High resolution ALMA observations of H$_2$S in LIRGS (Dense gas and shocks in outflows and CNDs)

TL;DR

The paper investigates how H$_2$S traces dense, possibly shocked gas in the nuclei of three nearby LIRGs, using high-resolution ALMA Band 5 observations of the ortho-H$_2$S 1$_{1,0}$–1$_{0,1}$ line and RADEX radiative transfer modelling to constrain physical conditions. By comparing H$_2$S with archival H$_2$S 2$_{2,0}$–2$_{1,1}$ and CO data, the study finds compact emission ($ obreak ext{≤}150$ pc) with broad wings indicative of outflows or shocks, and infers very high densities $n_{ m H_2} brack > 10^6$–$10^7$ cm$^{-3}$ and $T_{ m kin}$ in the range 40–200 K. The results show that H$_2$S traces denser gas than CO within the same regions, suggesting that grain-surface processing and subsequent desorption in shocks or radiative heating release H$_2$S into the gas phase in galactic nuclei. These findings position H$_2$S as a valuable tracer of dense, shocked gas in nuclear environments and galaxy-scale feedback, while highlighting the need for higher-resolution, multi-line observations to fully characterize H$_2$S chemistry and its relation to outflows and circumnuclear disks.

Abstract

Molecular gas plays a critical role in regulating star formation and nuclear activity in galaxies. Sulphur bearing molecules, such as H2S, are sensitive to the physical and chemical environments in which they reside and are potential tracers of shocked, dense gas in galactic outflows and active galactic nuclei (AGN). We aim to investigate the origin of H2S emission and its relation to dense gas and outflow activity in the central regions of nearby infrared luminous galaxies. We present ALMA Band 5 observations of the ortho H2S 1(1,0) 1(0,1) transition in three nearby galaxies: NGC 1377, NGC 4418, and NGC 1266. We perform radiative transfer modelling using RADEX to constrain the physical conditions of the H2S emitting gas and compare the results to ancillary CO and continuum data. We detect compact H2S emission in all three galaxies, arising from regions smaller than approximately 150 parsecs. The H2S spectral profiles exhibit broad line wings, suggesting an association with outflowing or shocked gas. In NGC 4418, H2S also appears to be tracing gas that is counterrotating. A peculiar red shifted emission feature may correspond to inflowing gas, or possibly a slanted outflow. RADEX modelling indicates that the H2S emitting gas has high densities (molecular hydrogen density greater than 10^7 cm^-3) and moderately warm temperatures (between 40 and 200 Kelvin). The derived densities exceed those inferred from CO observations, implying that H2S traces denser regions of the interstellar medium.

Figures (9)

