The project titled "Investigation of Starbursts in Interacting Spirals" examines the enhancement of star formation rates (SFR) in spiral galaxies undergoing gravitational interactions. Interacting galaxies are known to exhibit significant changes in their physical properties due to mutual gravitational effects, often triggering intense episodes of star formation referred to as starbursts. These starbursts substantially influence the galaxies’ evolutionary trajectories and spectral characteristics.
Context and Motivation
Galaxies evolve through various processes including passive evolution, secular changes, and interactions or mergers with other galaxies. Among these, gravitational interactions and mergers play a pivotal role in shaping galaxy morphology, star formation activity, and gas content. Interacting galaxies often display disturbed morphologies such as tidal tails and bridges—signatures of ongoing interactions. These events channel gas toward the galactic centers, creating favorable conditions for enhanced star formation.
Research Focus
The primary objective of this study was to quantify star formation rates in a selected sample of spiral galaxies, both interacting and isolated, and to identify starburst candidates within them. By comparing star formation indicators across these categories, the research aimed to elucidate the effect of gravitational interactions on star formation.
Data and Sample Selection
The investigation utilized spectral data from the Sloan Digital Sky Survey’s 17th data release (SDSS DR17), an extensive astronomical survey cataloging millions of galaxies. The sample was carefully selected to include spiral galaxies exhibiting clear emission line spectra, specifically requiring prominent Hα, Hβ, [NII], and [OIII] lines, which are essential for accurately calculating star formation metrics and classifying galaxies by ionization source.
From this dataset, a subset of 29 galaxies was chosen, with 10 identified as interacting systems, while excluding edge-on spirals to facilitate clearer detection of starburst activity.
From this dataset, a subset of 29 galaxies was chosen, with 10 identified as interacting systems, while excluding edge-on spirals to facilitate clearer detection of starburst activity.
Methodology
Star formation rates were computed using Hα emission line luminosities, a well-established tracer of recent star formation since Hα emission originates from ionized gas surrounding massive young stars. The study applied the Kennicutt relation to estimate the SFR of each galaxy.
To classify galaxies and distinguish starbursts, the Baldwin-Phillips-Terlevich (BPT) diagnostic diagram was employed. This diagram uses ratios of emission lines ([OIII]/Hβ versus [NII]/Hα) to categorize galaxies into star-forming, composite, active galactic nuclei (AGN), and starburst candidates. The boundaries adopted were based on established literature (Kewley et al. 2001; Kauffmann et al. 2003), enabling empirical classification of the sample galaxies.
To classify galaxies and distinguish starbursts, the Baldwin-Phillips-Terlevich (BPT) diagnostic diagram was employed. This diagram uses ratios of emission lines ([OIII]/Hβ versus [NII]/Hα) to categorize galaxies into star-forming, composite, active galactic nuclei (AGN), and starburst candidates. The boundaries adopted were based on established literature (Kewley et al. 2001; Kauffmann et al. 2003), enabling empirical classification of the sample galaxies.
Results
BPT diagram analysis revealed that approximately 27.6% of the galaxies are likely AGN candidates and thus unlikely pure starburst systems, while 10.3% fell into the composite category with mixed ionization sources. The majority (62%) were classified as star-forming galaxies.
The star formation rates spanned a wide range, with about 72.4% of all galaxies and 90% of interacting galaxies exceeding the threshold (SFR > 10 M⊙ yr⁻¹) designated for starburst classification. Several interacting galaxies demonstrated pronounced starbursts, evidenced by high SFRs, tidal features, and disturbed morphologies.
Specific systems such as NGC 2535 and NGC 4298 exhibited prominent tidal streams indicating gas redistribution that spurs starbursts. High-speed galaxy encounters, as in the Mice galaxies (NGC 4676), triggered shock-induced star formation, an exception to the usual expectation that only slow encounters enhance star formation.
The star formation rates spanned a wide range, with about 72.4% of all galaxies and 90% of interacting galaxies exceeding the threshold (SFR > 10 M⊙ yr⁻¹) designated for starburst classification. Several interacting galaxies demonstrated pronounced starbursts, evidenced by high SFRs, tidal features, and disturbed morphologies.
Specific systems such as NGC 2535 and NGC 4298 exhibited prominent tidal streams indicating gas redistribution that spurs starbursts. High-speed galaxy encounters, as in the Mice galaxies (NGC 4676), triggered shock-induced star formation, an exception to the usual expectation that only slow encounters enhance star formation.
Conclusions
The research concluded that tidal interactions and mergers significantly enhance star formation rates in spiral galaxies, fostering starburst phases. Influential factors include tidal bridges facilitating gas influx, the effects of ram-pressure stripping, and the duration and relative velocity of galactic encounters. The study’s results corroborate previous findings by Di Matteo et al. (2007) and provide further insights into the environmental factors driving galactic evolution.