Galactic Renaissance: A Surge in Star Formation During the Evolution of Galactic Collisions

Paper Title: Study of star formation in dual nuclei galaxies using UVIT observations

Authors: K. Rubinur, M. Das, P. Kharb, J. Yadav, C. Mondal, P.T. Rahna

First Author’s Institution: 1Institute of Theoretical Astrophysics, University of Oslo, PO box 1029 Blindern, Oslo 0315, Norway, 2National Centre for Radio Astrophysics-Tata Institute of Fundamental Research (NCRA-TIFR), S. P. Pune University Campus, Ganeshkhind, Pune 411007, India

Paper Status: Published in MNRAS [open access]

Image of Gourab Giri, sitting on grass and looking directly into the camera and smiling.

By Gourab Giri, who is wrapping up his graduation from Indian Institute of Technology as Prime Minister’s Research Fellow and preparing to join as SARAO postdoc fellow in South Africa. His broader interests encompass galaxy evolution, including topics such as the process of galaxy collisions and the feedback of jets on galaxies.

Collision of Galaxies

Two fundamental galaxy types with unique traits characterise the universe: spiral galaxies, showcasing youthfulness with features like dust, young stars, and gas; and early-type galaxies, encompassing elliptical and S0 types, typically older and exhibiting a reddish appearance due to ageing stars and reduced gas content. In the hierarchical model of galaxy evolution that explains the contemporary universe, galaxies are expected to evolve through galactic collisions. This prediction aligns with observations, particularly in the local universe, where collisions between galaxies occur frequently. Mergers play a crucial role in altering both the morphological appearance (e.g., deforming spiral arms or converting spirals into early-types) and intrinsic properties (e.g., enhancing star formation through the supply of fresh gas) of the galaxies involved.

Two pictures of galaxies. the one on the left shows a spiral galaxy with asymmetric arms. The one on the right has an image of a galaxy with zoomed frames showing the center of the galaxy with two bright spots, which are the two nuclei
Fig. 1: Illustration of galactic systems undergoing interactions: (left) the galaxy NGC 2623, representing the relic of ongoing merger between two spirals, showing asymmetric stellar and gaseous structures, a distinctive feature for detecting galaxy mergers. (Right) the highly disturbed galaxy NGC 6240, captured by the Hubble Space Telescope (background image) and Chandra X-ray Telescope (zoomed frames), results from the merger of two gas-rich spiral galaxies, giving rise to dual nuclei in the centre. Credits: ESA/Hubble & NASA (left), NASA/CXC/MPE/S.Komossa et al. & NASA/STScI/R.P.van der Marel & J.Gerssen (right).

Identifying galaxy merger systems relies on the observation of two key characteristics through imaging studies (Fig. 1). The first involves detecting asymmetric stellar or gaseous structures in and around the galaxy. The merger is anticipated to disrupt the galactic structure by twisting the spiral arms, resulting in deformed streams of stars, a broad fan of stars and gas, and shell-like features (see Fig.1). Although the detection of asymmetric structures is effective in the local universe, the faint emission property of such structures (< 10% of the host galaxy intensity) limits its utility in understanding mergers in the distant universe.

The second entails identifying dual nuclei separated by tens of kiloparsecs or less (one galaxy is expected to host one nucleus of compact stars and gas). In contrast to the first characteristic, the identification of dual nuclei proves to be a more effective tool in understanding the distant universe. 

Understanding merging systems, in a broader context, holds the potential to answer the important question of how galaxies have evolved throughout cosmic time, shaping the present-day universe that we observe. However, much remains to be unknown about these systems. We still don’t know how gas cloud collisions and shocks induced by mergers trigger star formation, whether there are morphological distinctions between head-on collisions and in orbit interactions of galaxies, and how certain supermassive black holes in merging systems get active and start accreting matter at an accelerated rate. Today’s paper delves into the study of star formation rates in nine dual-nuclei galaxies, aiming to enhance our understanding of the intrinsic star formation history of such interacting galactic systems.

