MAIN LIBRARYNo.63-March 1991 - A New Southern Hemisphere Galactic Extinction Map Based on Surface Brightnesses of External Galaxies

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MAIN LIBRARY No.63 (March 1991) reports a new galactic extinction study for the Southern Sky using about 12,000 homogeneously selected ESO-LV galaxies from surface brightness measurements in external galaxies. Precise B-band extinction values are converted into E(B-V) estimates, providing a 3°×3° resolution extinction map in polar equatorial coordinates. The method uses correlated two-band surface brightnesses (B and R) and an “analysis of variance” statistical technique to obtain extinctions simultaneously with the intrinsic unreddened surface-brightness curve.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages84

Reading Time3 h 32 min

AccessFree to download

1. Introduction

2. The Technique

3. The Map

# 1.Introduction The precise surface brightness valueslisted for about 12,000 homogeneouslyselected galaxies in"The Surface Pho-tometry Catalogue of the ESO-UppsalaGalaxies”(Lauberts and Valentijn 1989,hereafter ESO-LV;see also theMessenger 34,10,and 56,31,and thisissue)have been used to derive galacticextinction values for a large part of theSouthern Sky,cf.Figure 1.The newextinction measures are thought to re-flect the effect of the diffuse interstellar Figure 1:A map of the relative extinction inthe B band in polar equatorial coordinates.The derived extinction values have been av-eraged inside 3°×3°pixels and are heredisplayed using 1.5×1.5 pixels -someinterpolation has been applied to fill pixelswithout data(~5%of total).The credibility ofapparent structures can be assessed by us-ing the following information:the uncertaintyof Aginside one resolution element (inset boxat the lower right)is 0.13",roughly corre-sponding to one colour step on the map.Thezero point is somewhat arbitrary,but a lowerlimi could be deduced by avoiding negativeextinctions in regions with a minimum extinc-tion (not at the South Pole)and a deconvolu-tion of the observed frequency distribution ofAg with the measurement error function. MAIN LIBARYNo.63-March 1991 A New Southern Hemisphere Galactic Extinction MapBased on Surface Brightnesses of External Galaxies J.CHOŁONIEWSKI,Astronomical Observatory of the Warsaw University,Poland E.A.VALENTIJN,ESO and Laboratory for Space Research,Groningen,the Netherlands )9*90)×sin (a) -80 (8.90°)×cos(α) 18 IB Figure 2:Surface brightness (measured at the effective isophote)of ESO-LV galaxies in B andR bands. medium in our own Galaxy and the tech-nique we have employed should tracethe same component as has beenstudied before n so-called reddening’studies.The current analysis uses para-meters of detected galaxies,in similaritywith previous reddening studies,and isnot sensitive to those regions of the skythat may be so heavily obscured that westart to miss objects located behindthese regions.These heavily obscuredregions can be better traced by galaxycounts,a separate project,which is stillin progress.Here,we announce somefirst results of our extinction studies,inparticular a 3-degree resolution extinc-tion map of the Southern Sky with a o(Ag)≈0.13"and o(E(B-V)≈0.06". by at least a factor two,and is thoughtto deviate more locally.This motivatedBurstein and Heiles to include alsogalaxy counts,when deriving their ex-tinction model for the Northern Hemi-sphere,but this could unfortunately notbe done for the South.Motivated by therecent availability of the photometricdata in the ESO-LV catalogue,we haveundertaken some statistical experi-ments to see whether the data wouldallow an improvement of this situation. ## 2.The Technique The hitherto most frequently usedmodel for the extinction in the SouthernGalactic hemisphere(Burstein,Heiles)isexclusively based on the mapping ofneutral hydrogen.Since this mappingwas done with a rather poor spatialsampling,the resulting extinction mapwas grossly dominated by interpolationsof data.Furthermore,using HI data re-lies on a constant HI-to-extinction ratio.The overall value of this ratio is uncertain IB AB Figure 3:The idea of extinction measure-ments using the surface brightness of galax-ies in two bands. Although the surface brightness of anexternal galaxy at a particular singleband should respond in a linear way tothe extinction in the Galaxy,such single-band surface brightness values are notgood extinction indicators,since theyhave a too large scatter around themean value(o≈0.6 mag).If however,we have a sample of galaxies withmeasured surface brightness in twobands(B and R for ESO-LV),then wecan take into account the fact that theyare strongly correlated with each other(Fig.2);as a measure of extinction ofeach ndividual galaxy,we can use itsdistance from the average regressioncurve of unreddened surface bright-nesses,measured along the directio

FAQs

What observational data and catalogue are used to derive the extinction map?

The study uses about 12,000 homogeneously selected ESO-Uppsala galaxies from the “Surface Photometry Catalogue of the ESO-Uppsala Galaxies” (ESO-LV), with surface brightness measurements in the B and R bands.

How does the technique use surface brightness in two bands to estimate extinction?

Single-band surface brightness has too much scatter, so the analysis uses the strong correlation between B and R bands. It measures each galaxy’s displacement from the unreddened regression curve along the reddening vector, solved simultaneously with extinctions using an “analysis of variance” method.

What resolution and uncertainty characterize the produced extinction map?

Extinction values are averaged inside 3°×3° pixels and displayed with 1.5×1.5 pixels. Reported typical uncertainties include about σ(Ag)≈0.13 and σ(E(B-V))≈0.06 for the announced first results.

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