Abstract Introduction References Data Conclusion ...

Hence, the peak could be result of even distant galaxies that appear as point sources. Color-color plots help to differentiate between stars and other objects. (Fig. 4, 5, 6, 7, 8 & 9)

Abstract The primary objective of this work is to create an all sky extinction map of the Milky Way galaxy. We have so far cross-matched the SDSS photometric observations with that of GALEX, for high-latitude stars in the northern hemisphere (≥ 75°). This crossmatching provides a wide range of coverage from Far Ultra-Violet through the optical spectrum and gives one unique SDSS source for every GALEX source. The photometric observations of each star was then fit to the Castelli and Kurucz Atlas, best fit being determined using a chi-square test. Best fit parameters provide the temperature and E(B-V) towards each of the individual stars. The shift in magnitude obtained during the best-fit can be used to determine the distance to each of the stars. With this data, a comprehensive extinction map can be made for the high-latitude stars and later extended to allsky.

Introduction The GALEX satellite observes the sky at two ultraviolet passbands: the far-UV (FUV) and the near-UV (NUV) centered at 1539Å and 2316Å respectively. Although the satellite also takes grism spectra, in this work we are only concerned with the properties of the photometric observations. The GALEX measurements themselves, however, do not provide enough information for most studies due to the lack of angular resolution and narrow spectral coverage, which do not allow for a reliable star– galaxy separation. The solution is to cross-identify the GALEX sources to other catalogs, in particular to the Sloan Digital Sky Survey. Most GALEX observations are designed to cover regions of the sky already observed by the SDSS at a comparable depth (Budavári, T. et al 2009). SDSS takes images using photometric system of five filters, namely u, g, r, i and z. The central wavelengths of the filters are 3551Å, 4686Å, 6166Å, 7480Å and 8932Å respectively.

Fig 1. Polar distribution of stars Fig 4. Sample color-color plots

Analysis The photometric data obtained from GALEX and SDSS was fit to stellar models from the Castelli and Kurucz Atlas (Fig. 2). We assumed error bars of 0.1 magnitude in each of the bands and performed a χ² minimization to find the best fit parameters.

Fig 5. Blue band is the model as obtained from Castelli and Kurucz atlas and the black dots are data points from GALEX and SDSS

Fig 9. GALEX and SDSS data (black dots) superimposed over plot from Fan 1999 . Clump on the left contains white dwarfs, CELGs and quasars Fig 2. A sample fit to one of the stars E(B-V) =0.473 & Temperature = 39000K

The polar distribution of the resulting stars is shown in Fig 1. High star density regions are from deep observations with fainter stars while the others are observations of as little as 100s taken as part of the all-sky survey.

Conclusion

On analyzing the parameters obtained from the fitting, the histogram of extinction was found to show a peak at ~0.6, which is not expected near the north galactic pole (NGP) (Fig. 3).

Data Photometric data for high latitude stars (≥75°) have been obtained from GALEX and SDSS, to give a low resolution spectra of stars. Stars have been cross matched using the standard SQL tools on the CASJobs server at the Johns Hopkins University, giving one and only one unique match between GALEX and SDSS. Cross match has now given a total of 31883 stars. The position of the stars was used as the criteria for the cross match, with the search radius being 5 arcsec. Combining GALEX and SDSS gives a wider range of wavelength, from UV to optical.

Fig 8. GALEX and SDSS data (black dots) superimposed over plot from Fan 1999. Clump on the left contains white dwarfs, CELGs and quasars

Fig 6. GALEX and SDSS data (red dots) superimposed over plot from Zhang et al 2009. Points on the right represent ultra-cool dwarfs

• A large number of objects seem to be bluer than the main sequence stars • For the NGP, extinction is found to be high for many objects • Several objects do no fit well with a stellar spectrum • Data contains many objects misidentified as stars, which need to be removed

References • • •

Acknowledgement

Fig 3. Histogram of E(B-V)

Color-Color Diagrams Further analysis of the E(B-V) peak reveals that many of the objects classified as stars by SDSS might have been misidentified.

Budavári, T. et al. 2009, ApJ, 694, 1281 Zhang et al. 2009, A&A, 497, 619 Fan, X. 1999, AJ, 117, 2528

Fig 7. GALEX and SDSS data (red dots) superimposed over a plot from Fan 1999. The branch on the right contains halo stars, disk stars and BHB

We would like to thank Noah Brosch, Ani Thakar and T. Sivarani for their help. This work is supported by the Department of Science and Technology (DST), Ministry of Science and Technology, Government of India, New Delhi, India under the grant No. SR/S2/HEP-011/2009.