The brightest superassociations in spiral and irregular galaxies as extragalactic distance indicators

Abstract

The brightest blue knots (superassociations) detected by composite U,B,V color photography in spiral and irregular galaxies have absolute magnitudes B<SUB>1</SUB> ranging from -10 to -15, which are from 3 to 5 magnitudes brighter than the brightest single stars. The absolute magnitude of the brightest blue knot M<SUB>1</SUB><SUP>0</SUP> (corrected for galactic extinction) depends on the corrected color index (B-V)<SUB>T</SUB><SUP>0</SUP> and luminosity index Λ<SUB>c</SUB> of the galaxy, being brighter in the bluer and most luminous galaxies. In a first approximation a two-parameter solution gives <P />M<SUB>1</SUB><SUP>0</SUP> = M<SUB>1</SUB><SUP>C</SUP> + 2(Λ<SUB>c</SUB> -1)+2.5 [(B-V)<SUB>T</SUB><SUP>0</SUP>-0.5] <P />where 1<SUP>C</SUP>&gt; = -13.4 ± 0.2 from a sample of 78 galaxies having known distance moduli 18.3 &lt; μ<SUB>0</SUB> &lt; 32.0. In a second approximation allowing for internal extinction a three parameter solution gives <P />M<SUB>1</SUB><SUP>0</SUP> = M<SUB>1</SUB><SUP>*</SUP> +2(Λ<SUB>c</SUB> -0.8)+2.8 [(B-V)<SUB>T</SUB><SUP>0</SUP> -0.5] +0.7 (log R<SUB>25</SUB> -0.2) <P />where R<SUB>25</SUB> is the isophotal axis ratio of the galaxy and 1<SUP>*</SUP>&gt; &gt; = -13.0 ± 0.2. Distances derived from B<SUB>1</SUB> and M<SUB>1</SUB><SUP>C</SUP> or M<SUB>1</SUB><SUP>*</SUP> are in good systematic agreement with distances derived from primary, secondary and tertiary indicators, but with a rather large dispersion (σ ≅ 0.55 mag) due in part to errors in the preliminary photometry (σ ≅ 0.4 mag). With some refinements and better photometry this new distance indicator appears capable of providing distance moduli with mean errors ≲0.5 mag out to distances in excess of 100 megaparsecs.