Spectroscopic Study of Blue Compact Galaxies II. Spectral Analysis and Correlations

TL;DR

This study analyzes spectral data from 97 blue compact galaxies to explore their emission line properties, metallicities, and star formation indicators, revealing correlations between spectral features, luminosity, and metallicity.

Contribution

It provides a detailed spectral analysis of a large BCG sample, identifying key emission line trends and relationships useful for understanding galaxy evolution.

Findings

Equivalent widths correlate with galaxy luminosity.

Anti-correlation between [NII]/Hα ratio and Hα equivalent width.

Fluxes of certain emission lines are interconnected and serve as star formation tracers.

Abstract

This is the second paper in a series studying the star formation rates, stellar components, metallicities, and star formation histories and evolution of a sample of blue compact galaxies. We analyzed spectral properties of 97 blue compact galaxies, obtained with the Beijing Astronomical Observatory (China) 2.16 m telescope, with spectral range 3580\AA -- 7400\AA. We classify the spectra according to their emission lines: 13 of the total 97 BCGs sample are non-emission line galaxies (non-ELGs); 10 have AGN-like emission (AGNs), and 74 of them are star-forming galaxies (SFGs). Emission line fluxes and equivalent widths, continuum fluxes, the 4000 \AA Balmer break index and equivalent widths of absorption lines are measured from the spectra. We investigate the emission line trends in the integrated spectra of the star-forming galaxies in our sample, and find that: 1) The equivalent widths of emission lines are correlated with the galaxy absolute blue magnitude $M_B$; lower luminosity systems tend to have larger equivalent widths. 2) The equivalent width ratio [NII]6583/H$\alpha$ is anti-correlated with equivalent width H$\alpha$; a relationship is given that can be used to remove the [NII] contribution from blended H$\alpha$ + [NII]6548, 6583. 3) The [OII], H$\beta$, H$\gamma$ and H$\alpha$ fluxes are correlated; those can be used as star formation tracers in the blue. 4) The metallicity indices show trends with galaxy absolute magnitude and attenuation by dust; faint, low-mass BCGs have lower metallicity and color excess.