The Galactic Central Regions at 3.6 µM

Abstract

We have imaged fields of total area 189 arcmin²within 1° of the Galactic centre in narrow band L (~3.6µm) using UKIRT, with major-axis fields at l = ±1° and minor-axis fields at b = +0.4° and b =–1°.

Differential number counts show a marked asymmetry between at +ve and –ve longitude, with the counts at l =–1° being greater by a factor of ~1.6. We also find that for the two minor axis fields, number counts are nearly the same, despite the expected steep drop-off in numbers between |b|=0.4° and |b|=1.0° However, both these effects could be due to different amounts of extinction of order Δ Av 12. In addition, the b = –1° and l = 1° fields show a clear faint excess number count compared with the other fields, plausibly due to holes in the extinction.

On comparison of the L-band data with DENIS protosurvey K-band data, we deduce that the extinction Av varies between 5 and 40 on a scale of just arcminutes near 1 = –1° consistent with the known presence of large molecular clouds. The size of this effect suggests that the major contribution to observed asymmetries may be an extinction effect rather than a structural, possibly bar-like asymmetry.