Star Formation in Galactic Outflows

Evidence for star formation inside galactic outflows from X-shooter observations

Galactic outflows, powerful streams of gas expelled from galaxies, have long been thought to suppress star formation. However, recent theoretical models and simulations suggest that these outflows may contain dense, clumpy molecular gas—creating favorable conditions for a new mode of star formation. In this study, we examined 12 local galaxies with powerful Active Galactic Nuclei (AGN) using high-resolution spectroscopic data from the X-shooter spectrograph at the Very Large Telescope to search for observational evidence of star formation within outflows.

An example of simultaneously fitting the stellar continuum (red) and emission lines (orange) of the spectrum (black) of galaxy IRAS 20551-4250 extracted from the central region in the UVB range. Shown below the fit are the residuals (blue) from the PPXF fitting and the noise spectrum (magenta) from the observation.

Multi-component Spectral Analysis

We performed detailed spectral decomposition using multiple Gaussian components to separate emission from the galactic disk and outflowing gas. This allows us to independently measure the physical properties and excitation mechanisms in different kinematic components.

Subsections of continuum-subtracted X-shooter spectra (black) of the galaxy IRAS 20551-4250, extracted from the central region around the relevant emission lines for BPT-diagnostics, displaying the decomposition by 4 Gaussian components (see legend) representing the narrow components associated with the gas in the galactic disk and the broad components tracing the outflows. For clarity of presentation, the best-fit stellar continuum is subtracted in this figure. However, the full spectral fit with ppxf was performed on the stellar continuum and emission lines simultaneously.

BPT Diagnostic Diagrams

Using Baldwin-Phillips-Terlevich (BPT) diagnostic diagrams, we classified the ionization mechanism of gas in different regions. These diagrams allow us to distinguish between photoionization by young stars (star formation) versus harder radiation from AGN.

The flux ratios of the individual components extracted at different apertures for galaxy IRAS 20551-4250 are plotted on the [NII] (left), [SII] (middle) and [OI] (right) diagnostic diagrams. Most of the outflowing components are located in the star forming region of the diagram, especially in the [SII]-BPT and [OI]-BPT, which are less affected by metallicity and nitrogen enrichment dependence. The velocity of each component is colour-coded, from red (more redshifted) to blue (more blueshifted). The symbols are proportional to the FWHM of the component, as indicated by the legend above. The black line contours indicate the distribution of galaxies and AGN (10, 30, 68, 85 and 95 percent) from the SDSS.

Key Results

Out of seven galaxies with clearly detected outflows, we found:

  • Robust evidence for star formation within the outflow of IRAS 20551-4250
  • Additional candidates showing signatures consistent with outflow-driven star formation
  • Outflowing components predominantly located in the star-forming region of BPT diagrams, particularly in [SII] and [OI] diagnostics which are less affected by metallicity effects

This work provides observational support for theoretical predictions of star formation in galactic outflows, potentially representing a previously unrecognized mode of star formation in the universe.

References