• Figure 1: The results of 168 GHz observations towards NGC 1377. (a) The velocity-integrated line emission maps of H$_2$S 1$_{1,0}$-1$_{0,1}$: black contours: -70 km s$^{-1}$ to 70 km s$^{-1}$ (3,9,27, and 36 $\sigma$ where $\sigma_{\rm core} = 6.94 \times 10^{-3}$ Jy beam$^{-1}$ km s$^{-1}$), blue: -200 km s$^{-1}$ to -100 km s$^{-1}$, red: 100 km s$^{-1}$ to 200 km s$^{-1}$ (3,4, and 6 $\sigma$ where $\sigma_{\rm red} = 9.35 \times 10^{-3}$ Jy beam$^{-1}$ km s$^{-1}$ and $\sigma_{\rm blue} = 9.38 \times 10^{-3}$ Jy beam$^{-1}$ km s$^{-1}$, respectively). The beam size is shown at the bottom-left in grey-filled circle. The blue and red line segments indicate the position angle (PA) of the disk. The PA of the nuclear continuum is 90$^{\circ}$ (scales of r=2 pc) and also the orientation of the dynamics of the nuclear disk is close to 90$^{\circ}$Aalto2020ALMA1377. Aalto2020ALMA1377 note that this is different from the 104$^{\circ}$ found for the major axis on larger scales of r=10 pc Aalto2016. The direction of the molecular outflows detected in CO 3-2 Aalto2016 is indicated with black dotted line. (b) The Continuum emission at 168 GHz in greyscale and contours: (3,9,15, and 17 $\sigma$ where $\sigma_{\rm cont} = 1.1 \times 10^{-5}$ Jy beam$^{-1}$). (c) Spectrum in the unit of the flux density within the emitting region (over 3 $\sigma$ in moment 0 image) against the line velocity offset at the systemic velocity. The black histogram shows the observation result. The red-dotted line indicates the 1 $\sigma$ rms level. The black dashed line:narrow line component, dotted line: broad line component, and solid line: two components combined. (d) Spectrum in unit of the mean brightness at the peak intensity pixel against the line velocity.
• Figure 2: The results of 216 GHz observations towards NGC 1377. (a) The velocity-integrated line emission maps of H$_2$S 2$_{2,0}$-2$_{1,1}$: -100 km s$^{-1}$ to 100 km s$^{-1}$ (2, 3, 4, and 5 $\sigma$ where $\sigma = 2.1 \times 10^{-2}$ Jy beam$^{-1}$ km s$^{-1}$). The lines and symbol conventions are the same as in Fig.\ref{['fig:1377H2S11']}. (b) The Continuum emission at 216 GHz in greyscale and contours: (3,6, and 9 $\sigma$ where $\sigma_{\rm cont} = 1.8 \times 10^{-5}$ Jy beam$^{-1}$). (c) Spectrum in the unit of the flux density within the emitting region (over 3 $\sigma$ in moment 0 image) against the line velocity offset at the systemic velocity. The black histogram shows the observation result. The red-dotted line indicates the 1 $\sigma$ rms level. The black solid line: single Gaussian fit. (d) Spectrum in unit of the mean brightness at the peak intensity pixel against the line velocity.
• Figure 3: The results of 691 GHz observations towards NGC 1377. (a) The velocity-integrated line emission maps of CO 6-5: black contours: -70 km s$^{-1}$ to 70 km s$^{-1}$ (3,6,9,27, and 36 $\sigma$ where $\sigma_{\rm core}$ = 0.29 Jy beam$^{-1}$ km s$^{-1}$), blue: -200 km s$^{-1}$ to -100 km s$^{-1}$, red: 100 km s$^{-1}$ to 200 km s$^{-1}$ (3,6,9, and 12 $\sigma$ where $\sigma_{\rm red}$ = 0.13 Jy beam$^{-1}$ km s$^{-1}$ and $\sigma_{\rm blue}$ = 0.16 Jy beam$^{-1}$ km s$^{-1}$, respectively). The lines and symbol conventions are the same as in Fig.\ref{['fig:1377H2S11']}. (b) The Continuum emission at 691 GHz in greyscale and contours: (3,5,7, and 9 $\sigma$ where $\sigma_{\rm cont} = 4.07 \times 10^{-1}$ Jy beam$^{-1}$). (c) Spectrum in the unit of the flux density within the emitting region (over 3 $\sigma$ in moment 0 image) against the line velocity offset at the systemic velocity. The black histogram shows the observation result. The red-dotted line indicates the 1 $\sigma$ rms level. The black dashed line:narrow line component, dotted line: broad line component, and solid line: two components combined. (d) Spectrum in unit of the mean brightness at the peak intensity pixel against the line velocity.
• Figure 4: The velocity-integrated map of CO 3-2 Aalto2016. Greyscale shows the emission close to systemic velocity. The high velocity emission from the molecular jet is shown in contours (with the red and blue showing velocity reversals along the axis). The vertical bar indicates a scale of 100 pc. (For details on the figure see Aalto2016). We have indicated the extent and orientation of the CO 6-5 high velocity gas (from Fig. \ref{['fig:1377CO']}) with light blue and yellow ovals.
• Figure 5: The results of 168 GHz observations towards NGC 4418. (a) The velocity-integrated line emission maps of H$_2$S 1$_{1,0}$-1$_{0,1}$: black contours: -100 km s$^{-1}$ to 100 km s$^{-1}$ (3,9,27,81, and 243 $\sigma$ where $\sigma_{\rm core} = 2.63 \times 10^{-2}$ Jy beam$^{-1}$ km s$^{-1}$), blue: -200 km s$^{-1}$ to -100 km s$^{-1}$, red: 100 km s$^{-1}$ to 200 km s$^{-1}$ (3,4,6, and 9 $\sigma$ where $\sigma_{\rm red} = 2.41 \times 10^{-2}$ Jy beam$^{-1}$ km s$^{-1}$ and $\sigma_{\rm blue} = 2.28 \times 10^{-2}$ Jy beam$^{-1}$ km s$^{-1}$, respectively). The beam size is shown at the bottom-left in grey-filled circle. The blue and red line segments indicate the position angle (PA) of the disk Costagliola2013. (b) The Continuum emission at 168 GHz in greyscale and contours: (3,9,27, and 81 $\sigma$ where $\sigma_{\rm cont} = 7.6 \times 10^{-5}$ Jy beam$^{-1}$). (c) Spectrum in the unit of the flux density within the emitting region (over 3 $\sigma$ in moment 0 image) against the line velocity offset at the systemic velocity. The black histogram shows the observation result. The red-dotted line indicates the 1 $\sigma$ rms level. The black dashed line:narrow line component, dotted line: broad line component, and solid line: two components combined. (d) Spectrum in unit of the mean brightness at the peak intensity pixel against the line velocity.