Central Star Formation Rate Research Articles

Hα Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies

We describe the various Hα morphologies of Virgo Cluster and isolated spiral galaxies and associate the Hα morphologies with the types of environmental interactions that have altered the cluster galaxies. The spatial distributions of Hα and R-band emission are used to divide the star formation morphologies of the 52 Virgo Cluster spiral galaxies into several categories: normal (37%), anemic (6%), enhanced (6%), and (spatially) truncated (52%). Truncated galaxies are further subdivided on the basis of their inner star formation rates into truncated/normal (37%), truncated/compact (6%), truncated/anemic (8%), and truncated/enhanced (2%). The fraction of anemic galaxies is relatively small (6%-13%) in both environments, suggesting that starvation is not a major factor in the reduced star formation rates of Virgo spiral galaxies. The majority of Virgo spiral galaxies have their Hα disks truncated (52%), whereas truncated Hα disks are rarer in isolated galaxies (12%). Most of the Hα-truncated galaxies have relatively undisturbed stellar disks and normal to slightly enhanced inner disk star formation rates, suggesting that intracluster medium-interstellar medium (ICM-ISM) stripping is the main mechanism causing the reduced star formation rates of Virgo spiral galaxies. Several of the truncated galaxies are peculiar, with enhanced central star formation rates, disturbed stellar disks, and barlike distributions of luminous H II complexes inside the central 1 kpc but no star formation beyond, suggesting that recent tidal interactions or minor mergers have also influenced their morphology. Two highly inclined Hα-truncated spiral galaxies have numerous extraplanar H II regions and are likely in an active phase of ICM-ISM stripping. Several spiral galaxies have one-sided Hα enhancements at the outer edge of their truncated Hα disks, suggesting modest local enhancements in their star formation rates due to ICM-ISM interactions. Low-velocity tidal interactions and perhaps outer cluster H I accretion seem to be the triggers for enhanced global star formation in four Virgo galaxies. These results indicate that most Virgo spiral galaxies experience ICM-ISM stripping, many experience significant tidal effects, and many experience both.

Star Formation Properties of a Large Sample of Irregular Galaxies

We present the results of Hα imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters, including integrated absolute magnitude (MV of -9 to -19), average surface brightness (20–27 mag arcsec-2), current star formation activity (0–1.3 M⊙ yr-1 kpc-2), and relative gas content (0.02–5 M⊙/LB). The Hα images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthally averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 104 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other cataloged galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H II regions in these galaxies are most often found within the Holmberg radius RH, although in a few systems H II regions are traced as far as 1.7RH. Similarly, most of the star formation is found within three disk scale lengths RD, but in some galaxies H II regions are traced as far as 6RD. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops with radius. In approximately half of the Im galaxies Hα and V correspond well, and in the rest there are small to large differences in the relative rate of falloff with radius. The cases with strong gradients in the LHα/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to migrate to the center in the last gigayear. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or H I clouds. There is now a substantial amount of evidence for these processes among many surveys of BCDs. We note that such gas migration will also increase the local pressure and possibly enhance the formation of massive dense clusters but conclude that the star formation process itself does not appear to differ much among BCD, Im, and Sm types. In particular, there is evidence in the distribution function for Hα surface brightness that the turbulent Mach numbers are all about the same in these systems. This follows from the Hα distribution functions corrected for exponential disk gradients, which are log-normal with a nearly constant dispersion. Thus, the influence of shock-triggered star formation is apparently no greater in BCDs than in Im and Sm types.

Sustained star formation in the central stellar cluster of the Milky Way

THE central stellar bulge of our Galaxy is commonly thought to consist predominantly of a coeval population of very old (~1010yr) stars1–3. But evidence for young4 and intermediate-age stars5,6 in the bulge is accumulating. Here we argue that star formation has been occurring in the molecular gas near the centre of the bulge throughout the lifetime of the Galaxy, and that the resulting stellar population is evident as the prominent infrared7,8 'r−2 central star cluster' (a cluster whose space density decreases as the square of the distance from the centre). Although our hypothesis is at odds with the standard view that this central cluster is the innermost part of the more extended and ancient bulge9–11, sustained star formation is consistent with both the available observational data and simple models of mass inflow and collisionally induced star formation in this region. Even if the central star-formation rate is relatively modest, it appears that the stellar population of the Galactic bulge has been augmented substantially since its birth.