Signatures of star formation inside galactic outflows

TL;DR

The paper tackles whether star formation can occur inside galactic outflows, a scenario predicted by theory and hinted at in previous observations. It analyzes 12 local (U)LIRGs with powerful AGN using high-resolution X-shooter spectra, performing multi-component fits with pPXF to separate disc and outflow gas, and employing BPT/VO diagnostics to classify ionisation. The results show robust evidence for in situ star formation within the outflow of IRAS 20551-4250 and tentative signs in IRAS 13120-5453 and F13229-2934, while ruling out shock-driven and external photoionisation scenarios; this reinforces the view that SF in outflows is a relatively common phenomenon. Quantitatively, IRAS 20551-4250 hosts an outflow SFR of $5.24 \,\pm\,0.06$ (stat) $\,\pm\,2.62$ (sys) $M_{ ext{O}}$ yr$^{-1}$, implying that outflow SF accounts for a few percent of the host galaxy’s SF budget, with implications for the growth of spheroidal components and circumgalactic/ intergalactic chemical enrichment. Overall, the work strengthens the case that star formation inside galactic outflows is a non-negligible channel in the evolution of AGN-host galaxies and aligns with theoretical predictions and simulations.

Abstract

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.

Figures (26)

• Figure 1: 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 (a) UVB range and (b) VIS range. Shown below the fit are the residuals (blue) from the PPXF fitting and the noise spectrum (magenta) from the observation. Emission lines (He I line in this example) unrelated to the analysis has been masked (grey vertical strip) to avoid interrupting the stellar continuum fitting.
• Figure 2: 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.
• Figure 3: A typical case (4 out of 7 galaxies analysed) where outflows are in the AGN excitation region. The flux ratios of the individual components extracted at different apertures for galaxy NVSSJ151402+015737 are plotted on the [NII] (left), [SII] (middle) and [OI] (right) diagnostic diagrams. 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.
• Figure 4: Same as Fig. \ref{['fig:NVSSJ151402+015737_bpt']}, for the galaxy IRAS 20551-4250, in which case 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.
• Figure 5: The broad components (blue hatched) tend to have lower [SII] flux ratio, hence higher density than the narrow components (orange). The histogram shows 7 galaxies analysed.