Massive star formation in the central regions of spiral galaxies

Abstract

Context. The morphology of massive star formation in the central regions of galaxies is an important tracer of the dynamical processes that govern the evolution of disk, bulge, and nuclear activity. Aims. We present optical imaging of the central regions of a sample of 73 spiral galaxies in the Hα line and in optical broad bands, and derive information on the morphology of massive star formation. Methods. We obtained images with the William Herschel Telescope, mostly at a spatial resolution of below one second of arc. For most galaxies, no Hα imaging is available in the literature. We outline the observing and data reduction procedures, list basic properties, and present the I-band and continuum-subtracted Hα images. We classify the morphology of the nuclear and circumnuclear Hα emission and explore trends with host galaxy parameters. Results. We confirm that late-type galaxies have a patchy circumnuclear appearance in Hα, and that nuclear rings occur primarily in spiral types Sa-Sbc. We identify a number of previously unknown nuclear rings, and confirm that nuclear rings are predominantly hosted by barred galaxies. Conclusions. Other than in stimulating nuclear rings, bars do not influence the relative strength of the nuclear Hα peak, nor the circumnuclear Hα morphology. Even considering that our selection criteria led to an over-abundance of galaxies with close massive companions, we do not find any significant influence of the presence or absence of a close companion on the relative strength of the nuclear Hα peak, nor on the Hα morphology around the nucleus.

On the morphology of sigma-drop galaxies

“…Both subsamples have galaxies with types ranging between zero and six. In Knapen et al (2006) galaxies with no circumnuclear emission tend to be early type and patchy emission tends to be in later-type galaxies which is coherent with what we have found in the two subsamples. Diffuse emission is also more prevalent in the control sample.…”

“…As the diffuse emission is an intermediate case between no emission and patchy emission, the overabundance of this type in the control sample is probably due to the fact that it contains earlier-type galaxies. But in Knapen et al (2006) the nuclear starburst ring distribution peaks around T = 1, so the control sample should have a higher number of nuclear rings, which is not the case. According to the statistics of Knapen et al (2006), corrected for the nuclear ring fractions reported there for each morphological type of the host galaxy, four of our σ-drop galaxies and six control galaxies should have a circumnuclear ring -the actually observed numbers are five and zero.…”

“…But in Knapen et al (2006) the nuclear starburst ring distribution peaks around T = 1, so the control sample should have a higher number of nuclear rings, which is not the case. According to the statistics of Knapen et al (2006), corrected for the nuclear ring fractions reported there for each morphological type of the host galaxy, four of our σ-drop galaxies and six control galaxies should have a circumnuclear ring -the actually observed numbers are five and zero. Therefore it is reasonable to think that the higher proportion of circumnuclear star-forming rings in σ-drop galaxies is a true effect correlated with the σ-drop.…”

“…In the absence of Hα line emission, the number of counts in the narrow-band filter will be proportional to the number of counts in the continuum, and the constant of proportionality is the factor by which the continuum has to be scaled before subtracting from the narrow-band image. Since even for a star-forming galaxy most pixels trace only continuum emission, those pixels without Hα emission can easily be recognised in the graph, and a fit to them yields the continuum scaling factor (see Knapen et al 2005Knapen et al , 2006, for details and a graphic illustration). In most cases, there is a contribution from the [N ii] line in the Hα images but this is not a problem because we do not use the Hα images for photometry, and [N ii] emission is unlikely to affect the morphology on the scales we are interested in here.…”

Supernovae without host galaxies?

“…This is of course a much poorer assumption than the starting point being the center of the galaxy because the star would then have had to have formed in the direct vicinity of the central SMBH where, in non-active galaxies, not much material which is sufficient for massive star formation usually is present (e.g. Knapen et al 2006). Hence, this assumption causes the travel time to become an upper limit which then causes the derived velocity to be a lower limit.…”