Edge-on galaxies at ultraviolet wavelengths

Abstract

Recent rest-frame ultraviolet imaging of galaxies at apparently high redshift have revealed a population of very blue galaxies with aspect ratios approaching 30 which have been interpreted to be chains of star formation regions in the initial stages of galaxy formation [1]. We compare these optical observations with rest-frame far-ultraviolet observations of nearby edge-on galaxies with a view to illustrating their noteworthy similarities. For the comparison we use far ultraviolet (FUV) images of NGC 253 (SAB(s)c), NGC 891 (SA (s)b), NGC 3034 (M82)(I0), NGC 4248 (I0), UGC 6697 (Im), and NGC 4631 (SB(s)d) which were recorded by the Ultraviolet Imaging Telescope. We focus primarily on NGC 4631, 7.5 Mpc distant, which in the FUV exhibits a linear beaded structure, an axial ratio of ∼24 and a (λλ 1520 Å)-U color of −0.70. At z≈1.9 (λλ 1520 Å)-U would approximate (B–I) and would appear to be very blue. NGC 4248 and UGC 6697 are additional examples of edge-on galaxies which give the appearance of “chain galaxies” in the far ultraviolet. NGC 891 and NGC 253 illustrate a variation on the theme. These galaxies were barely detectable at λλ 2500 Å, very likely due to extinction. However, their linear morphology is clearly shown. The small starburst galaxy M82 is not completely linear, but would be seen as a very blue object at faint magnitudes. Our conclusion is that at least a portion of the faint blue objects seen at redshifts between 0.5 and 3 are not exotic objects, but are galaxies with familiar morphologies seen in the rest-frame ultraviolet.