DIISC-II: Unveiling the Connections between Star Formation and Interstellar Medium in the Extended Ultraviolet Disk of NGC 3344

We present our investigation of the extended ultraviolet (XUV) disk galaxy, NGC 3344, conducted as part of Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium survey. We use surface and aperture photometry of individual young stellar complexes to study star formation and its effect on the physical properties of the interstellar medium. We measure the specific star formation rate (sSFR) and find it to increase from 10−10 yr−1 in the inner disk to >10−8 yr−1 in the extended disk. This provides evidence for inside-out disk growth. If these sSFRs are maintained, the XUV disk stellar mass can double in ∼0.5 Gyr, suggesting a burst of star formation. The XUV disk will continue forming stars for a long time due to the high gas depletion times (τ dep). The stellar complexes in the XUV disk have high-ΣH I and low-ΣSFR with τ dep ∼ 10 Gyr, marking the onset of a deviation from the traditional Kennicutt–Schmidt law. We find that both far-ultraviolet (FUV) and a combination of FUV and 24 μm effectively trace star formation in the XUV disk. Hα is weaker in general and prone to stochasticities in the formation of massive stars. Investigation of the circumgalactic medium at 29.5 kpc resulted in the detection of two absorbing systems with metal-line species: the stronger absorption component is consistent with gas flows around the disk, most likely tracing inflow, while the weaker component is likely tracing corotating circumgalactic gas.

Distribution of very young stellar clusters in grand-design spirals

Many grand-design spiral galaxies display strings of knots along their arms on K-band images. Near-infrared (NIR) spectra and broad-band colours of such knots have identified them as very young, massive stellar complexes. The low absorption in the NIR makes it possible to derive complete statistics of such complexes and thereby estimate the associated star-formation rate. We have obtained deep NIR maps of eight grand-design spirals using VLT/HAWK-I and identified massive complexes with ages of < 10 Myr using NIR colour–colour diagrams. The youngest, most massive complexes are well-aligned and concentrated in the arm regions of the grand-design galaxies with strong spiral perturbations. Their absolute magnitudes have a bright tail reaching almost MK = −16 mag. Both the fraction of young to old sources and the ratio of diffuse to more compact objects suggest a dependence on the strength of the spiral pattern in the host galaxy.

Star-Burst Regions in the LMC

Filamentary structures of early type stars are found to be a common feature of the MCs formed ~0.9 − 2 × 108yr ago. As we go to younger ages these large structures appear fragmented and sooner or later form young clusters and associations. In the optical domain we have detected 56 such large structures of young objects, known as stellar complexes in the LMC (Maragoudaki, Kontizas, Kontizas, et al. (1998)) and investigate their properties. IRAS data of the LMC stellar complexes were compared with observations of starburst galaxies (Helou (1986); Lehnert & Heckman (1996)). It has been found that nearly 1/4 of the stellar complexes are extremely active resembling the IR behaviour of starburst galaxies and HII regions. These stellar complexes are called here “starburst regions”. They host an increased number of HII regions, SNRs, stellar associations and nebulae. The main starburst tracers are their IR luminosity F60 and the 8.6-GHz radio emission (8.6-GHz data: Dickel, McIntire, Gruendl, et al. (2005)). In Table 1 the characteristics of the various types of complexes are given. Finally the evolution of all stellar complexes is discussed based on the CO emission (CO data: Fukui, Mizuno, Yamagushi, et al. (1999)). More than 50% of the starburst and starburst candidate regions show enhanced CO emission, indicating ongoing and future evolution.