Imaging and Challenges

Scientists from India, Norway, and Spain have collaborated to utilise India’s first space-based astronomical observatory, AstroSat, to observe galaxies with dual nuclei in the ultraviolet (UV) wavelength. Given the study’s focus on monitoring star formation activity in these merging systems, the selection of ultraviolet light is ideal considering that massive stars emit a significant portion of their radiation in this wavelength range from burning at temperatures nearly twice as high as our sun. 

However, the analysis of such data to extract scientific insights is often challenging due to associated physical processes that can interfere with the targeted investigation. For instance, UV emissions from stars are frequently absorbed by surrounding dust particles, leading to emission in the infrared wavelength from these particles as they release their excitation energy (this is called extinction). This phenomenon can also occur as light travels through our own galaxy, the Milky Way. Properly accounting for the extinction of UV light, the authors observed that the estimated star formation rate increased by 2 to 48 times higher for different galaxies in the sample in comparison to situations where appropriate correction was not applied. 

Moreover, as previously discussed, certain supermassive black holes at the cores of galaxies are triggered during mergers, leading to active accretion of matter. In this phase, they also contribute to UV emission, which needs to be subtracted for a fair estimation of star formation. This can be done by masking out the UV contribution of the core region. However, beforehand, one needs to identify which cores are showing such active phases. The authors accomplished this by analysing the infrared colour of the source, which is primarily the detected emission difference between two infrared observing bands of WISE telescope. This difference would be higher (redder) for AGNs and lower (bluer) for star-forming cores with respect to a standard value. 

To name a few other corrections, one must also subtract the background UV photons generated by uncorrelated background sources, as well as accurate counting of the photons originating from the galaxy, accounting for both disturbed and symmetric shapes.

Set of 3 plots with images of this galaxy. On left, a plot that shows the spiral arms of the galaxy, with more flux at the center from the dual nuclei, in the center a plot of the same galaxy, but in near UV. On the right is a plot which shows more flux in far UV at the center of the galaxy (red) and more flux in near UV away from the center (blue)
Fig. 2: (Left) Image of a galaxy (MRK 306) taken from the SDSS telescope, displaying less compact spiral arms and the core region with dual nuclei N1 and N2. (Middle) The same galaxy observed through AstroSat in the (near) UV wavelength, with white ellipses representing star-forming regions. (Right) The colour map (i.e., emission difference between far and near UV bands), indicating a bluer colour with active star formation in the outskirts, whereas the cores exhibit a redder colour, indicating the presence of primarily older stars. Images adopted from Figure 2 of the article discussed here.

Mergers Triggering Galactic Renaissance

Addressing the above challenges, the study offered insights into the star formation activities and associated effects on dual-nuclei galaxies. This was achieved by identifying star-forming clumps or regions, and analysing the colour of the galaxies in UV bands (i.e., emission difference in Far-UV (130-180 nanometres) and Near-UV wavelengths (200-300 nanometres); similar to the infrared colour estimation mentioned above). The authors have detected a higher rate of star formation occurring in these galaxies, with a yearly production rate of up to 32 solar masses (a number often associated with starburst galaxies). Several galaxies have also been found to show many star-forming regions (sometimes as many as 20) and the largest one extending nearly 140,000 light years in size. An interesting point here is that this increased star formation activity is primarily happening in the disk areas far from the nucleus of the galaxy, as has also been observed in the colour maps with a bluer appearance in the outskirts of the galaxy. This is interesting because it infers that dual-nuclei galaxies are an ongoing process of merger, supplying gas and dust in the outskirts, thereby boosting star formation in such regions. Meanwhile, the central region continues to harbour older stellar populations. 

This is a pilot study, and nine galaxies may not constitute a statistically complete sample to predict any general trend in such systems. However, this study highlights the importance of high-resolution mapping when studying these systems. AstroSat provides three times better resolution than other UV telescopes, such as GALEX. Without such high-resolution, faraway galaxies may not be studied comprehensively. 

Astrobite edited by Jessie Thwaites

Featured Image credit: From Figure 2 of Rubinur et al. 2024.

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