Characteristic differences between star formation in spiral galaxies and irregular galaxies identified by scientists has raised important questions regarding the role of dark matter in regulating star formation and the growth of gravitational instabilities.
Gravitational instabilities are at the heart of different physical processes like tidal interaction, galaxy mergers, fragmentation of the gas disc, turbulence, and feedback which play an important role in shaping the observed morphology of the galaxies. The mystery of how gravitational instabilities are connected with the star formation rate has intrigued scientists for long.
K. Aditya, a post-doctoral fellow at the Indian Institute of Astrophysics (IIA), an autonomous institute of the Department of Science and Technology (DST) tried to explore how gravitational instabilities are connected with the star formation, gas fraction, time scale for growth of gravitational instabilities and finally the observed morphology. They then compared the star formation rate, gas fraction, time scale for growth of gravitational instabilities of nearby galaxies and investigated the stability levels of a sample of 175 galaxies taken from the Spitzer Photometry and Accurate Rotation Curves (SPARC) data base. This helped them trace the role of dark matter in regulating the stability levels of the galaxies and understand if stars and gas can self-regulate the stability levels.
They found that Spiral galaxies like Milky Way exhibit a higher median star formation rate, lower stability, lower gas fraction, and a smaller time scale for the growth of gravitational instabilities. This indicates that gravitational instabilities in spirals rapidly convert a substantial amount of gas into stars, depleting the gas reservoirs. Conversely, irregular galaxies like Large and Small Magellanic Clouds found around the Milky Way, demonstrated a more gradual and prolonged star formation process over extended timescales, which explains the higher gas fraction observed in irregulars.
The study published in the journal Monthly Notices of the Royal Astronomical Society shows that the net stability levels of nearby galaxies are primarily regulated by the stellar disc, highlighting an
inherent self-regulation mechanism. Furthermore, upon isolating the contribution of dark matter to the total potential, the net stability remains unchanged, which raises important questions about the role of dark matter in regulating the stability and star formation in disc galaxies.
The study which compared the stability levels in the nearby galaxies with those observed at high redshift, which are precursors to the galaxies in the local universe could help understand how gravitational instabilities are connected to galaxy evolution.
The results obtained in this work suggest a simple mechanism in which galaxies characterized by marginal stability levels undergo intense star formation activity for a short time scale, depleting the gas reserves. Whereas in galaxies which are highly stable, the star formation proceeds more slowly over longer time scales in galaxies with a relatively higher stability gradually converting the available gas into stars.
The study not only contributes to the understanding of gravitational instabilities but also emphasizes the need for future investigations into the impact of these instabilities on the morphological evolution of galaxies across different redshifts. The Square Kilometer Array (SKA) and James Webb Space Telescope (JWST) hold promise for unraveling this intriguing relationship.
Image Courtesy: HYPERLEDA
Figure 1: Morphological classification of galaxies
Figure 2: In the first panel, we plot the time scale for the growth of gravitational instabilities (て) as a function of gas fraction (fGas). In the second panel, we show (て) as a function of the star formation rate (SFR) and plot (て) as a function of stability parameter (QRW) in the third panel. The vertical dashed line indicates fGas =0.5. Spiral galaxies have fGas < 0.5 and irregulars have a fGas > 0.5. The galaxies are color-coded according to their morphological type [Type 0=S0, 1=Sa, 2 = Sab, 3 = Sb, 4 = Sbc, 5 = Sc, 6 = Scd, 7 = Sd, 8 = Sdm, 9 = Sm, 10 = Im, 11 = BCD]. Types = 0 - 7 are spiral galaxies, and Type = 8 - 11 are irregular galaxies.