1 GALEX Angular Correlation Function … or about the Galactic extinction effects

2 Two ways to compute the ACF from a given set of fields    Combined Field Method (CF) Pair Weighted average Method (PW) Keep track of the relative positions of the fields within the angular scales relevant for the ACF Minimize Integral Constraint bias and best signal to noise ratio (use of cross- pairs) Compute ACF for each field and then average the results Weight: number of random-random pairs Less sensitive to large scale problems, but poor signal to noise ratio (reach only field scales)

3 G2 paper – GALEX IR 0.9 ACF method and extinction CF method: sensitivity to extinction (and flattening at large scales) PW method: no trend with extinction Method used in G2 papers NUV < 22

4 GALEX IR 1.1 ACF method and extinction (1/3: m UV < 22)

5 GALEX IR 1.1 ACF method and extinction (2/3: m UV < 21.5)

6 GALEX IR 1.1 ACF method and extinction (3/3: m UV < 21)

7 GALEX IR 1.1 ACF method and extinction  Trend weaker than with IR0.9 but still observed  Trend present at faint (m UV < 22) as well as at bright (m UV < 21) apparent magnitudes  Continue to use PW method to do science ?  Problem: PW method yields noisy results with low number of galaxies per field  volume-limited studies difficult for instance

8 SDSS counts and extinction  See Yahata et al (astro-ph/ ) study on SDSS galaxy counts  They propose that the Schlegel et al maps are contaminated by FIR emission of background galaxies in the low E(B-V) regions Yahata et al (2006) Increase of counts with A r at low extinction (A r < 0.1)

9 GALEX IR 1.1 GALEX counts and extinction  No rise of counts at low extinction similar to that observed by Yahata et al (except for NUV<22 corrected ??)  Yahata et al observe it for A r < 0.1  A UV < 0.3

10 SDSS colors and extinction Yahata et al (2006) Effect similar in corrected colors

11 GALEX IR 1.1 GALEX colors and extinction  Trend less obvious within GALEX, if any …  N.B.: corrected fuv-nuv redder because A NUV /E(B-V) > A FUV /E(B-V)

12 GALEX IR 1.1 PDF of galaxy counts and extinction PDF of galaxy counts lower in the fields with highest E(B-V) (for counts ≥ mean N gal )

13 GALEX IR 1.1 Images and extinction Features in the images correlated with extinction maps FUV and NUV images of a high extinction field (mean E(B-V) = 0.12) blue, green and red contours are E(B-V) contours from Schlegel et al maps at E(B-V) = 0.1, 0.12 and 0.14 respectively

14 GALEX IR 1.1 Cross-correlation of galaxies and extinction map (1/3: m UV < 22)

15 GALEX IR 1.1 Cross-correlation of galaxies and extinction map (2/3: m UV < 21.5)

16 GALEX IR 1.1 Cross-correlation of galaxies and extinction map (3/3: m UV < 21)

17 GALEX IR 1.1 Cross-correlation of galaxies and extinction map  Method: random catalogs on the sky weighted by E(B-V) values at the corresponding (ra, dec); galaxies have weight = 1  No obvious trend of the cross-correlation with E(B-V)  Results obtained with the PW method are roughly consistent with 0  CF method yields a negative Xcorrelation for FUV<22, but positive for NUV<22 …

18 GALEX IR 1.1 Sky Background and Galactic extinction Background increases with E(B-V) (but not an issue in itself)

19 GALEX IR 1.1 Cross-correlation of galaxies and background (1/3: m UV < 22)

20 GALEX IR 1.1 Cross-correlation of galaxies and background (2/3: m UV < 21.5)

21 GALEX IR 1.1 Cross-correlation of galaxies and background (3/3: m UV < 21)

22 GALEX IR 1.1 Cross-correlation of galaxies and background  Results are color-coded according to the mean E(B-V) in the fields  Method: same as before with E(B-V) (weighted random catalogs)  Results obtained with PW method consistent with 0  CF method: no monotonic trend with E(B-V); but highest E(B-V) fields have the strongest amplitude of Xcorrelation. For this group, the amplitude roughly doubles between m UV < 22 and m UV < 21 (not observed for all the other E(B-V) groups  not linked to the projection effects on the amplitude of the autocorrelation of the galaxies)

23 GALEX IR 1.1 Conclusions …  Clustering measurements are not independent on Galactic extinction within the GALEX IR1.1 sample  Trends appear to be weaker than with previous GALEX releases; however, accurate comparisons with modeling (analytic or else) require to use the cleanest measurements  Trends observed in the optical (based on galaxy counts and colors) not observed as clearly with GALEX: use AIS to get better statistics ? Redshift selection effects (see Yahata et al (2006)) ?  Cross-correlation studies point towards an imprint of the sky background in the high Galactic extinction regions  Solutions: use PW method in the lowest Galactic extinction regions ? Attempt to correct measurements (CF ones in particular) using sky background autocorrelation ?