Exploring Spatially-Resolved Metallicities, Dynamics and Outflows in Low-Mass Galaxies at $z \sim 7.6$

TL;DR

Using deep JWST/NIRSpec/IFU observations of two low-mass galaxies at $z ~ 7.6$, the paper characterizes spatially resolved ISM properties, metallicities, and gas kinematics, including a Type-II AGN host. The authors find flat strong-line gradients ($-0.01\pm0.01$ and $-0.00\pm0.02$ dex/kpc) but a negative direct-$T_e$ gradient in one system ($-0.11\pm0.03$ dex/kpc) and identify a broad, non-rotational component in [O III]5007 and H that traces outflows with $v_{out} \approx 500$ and $250$ km s$^{-1}$, respectively. They infer ionised outflows with radii $r_{out} \sim 1$ kpc, mass outflow rates $\dot{M}_{out} \sim 14$ and $8$ M_sun/yr, and mass loading factors $\eta \sim 0.3-0.4$, with $v_{out}/v_{esc} \sim 1$, consistent with AGN-driven feedback in dwarf galaxies according to AESOPICA simulations. The results underscore the importance of deep IFU data to disentangle complex high-z kinematics and constrain feedback in the early Universe.

Abstract

A majority of JWST/NIRSpec/IFU studies at high redshifts to date have focused on UV-bright or massive objects, while our understanding of low-mass galaxies at early cosmic times remains limited. In this work, we present NIRSpec/IFS high-resolution observations of two low-mass ($M_* < 10^9 \ M_\odot$), low-metallicity ($[12 + \log(\text{O/H})] < 8$) galaxies at $z \sim 7.66$, one of which we identify as hosting a Type-II AGN. We measure flat strong-line metallicity gradients, suggestive of ISM redistribution by outflows or past merging, but also identify tension with the direct-$T_\text{e}$ metallicity gradient in one galaxy. We measure $v_\text{rot}/σ< 1$ in both galaxies, consistent with observations of lower rotational support at early cosmic times. We identify broad kinematical components decoupled from galactic rotation with velocities of $\sim 250 - 500 \ \text{km} \ \text{s}^{-1}$ and argue these components trace outflows, for which we infer outflow rates of $\sim 8 - 14 \ M_\odot \ \text{yr}^{-1}$ with $v_\text{out}/v_\text{esc} \sim 1$. We compare our findings to results from the new large-volume AESOPICA simulations, which fully incorporate different models of black hole growth and AGN feedback. We find that our observational results of $v_\text{out}/v_\text{esc}$ are consistent with the simulated dwarf AGN population, hinting AGN-driven feedback may contribute to quenching both in our systems and in a wider population of low-mass galaxies in the early Universe. This novel study illustrates the necessity of deep IFU observations to decompose the complex kinematics and morphology of high-$z$ galaxies, trace outflows, and constrain the effect of feedback in the early Universe.

Figures (18)

