Properties of the Magellanic-type spirals. I - Surface photometry of NGC 4618 and NGC 4625

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Abstract

Detailed BVRI broadband surface photometry is presented for the close pair of SBm galaxies NGC 4618 and NGC 4625. These galaxies exhibit clearly the morphological properties of Magellanic spirals, namely, an offset stellar bar and asymmetric one-armed spiral structure. Since the Magellanic spirals represent a transitional stage between the pure disk and irregular galaxies, an understanding of their photometric and kinematic properties will be essential to a full understanding of the physics underlying the Hubble sequence. Standard photometric parameters are derived for both galaxies using image material from a variety of CCD detectors and telescopes, as well as one deep photographic plate. A mean distance of δ=6.3+/-0.6 Mpc is derived using five independent distance indicators. Disk orientation parameters are derived through the use of interactive ellipse fitting routines. It is shown that the bar center in NGC 4618 is offset approximately 0.7 kpc from the disk center. The radial- luminosity profiles are studied in several bandpass regions and it is found that both galaxies exhibit a characteristic segmented profile composed of two linear sections. This feature has been observed in the cases of NGC 4027 and LMC by previous workers. Color distribution maps are constructed in several indices and used to show that a significant amount of massive star formation is occurring in the dominant spiral arm. It is shown that the integrated color of the bar region seems to be excessively blue and may indicate the presence of massive star formation in the bar, possibly resulting from inflow along the bar. Detailed studies of the luminosity distribution along the bar major axis, as well as a Fourier decomposition of the bar luminosity distribution are presented. A two-dimensional luminosity decomposition in the B band shows that 50% of the total light originates in the underlying disk component, 25% from the arm, 15% from the bar and 5%-10% from the discrete components such as OB star complexes and HII regions. The similarity in photometric and morphological properties between these two galaxies is discussed in the light of recent theoretical work dealing with the growth of one-armed spiral modes and bars in late- type galaxy disks. The high frequency of companion galaxies near SBm galaxies such as NGC 4618, and the high frequency of bar formation in late galaxies in general, motivates an interest in the role of tidal interactions in explaining the classical SBm morphology.

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