UNCERTAINTIES IN THE DEPROJECTION OF THE OBSERVED BAR PROPERTIES - - PowerPoint PPT Presentation
UNCERTAINTIES IN THE DEPROJECTION OF THE OBSERVED BAR PROPERTIES Yanfei Zou 1 Juntai Shen 1 and Zhao-Yu Li 1 1.Shanghai Astronomical Observatory, China 2013.10.21 MOTIVATION ~2/3 of disk galaxies are barred Composition of the bar
Juntai Shen1 and Zhao-Yu Li1
1.Shanghai Astronomical Observatory, China 2013.10.21
~2/3 of disk galaxies are
barred
Composition of the bar
old stars
Bar pattern rotates rapidly Internal driver of disk galaxies
gas inflow, central starburst, etc. energy and angular momentum
exchange
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Deproject the inclined bar to
face-on.
Basic assumption of the
deprojection
The outer parts of the bar are
vertically thin.
Observed galaxies: projected
2D information
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Simulated galaxies: 3D
information
So we know the true face-on
values of the bar length and ellipticity.
Observe the simulated galaxy
from different viewing angles
The uncertainties of
deprojection can be examined.
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3D information
true face-on values of
the bar length and ellipticity.
Observe the
simulated galaxies from different viewing angles
We reduce the data
in the same way as
Model A Model B
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106 particles rigid dark matter halo potential T = 1.8 Gyr 2*106 particles live halo T = 2.4 Gyr
Data
IRAF ellipse fitting
amax : Maximum ellipticity (Sheth et al. 2003) amin : Minimum ellipticity (Erwin 2005) a10 : Position angle (PA) deviates by 10° (Erwin 2005)
Measure bar properties (length and ellipticity)
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1-D analytical deprojection
(Martin 1995)
Assuming the bar is a straight line
segment
2-D analytical deprojection
(Gadotti et al. 2007)
Assuming the bar is a planar
elliptical structure
2-D image deprojection Other Fourier-based
deprojections
Fourier decomposition
(Li Z-Y et al. 2011)
Bar-interbar contrast
(Ohta et al. 1990)
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Deproject bar properties to face-on
1. Deproject the image using GEOTRAN 2. Measure the deprojected bar properties
Assuming the bar is a planar
elliptical structure
The deviation of adep/aint
increases with the i
The deprojected amax tends to
for amin and a10.
i >60°, the deviation becomes
large
~10%
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2-D analytical deprojection of the bar length
Φbar
A reflection of the strength of
the bar
The deviation of edep/eint
becomes large with i
As Φbar increases from 0° to
90°, the deprojected ellipticity gradually transition from under-estimate to over- estimate
~10%
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2-D analytical deprojection of the bar ellipticity
Φbar
1D deprojection has the
largest uncertainty ~20%.
2D deprojection is more
~10%.
Uncertainty of Fourier
decompostion：~5%.
Bar-interbar contrast has large
uncertainty, which is ~10%.
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Typical scatter in the deprojection
Bar structure: triaxial ellipsoid
shell.
Axis ratio: a:b:h
Project the shell of the triaxial
ellipsoid from different i and Φbar.
1D and 2D analytical
deprojection are tested using this simple toy model
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2D analytical deprojection of
the toy bar length
Uncertainties increase with i
and Φbar.
Similar trend and scatter as
the simulation results of 2D analytical deprojection.
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2D analytical deprojection of
the toy bar ellipticity
Uncertainties increase with i. Deprojected ellipticity
transition from under-estimate to over-estimate in the ellipticity deprojection as Φbar increases from 0◦ to 90◦.
Similar trend and scatter as
the simulation results of 2D analytical deprojection.
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Solid line: thick bar. Dashed line: 2D planar bar. Small residual at small i. Large residual at large i. Uncertainties mainly stem from
the vertical structure of the bar.
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Uncertainties of deprojection increase with i. All deprojection methods have
trouble recovering the bar properties when i>60°.
For amax, both the 1D and 2D methods overestimate the intrinsic bar length.
There is no clear trend for amin and a10.
As Φbar increases from 0 ° to 90 °, the deprojected emax and emin from 2D
methods transition from underestimate to overestimate. The deprojected e10 is generally underestimated.
Uncertainties of the deprojection can be reproduced by a simple toy model,
which confirms that it mainly stems from the vertical structure of the bar.
The uncertainty and application range of popular methods are given (lower
limit).
Provide guidelines for the sample selection and error estimation of future
statistical research on barred galaxies.
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