The MALATANG survey: star formation, dense gas, and AGN feedback in NGC 1068

TL;DR

NGC 1068 is studied to understand how dense molecular gas relates to star formation and AGN feedback on sub-kpc scales. The MALATANG survey maps HCN(4-3) and HCO+(4-3) with JCMT and combines these data with CO, infrared, and Chandra X-ray observations to examine the dense-gas–IR and X-ray–SFR connections, including the center and outer disk. The results show the dense-gas–IR relation is nearly linear across the galaxy, while the X-ray emission from hot gas scales nonlinearly with SFR in the center and shows a distinct heating regime in the outer disk; HCN(4-3) appears more sensitive to high-energy feedback than HCO+(4-3) or CO(3-2). The findings suggest AGN feedback does not dominate star formation on sub-kpc scales, and the dense-gas tracer HCN(4-3) may be more strongly influenced by high-energy processes than the others.

Abstract

We aim to investigate the interplay between dense molecular gas, star formation, and active galactic nucleus (AGN) feedback in the luminous infrared galaxy (LIRG) NGC 1068 at sub-kiloparsec scales. We present the HCN (4-3) and HCO$^+$ (4-3) maps of NGC 1068, obtained with JCMT as part of the Mapping the dense molecular gas in the strongest star-forming galaxies (MALATANG) project, and perform spatially resolved analyses of their correlations with infrared luminosity and soft X-ray emission. Spatially resolved relations between the luminosities of infrared dust emission and dense molecular gas tracers ($L_{\rm IR}-L'_{\rm dense}$) are found to be nearly linear, without clear evidence of excess contributions from AGN activity. The spatially resolved X-ray emission ($L^{\rm gas}_{0.5-2\,\mathrm{keV}}$) displays a radially-dependent twofold correlation with the star formation rate (SFR), suggesting distinct gas-heating mechanisms between the galaxy center and the outer regions. A super-linear scaling is obtained in galactic center regions with SFR surface density ($Σ_{\rm SFR}$) $>$ 8.2 $\times$ 10$^{-6}$ $M_\odot$ yr$^{-1}$ kpc$^{-2}$: log($L^{\rm gas}_{0.5-2\,\mathrm{keV}}$/erg s$^{-1}$) = 2.2 log(SFR/$M_\odot$ yr$^{-1}$) + 39.1. We further found a statistically significant super-linear correlation ($β= 1.34$ $\pm$ 0.86) between $L^{\rm gas}_{0.5-2\,\mathrm{keV}}$/SFR and HCN(4-3)/CO(1-0) intensity ratio, whereas no such trend is seen for HCO$^+$(4-3)/CO(1-0) or CO(3-2)/CO(1-0). These findings indicate that AGN feedback does not dominate star formation regulation on sub-kiloparsec scales, and that the excitation of dense gas traced by HCN (4-3) may be more directly influenced by high-energy feedback processes compared to HCO$^+$ (4-3) and CO (3-2).

Figures (7)

• Figure 1: Upper panel: JCMT observing positions of NGC 1068 overlaid on Herschel PACS 70 $\mu$m greyscale map. The black contours indicate the 70 $\mu$m continuum, starting from 100 $\sigma$ and increasing in steps of 3 $\sigma$, where 1 $\sigma = 3\times10^{-4}$ Jy/pixel. The white cross denotes the position of the central AGN in the galaxy. Orange circles indicate the observed positions using jiggle mapping mode. The red square shows the area (1.5$^{\prime}$$\times$ 1.5$^{\prime}$) studied in this work. Bottom panel: A zoom-in view of the central region obtained from the ALMA high-resolution CO (1–0) map (project ID: 2018.1.01684.S, Saito2022ApJ...935..155S; see also Nakajima2023ApJ...955...27N). Black contours represent CO (1–0) integrated intensity levels, ranging from 4$\sigma$ to 20$\sigma$ in steps of 4$\sigma$, where 1$\sigma$ = 0.25 Jy beam$^{-1}$ km s$^{-1}$. The red dotted-dashed circle here represents the JCMT beam size of $14^{\prime \prime}$ (approximately 1.1 kpc).
• Figure 2: Integrated intensity (moment 0) maps of molecular gas tracers, shown with a pixel size of 10$^{\prime \prime}$. (a): CO ($1-0$) map with contours at levels of 15 to 35 $\sigma$ in steps of 5 $\sigma$ ($\sigma = 4.34$ K km s$^{-1}$); (b): CO ($3-2$) map with contours from 13 to 57 $\sigma$ in steps of 11 $\sigma$ ($\sigma = 2.39$ K km s$^{-1}$); (c): HCN ($4-3$) map with contours from 3 to 12 $\sigma$ in steps of 3 $\sigma$ ($\sigma = 0.38$ K km s$^{-1}$); (d): HCO$^{+}$ ($4-3$) map with contours from 3 to 6 $\sigma$ in steps of 1 $\sigma$ ($\sigma = 0.41$ K km s$^{-1}$). The white cross marks the location of the AGN in NGC 1068. Pixels with S/N below 3 are masked. The beam size and scale bar are shown in the lower-left corner of each panel.
• Figure 3: Relations between IR luminosities and dense molecular line luminosities in logarithmic scale. (a) $L_{\rm IR} - L^{'}_{\rm HCN (4-3)}$; (b) $L_{\rm IR} - L^{'}_{\rm HCO^+ (4-3)}$. Detected data are shown as solid points, data points obtained from stacked spectra are shown as diamonds, and upper limits are marked by grey leftward arrows. The black circles highlight the central region of NGC 1068, which hosts the AGN. The Spearman rank correlation coefficients ($\rho_{\rm sp}$) are displayed in each panel. The solid lines and their slopes ($\beta$) represent the the best-fit results from Bayesian regression, as adopted from Tan2018ApJ...860..165T.
• Figure 4: Relation between SFR and thermal X-ray luminosity of hot gas on sub-kpc scale. Color scale represents the distance from the galactic center, and the marker size represents the SFR surface density. The central data point containing the AGN has been exclude here. The vertical black dashed line marks SFR = 1 $M_{\odot}$ yr$^{-1}$, corresponding to $\Sigma_{\rm SFR}$ = 8.2 $\times$ 10$^{-6}$$M_\odot$ yr$^{-1}$ kpc$^{-2}$. The black solid line represents the best-fit scaling relation as expressed in Eq. \ref{['equ:Lx_SFR']}, derived from the central 12 data points with SFR $>$ 1 $M_{\odot}$ yr$^{-1}$. The Spearman rank correlation coefficient ($\rho_{\rm sp}$) is given in the panel. The gray crosses represent data from the central regions of nearby galaxies Zhang2024ApJ...967L..25Z, while the gray dashed line indicates the corresponding best-fit relation for these galaxies.
• Figure 5: (a): Correlation between dust temperature and gas excitation (traced by $R_{\rm 31}$); (b): Correlation between dust temperature and dense and warm gas fraction, denoted by HCN $(4-3)$/CO $(1-0)$ and HCO$^{+}$$(4-3)$/CO $(1-0)$ integrated intensity ratios. Black circles denote the central position of NGC 1068, which hosts the AGN. The Spearman rank correlation coefficients are shown at each panel.