Signatures of star formation inside galactic outflows

Observations have suggested that galactic outflows contain substantial amounts of dense and clumpy molecular gas, creating favourable conditions for igniting star formation. Indeed, theoretical models and hydrodynamical simulations have suggested that stars could form within galactic outflows, representing a new mode of star-formation that differs significantly from the typical star formation in star forming discs. In this paper, we examine 12 local galaxies with powerful Active Galactic Nuclei and high star-formation rate using spectroscopic data from the X-shooter spectrograph at the Very Large Telescope. We investigate the excitation mechanism and physical properties of these outflows via spatially resolved diagnostic diagrams (along with tests to rule out contribution by shocks and external photoionisation). Out of the seven galaxies with clearly detected outflows, we find robust evidence for star formation within the outflow of one galaxy (IRAS 20551-4250), with two additional galaxies showing tentative signs (IRAS 13120-5453 and F13229-2934). Therefore, our findings support previous results that star formation inside outflows can be a relatively common phenomenon among these active galaxies and may have played an important role in the formation and evolution of the spheroidal component of galaxies.

💡 Research Summary

This paper presents a detailed investigation into the phenomenon of star formation occurring within powerful galactic outflows, a process distinct from traditional star formation in galactic disks. The study focuses on a sample of 12 local luminous and ultra-luminous infrared galaxies (LIRGs/ULIRGs) known to host powerful Active Galactic Nuclei (AGN) and exhibit high star-formation rates. These galaxies were selected based on prior evidence of outflows from VIMOS integral field spectroscopy.

The core of the methodology relies on high-quality, spatially resolved spectroscopic data obtained with the X-shooter spectrograph on the Very Large Telescope (VLT). X-shooter provides superior spectral resolution (R = 3000 - 18000) and sensitivity compared to instruments used in previous similar studies, enabling the detection and clean separation of faint emission from outflowing gas from the bright stellar continuum of the host galaxy. The team extracted spectra at various positions along the slit, moving outward from the galactic center.

The critical analytical step involved using the ppxf software to perform a sophisticated spectral decomposition. This process simultaneously fits the stellar continuum and multiple kinematic components of emission lines, allowing for the isolation of the pure emission line flux associated specifically with the outflowing gas, which often shows blueshifted or redshifted velocities relative to the systemic disk component.

With the purified emission line fluxes from the outflow regions, the researchers constructed spatially resolved Baldwin-Phillips-Terlevich (BPT) diagnostic diagrams. By plotting line ratios like