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GALEX Angular Correlation Function

… or about the Galactic extinction effects

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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)

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G2 paper – GALEX IR 0.9ACF 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

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GALEX IR 1.1ACF method and extinction (1/3: mUV < 22)

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GALEX IR 1.1ACF method and extinction (2/3: mUV < 21.5)

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GALEX IR 1.1ACF method and extinction (3/3: mUV < 21)

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GALEX IR 1.1ACF method and extinction

Trend weaker than with IR0.9 but still observed

Trend present at faint (mUV < 22) as well as at bright (mUV < 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

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SDSS counts and extinction

See Yahata et al (astro-ph/0607098) 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 Ar at low extinction (Ar < 0.1)

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GALEX IR 1.1GALEX 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 Ar < 0.1 AUV < 0.3

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SDSS colors and extinction

Yahata et al (2006)

Effect similar in corrected colors

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GALEX IR 1.1GALEX colors and extinction

Trend less obvious within GALEX, if any …

N.B.: corrected fuv-nuv redder because ANUV/E(B-V) > AFUV/E(B-V)

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GALEX IR 1.1PDF of galaxy counts and extinction

PDF of galaxy counts lower in the fields with highest E(B-V) (for counts ≥ mean Ngal )

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GALEX IR 1.1Images 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

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

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

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

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GALEX IR 1.1Cross-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 …

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GALEX IR 1.1Sky Background and Galactic extinction

Background increases with E(B-V) (but not an issue in itself)

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

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

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

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GALEX IR 1.1Cross-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 mUV < 22 and mUV < 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)

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GALEX IR 1.1Conclusions …

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 ?