Spiral arms in near-infrared bands^*

Broad- and narrow-band NIR photometry

¹ Research Center for Astronomy, Academy of Athens, 14 Anagnostopoulou St., 10673, Athens, Greece
² Astronomisches Rechen-Institut Heidelberg, Mönchhofstrasse 13, 69120 Heidelberg, Germany
³ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Corresponding author: P. A. Patsis, ppatsis@cc.uoa.gr

Received: 31 October 2000
Accepted: 15 February 2001

Abstract

We investigate the contribution of Brγand H₂ emission due to young objects in the arms of spiral galaxies observed in the filter. Out of a sample of disk galaxies for which we obtained deep surface photometry in broad- and narrow-band near-infrared filters, we selected two grand design spirals (NGC 5861, NGC 7412), which clearly have sharp knots along their arms both in optical and NIR images. For these galaxies we estimate the amount of light coming from Brγand H₂ emission and we conclude that it represents only a few percent of the observed light. For comparison we used the spiral galaxy NGC 4603, which has high recessional velocity. In this case the emission lines we study are practically shifted out of the narrow-band filter. Comparing its flux with what we found in the two former cases, we conclude that a major contribution from young objects in comes from continuum radiation which in the arm regions can amount to 20% .