtosis: 3.983 Cond. No. 34.7
==============================================================================
This appears to be a better fit! Also, note that the low_cut and high_cut parameters can be given in pixel or spectral frequency units. We estimated low_cut from the previous example, where the plot was already in spectral frequency units.
Based on the power spectrum slope of \(-3.2\pm0.1\) we found using the zeroth moment map (Power Spectrum tutorial), this data is in the steep regime, where density fluctuations do not dominate at any spectral scale. Using the asymptotic case from Fig. 2 in Lazarian & Pogosyan 2006, the slopes should be close to \(-6 / m\) at small scales and \(-2 / m\) on large scales, where \(m\) is the index of the velocity field. The second slope in the fit summary (x2) is defined relative to the first slope (x1). The true slopes can be accessed through:
>>> vcs.slope
array([ -9.19479557, -21.58069847])
>>> vcs.slope_err
array([ 0.21700618, 0.53366172])
Since, in this regime, both components only rely on the velocity field, they should both give a consistent estimate of \(m\):
>>> -2 / vcs.slope[0]
0.21751435186363388
>>> - 6 / vcs.slope[1]
0.2780262190776282
Each component does give a similar estimate for \(m\). There is the additional issue with the simulated data as to how the inertial range should be handled. Certainly the slope at smaller scales is made steeper if portions are outside the spatial inertial range.
While we find a good fit to the data, the VCS transition between the two regimes is smoothed over. This is a break down of assuming the asymptotic regimes, and is a break down of the simplified segmented linear model that has been used. The model presented in Chepurnov et al. 2010 and Chepurnov et al. 2015, which account for a smooth transition over the entire spectrum, will be a more effective and useful choice. This model will be included in a future release of TurbuStat.