Multi-wavelength study of star formation In nearby galaxies

Abstract

The galaxies in the Universe are changing constantly through secular evolution, mergers and interactions. Recent star forming activities in galaxies convey important message about the current evolutionary state of these systems. The study of star formation can be explored well in galaxies, which are nearby, where star forming regions are resolved up to smaller scales. This thesis is aimed to understand the nature of star formation and young star forming regions in four nearby galaxies, which are part of the Local Volume. Among the four selected galaxies, two (WLM and IC 2574) are dwarf irregular and two (NGC 7793 and NGC 300) are spiral galaxies. We primarily used FUV and NUV imaging observations to identify and characterise young star forming regions in each galaxy, supplemented with multi-wavelength data. The major part of the UV data is acquired using the Ultra-Violet Imaging Telescope (UVIT). In order to explore the properties of neutral interstellar medium (ISM) and its connection with the active star forming regions, we used H I intensity maps. We also used other multi-wavelength data, such as Hα, optical, infra-red and molecular CO in specific cases, to understand the properties of star forming regions in more detail.

Using UVIT data, we identified several possible OB associations with sizes between 4 - 50 pc in the galaxy WLM. The UV, Hα and H I multi-wavelength data together helped to infer a vigorous recent star formation in the western part of the galaxy. Our study concludes that being a dwarf system, WLM is forming stars quite efficiently in the recent times. In the galaxy IC 2574, we found 28.6% of the identified FUV bright star forming regions to be located in H I shells, 12.6% inside H I holes and 60.1% to be away from any H I hole. Cross-matching with Hα emission, we found that 23 H I holes have both FUV and Hα emission in their shells signifying very recent trigger. Therefore, star formation in IC 2574 has been partly triggered due to the expanding H I holes, whereas in majority of the sites, it is driven by other mechanisms.

Both the spiral galaxies, NGC 7793 and NGC 300, studied in this thesis are members of the nearby sculptor group. Utilising multi-band data, we found that the disk of both the galaxies are more extended towards the shorter wavelengths. The young star forming clumps identified in the galaxy NGC 7793 have radii between 12 - 70 pc, which is similar to the size of giant molecular clouds identified in the galaxy. The clumps younger than 10 Myr are specifically found along the flocculent arms, which signifies the enhancement of recent star formation along the arms in NGC 7793. In the galaxy NGC 300, we studied the stellar populations in the outer disk of the galaxy between the radii 5.3 kpc and 10 kpc (1 ∼ 2 R25) and noticed an enhancement of star formation during the last 25 Myr. The UV emission beyond the R25 radius has contribution from most of the low mass sources and is extended up to ∼ 2R25 radius. We conclude that NGC 300 has an extended UV disk, mainly populated by young low mass sources.

Our study brings out the importance of UV images in tracing the recent star formation in galaxies. It highlights the unique capability of the UVIT to resolve star forming regions up to size ∼ 10 pc for a distance up to 4 Mpc and study their properties. Our study also demonstrated how it can be useful to identify the triggering mechanism, if we combine the UV images with Hα images and H I and CO maps.