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  old stars  Bar pattern rotates rapidly  Internal driver of disk galaxies  gas inflow, central starburst, etc.  energy and angular momentum exchange 2

MOTIVATION  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 3

MOTIVATION  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. 4

METHODS Data  3D information Model A Model B  true face-on values of the bar length and ellipticity.  Observe the simulated galaxies from different viewing angles 10 6 particles 2*10 6 particles  We reduce the data rigid dark matter halo potential live halo in the same way as T = 1.8 Gyr T = 2.4 Gyr observations 5

METHODS Measure bar properties (length and ellipticity)  IRAF ellipse fitting  a max : Maximum ellipticity (Sheth et al. 2003)  a min : Minimum ellipticity (Erwin 2005)  a 10 : Position angle (PA) deviates by 10° (Erwin 2005) 6

METHODS Deproject bar properties to face-on  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) 1. Deproject the image using GEOTRAN 2. Measure the deprojected bar properties  Bar-interbar contrast 7 (Ohta et al. 1990)

RESULTS 2-D analytical deprojection of the bar length  Assuming the bar is a planar elliptical structure  The deviation of a dep /a int increases with the i  The deprojected a max tends to overestimate the true face-on a max . There is no clear trend for a min and a 10 .  i >60°, the deviation becomes large  ~10% Φ bar 8

RESULTS 2-D analytical deprojection of the bar ellipticity  A reflection of the strength of the bar  The deviation of e dep /e int becomes large with i  As Φ bar increases from 0° to 90°, the deprojected ellipticity gradually transition from under-estimate to over- estimate  ~10% Φ bar 9

RESULTS Typical scatter in the deprojection  1D deprojection has the largest uncertainty ~20%.  2D deprojection is more accurate. The uncertainty is ~10%.  Uncertainty of Fourier decompostion ： ~5%.  Bar-interbar contrast has large uncertainty, which is ~10%. 10

TOY MODEL  Bar structure: triaxial ellipsoid shell.  Axis ratio: a:b:h  Project the shell of the triaxial ellipsoid from different i and Φ bar .  1D and 2D analytical deprojection are tested using this simple toy model 11

TOY MODEL  2D analytical deprojection of the toy bar length  Uncertainties increase with i and Φ bar .  Similar trend and scatter as the simulation results of 2D analytical deprojection. 12

TOY MODEL  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 Φ bar increases from 0 ◦ to 90 ◦ .  Similar trend and scatter as the simulation results of 2D analytical deprojection. 13

TOY MODEL  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. 14

SUMMARY  Uncertainties of deprojection increase with i . All deprojection methods have trouble recovering the bar properties when i >60°.  For a max , both the 1D and 2D methods overestimate the intrinsic bar length. There is no clear trend for a min and a 10 .  As Φ bar increases from 0 ° to 90 °, the deprojected e max and e min from 2D methods transition from underestimate to overestimate. The deprojected e 10 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. 15