Deep photometry of galaxies : to see or not to see /Magnus Näslund

Abstract: The edge-on spiral galaxy NGC 4565 and the distant galaxy cluster MS2255.7+2039 have been studied at optical wavelengths with deep photometry using the 2.56 m Nordic Optical Telescope on La Palma.By careful data reductions we traced the surface brightness of NGC 4565 to faint levels. The vertical colour profiles display a reddening in the dust lane close to the galactic plane and a hint of a reddening at large galactic heights. The presence of the previously detected photometric thick disk was confirmed. The radial colour profiles show a clear transition from the relatively red inner region to the bluer outer parts of the thin disk. This is to some extent explained by the prominent dust lane that is present in this galaxy, which causes a reddening of the inner regions.We have found, with our deep photometry, that there is no wavelength dependence of the apparent disk bending in NGC 4565. We conclude that the evidence in favour of the proposed magnetic-field hypothesis for the generation of warps is weakened.We have confirmed, by tracing the radial luminosity profiles, the clear disk cutoff found in previous studies. Outside the optical disk a faint optical counterpart to the gaseous warp was found. This is, to our knowledge, the first detection of such a structure. This warp was subsequently observed in greater detail in two wavelength bands (B and V). With the colour information, a crude estimate of the age could be achieved by use of the results from stellar-population synthesis.The results obtained for NGC 4565 fit into a picture where infall of gas, continuously or in lumps, is responsible for the warp, and furthermore makes the outer disk bluer by triggering star formation.Modelling of the light distribution revealed no clear signs of an optical halo component, as that detected in NGC 5907. This could in part be explained by the more complex structure of NGC 4565. A mass model was made from three components; a bulge, a disk, and a dark-matter halo. Although the luminous disk is dominating in the inner regions, the extended rotation curve assures that the dark-matter halo provides the largest mass.The luminosity function of the galaxy cluster MS2255.7+2039, at redshift z = 0.288, was determined. By modelling the galaxy population we converted the isophotal magnitudes to total magnitudes. The luminosity function was found to increase steeply at the faint end (M_R > -19), similar to the luminosity functions of more nearby clusters.