A Rigorous Reanalysis of the IRAS Variable Population: Scale Lengths, Asymmetries, and Microlensing

Details A Rigorous Reanalysis of the IRAS Variable Population: Scale Lengths, Asymmetries, and Microlensing A Rigorous Reanalysis of the IRAS Variable Population: Scale Lengths, Asymmetries, and Microlensing

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...487..885n&link_type=abstract

Astrophysical Journal v.487, p.885

Statistics

54

Galaxy: Structure, Infrared: Stars, Methods: Statistical, Stars: Statistics

Scientific paper

Previous work reported a bar signature in color-selected IRAS variable stars. Here, we estimate the source density of these variables while consistently accounting for spatial incompleteness in the data using a likelihood approach. The existence of the bar is confirmed with a shoulder at a ~ 3 kpc, an axis ratio of a:b = 2.2--2.7, and a position angle of 19 deg +/- 1 deg. The ratio of nonaxisymmetric to axisymmetric components gives a similar estimate for the bar size of a = 3.3 +/- 0.1 kpc and a position angle of phi 0 = 24 deg +/- 2 deg. We estimate a scale length of 4.00 +/- 0.55 kpc for the IRAS variable population, suggesting that these stars represent the old disk population. We use this density reconstruction to estimate the optical depth to microlensing for the large-scale bar in the Galactic disk. We find an enhancement over an equivalent axisymmetric disk by up to 40%, but this still too small to account for the MACHO result. In addition, we note a possibility for a significant asymmetry at positive and negative longitudes with optical depths comparable to those in Baade's window. Independent of our reconstruction, an infrared microlensing survey may be a sensitive tool for detecting or constraining structural asymmetries. More generally, this is a pilot study for Bayesian star count analyses. The Bayesian approach allows the assessment of prior probabilities to the unknown parameters of the model; the resulting likelihood function is straightforwardly modified to incorporate all available data.

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