Stratification of the AGN-Driven multi-phase outflows in the dwarf Seyfert galaxy NGC 4395

TL;DR

This work presents a spatially resolved, multi-phase view of AGN-driven outflows in the dwarf Seyfert NGC 4395 by combining JWST/NIRSpec and MIRI IFU spectroscopy with ALMA CO(2–1) and GMOS optical data. The authors identify 134 emission lines across ionised and molecular gas, decompose line profiles to isolate outflow components, and derive detailed physical conditions (densities, temperatures) and masses for each phase. They uncover a stratified outflow: high-ionisation gas near the AGN shows higher velocities and extinction-affected near-side emission, while cold molecular gas carries the largest mass outflow rate, yielding a complex, multi-phase feedback picture with modest overall kinetic coupling efficiencies. The findings demonstrate that even low-luminosity AGN in dwarf galaxies can drive substantial multiphase outflows, underscoring the importance of high-resolution infrared and millimeter observations for understanding AGN feedback in low-mass systems.

Abstract

We present a multi-wavelength study of nuclear outflows in the nearby dwarf Seyfert galaxy NGC~4395, which hosts an intermediate-mass black hole. Using \textit{JWST}/NIRSpec and MIRI IFU spectroscopy (1.66--28.6~$μ$m), together with ALMA and Gemini/GMOS data, we probe the ionised and molecular gas on parsec scales. The JWST nuclear spectra reveal 134 emission lines, including H\,\textsc{i}, He, numerous fine-structure lines, H$_2$ rotational/ro-vibrational transitions, and several PAH bands. Modelling of the H$_2$ rotational lines reveals three warm/hot molecular components ($T\!\approx\!580$, 1480, and 2900~K), along with a cold ($<50$~K) phase traced by ALMA CO(2--1). Outflow signatures are detected in cold and warm/hot molecular gas, in H\,\textsc{i}, and in 36 fine-structure lines spanning ionisation potentials of 7.6--300~eV. Ionised outflow velocities range from 127 to 716~km\,s$^{-1}$, with blueshifted and redshifted components consistent with a stratified biconical geometry. The cold molecular gas shows a mass outflow rate nearly 1--2 orders of magnitude larger than that of the warm/hot molecular and ionised phases. The kinetic coupling efficiency is 0.003--0.12\% for the coronal-line gas and 0.4--1.4\% for the H\,\textsc{i} outflow, indicating that only the low-ionisation gas significantly impacts the surrounding ISM. Outflow velocity and the fraction of flux in the outflowing component increase with ionisation potential, implying that the most highly ionised gas originates closest to the AGN and is most efficiently accelerated.

Figures (12)

• Figure 1: The aperture-corrected nuclear spectrum of NGC 4395, extracted from the central region, spans the rest-frame wavelength range 1.66--28.6 $\mu$m, combining observations from JWST/NIRSpec and MIRI Channels 1--4. Some of prominent emission lines are indicated along the spectrum. Molecular features, including PAH and H$_2$ lines, together with H$\textsc{i}$ recombination lines, are shown by solid lines, while ionized gas lines are denoted by dashed lines. Low-ionization species (e.g., [Fe ii], [Ne ii], [S iii], [O iv]) are marked in purple, whereas high-ionization lines are indicated in dark green.
• Figure 2: Distribution of the observed FWHM values (uncorrected for instrumental resolution) for the different gas phases: H$_2$, H i, and fine-structure lines.
• Figure 3: Variation of different line ratios with density for different particular temperatures, which represent different curves in each plot and are denoted by individual legends in each plot. The horizontal lines in each plot represents the observed line ratio.
• Figure 4: Variation of [Fe vii]$\lambda$6087 $\AA$ and [Fe vii]$\lambda$7.81 $\mu$m line ratio with electron temperature for different particular electron densities, which represent different curves in each plot and are denoted by individual legends in each plot. The horizontal line in the plot represents the observed line ratio.
• Figure 5: Upper panel: Rotational excitation diagram of the pure molecular hydrogen emission lines. The solid lines indicate the best-fit models, while the points represent the observed data. The residual panels show the normalized residuals, defined as (data$-$model)/data. Lower panel: Lower panel: Excitation diagram of the H$_2$ ro-vibrational line ratios, H$_2$(2--1) S(1)/H$_2$(1--0) S(1) versus H$_2$(1--0) S(2)/H$_2$(1--0) S(0). Ratios with H$_2$(2--1) S(1)/H$_2$(1--0) S(1) $\lesssim 0.2$ indicate predominantly thermal excitation, while ratios $\gtrsim 0.5$ are characteristic of non-thermal excitation dominated by UV fluorescence 1994ApJ...427..777M. The black points connected by dotted lines represent LTE predictions for temperatures between 1000 and 4000 K 2021MNRAS.506.2950R. The pink shaded region corresponds to AGN photoionization 2012MNRAS.422..252D, the grey diamond symbols mark shock-excited regions 1977ApJ...216..713K1995AA...296..789S, and the blue shaded region denotes thermal UV excitation 1989ApJ...338..197S. The brown star symbols indicate thermal X-ray excitation 1990ApJ...363..464D. The solid black square with error bars represents the measured line ratios for NGC 4395.