• Figure 1: Summary of background-subtracted integrated spectra, fitted models and model residual comparisons in both target galaxies. Top panels: Full R2700 integrated spectra from the NIRSpec/IFU observations, extracted from circular apertures of radius 5 pixels (0.25"), centred on the $\forbiddenEL[O][iii]\lambda5007$ brightest spaxel in each galaxy. The background-subtracted integrated spectrum is plotted in blue, and the fitted narrow-line model (see Sec. \ref{['sec:ELF']}) in red. The axes have been cut to eliminate the detector gap in each spectrum from the plots. The spectra shown here have not been continuum-subtracted, as the spectral fitting accounts for the weak continuum with a linear continuum model. Middle panels: A zoom-in on the H and [O][iii][][][4959,][5007] emission lines, showing how these lines are now fitted with the 2-Gaussian model. a) shows the fitted broad component in ID6355 is clearly offset from the narrow line component, suggesting the presence of non-rotational kinematics such as an outflow or merger. The small feature visible blueward of H in the spectrum of ID6355 is a noise feature close to the detector gap, and our analysis based on SNR and BIC also shows that this feature does not meet our criteria for a detected emission line wing. b) shows the offset between the broad and narrow components is less distinct in ID10612, as shown in .Bottom panels: A comparison of narrow-line and 2-Gaussian model $\chi$ residuals, illustrating the necessity of including broad components for H and [O][iii][][][4959,][5007] in our fit.
• Figure 2: Resolved ($\mathrm{SNR}>3$) emission line maps of ID6355. Integrated fluxes, measured per spaxel, are plotted in $\log$ space to illustrate the relative brightness of each line. The magenta hatched ellipse in the bottom left of each plot illustrates the JWST/NIRSpec PSF of the relevant emission line. The red cross on each figure corresponds to the centroid of $\forbiddenEL[O][iii]\lambda5007$ integrated flux. Fluxes are uncorrected for dust attenuation. Top row: Detected forbidden lines, from left to right: $\forbiddenEL[O][ii]\lambda\lambda3727,29$, $\forbiddenEL[Ne][iii]\lambda3869$ and $\forbiddenEL[O][iii]\lambda4363$. Bottom row: Detected Balmer lines, from left to right: H, H and H.
• Figure 3: Resolved ($\mathrm{SNR}>3$) emission line maps of ID10612. Integrated fluxes, measured per spaxel, are plotted in $\log$ space to illustrate the relative brightness of each line. The magenta hatched ellipse in the bottom left of each plot illustrates the JWST/NIRSpec PSF of the relevant emission line. The red cross on each figure corresponds to the centroid of $\forbiddenEL[O][iii]\lambda5007$ integrated flux. Fluxes are uncorrected for dust attenuation. Top row: Detected forbidden lines, from left to right: $\forbiddenEL[O][ii]\lambda\lambda3727,29$, $\forbiddenEL[Ne][iii]\lambda3869$ and $\forbiddenEL[O][iii]\lambda4363$. Bottom row: Detected Balmer lines, from left to right: H, H and H.
• Figure 4: $\forbiddenEL[O][iii]\lambda4363$-based AGN diagnostic diagrams Mazzolari24. ID6355 and ID10612 are plotted as the pink and purple stars, respectively. Orange points correspond to high-$z$ AGN, blue points to low-$z$ SFGs, and grey points to high-$z$ sources not classified as AGN; the sample shown in these plots is compiled from Izotov07Amorin15Dors20Nakajimaempgs22Nakajima23censusKokorev23Ubler23Juodzbalis24Topping24Ubler24 and Scholtz25-COS3018. The Low-OH compilation comprises sources from Izotov06lowzIzotov18Izotov19Berg12 and Pustilnik20Pustilnik21. Orange and blue contours show the distribution of SDSS AGN and SFGs respectively, including the 90%, 70%, 30% and 10% of the populations. Demarcation lines between AGN-only and SFR/AGN regions are plotted as dashed black lines. Fluxes were not corrected for dust attenuation.
• Figure 5: Strong-line metallicity maps of each galaxy calculated from $\forbiddenEL[O][iii]\lambda5007$ PSF-matched cubes, using calibrations from Cataldi25 and the line ratios from Table \ref{['table:diagrat']}, in spaxels where SNR$>3$ for all relevant emission lines. The $\forbiddenEL[O][iii]\lambda5007$ flux centroid of each galaxy is indicated by a black cross on the relevant plot. The morphological centre obtained from PySersic fitting (see) is shown as the white-outlined cross on each plot. The annular regions from which the metallicity gradients are derived (Fig. \ref{['fig:gradmet']}) are highlighted by the red dashed circles on each plot. The average error in metallicity for an individual spaxel is $\sim0.25$ dex for ID6355 and $\sim0.16$ dex for ID10612; hence, both maps are consistent with flat metallicity profiles.