GOALS-JWST: The Warm Molecular Outflows of the Merging Starburst Galaxy NGC 3256 (2403.14751v3)

Abstract: We present James Webb Space Telescope (JWST) Integral Field Spectrograph observations of NGC 3256, a local infrared-luminous late-stage merging system with two nuclei roughly 1$\;\rm{kpc}$ apart, both of which have evidence of cold molecular outflows. Using JWST/NIRSpec and MIRI datasets, we investigate this morphologically complex system on spatial scales of $<$100$\;\rm{pc}$, where we focus on the warm molecular H$2$ gas surrounding the nuclei. We detect collimated outflowing warm H$_2$ gas originating from the southern nucleus, though we do not find significant outflowing H$_2$ gas surrounding the northern nucleus. We measure maximum intrinsic outflow velocities of $\sim$1,000$\;\rm{km}\;\rm{s}^{-1}$, which extend out to a distance of 0.7$\;\rm{kpc}$. Based on H$_2$ S(7)/S(1) ratios, we find a larger fraction of warmer gas near the S nucleus, which decreases with increasing distance from the nucleus, signifying the southern nucleus as a primary source of H$_2$ heating. The gas mass of the warm H$_2$ outflow component is estimated to be $M\rm{{warm,out}}=(1.4\pm0.2)\times10^6\;\rm{M}_{\odot}$, as much as 6$\%$ of the cold H$2$ mass estimated using ALMA CO data. The outflow time scale is about $7\times10^5\;\rm{yr}$, resulting in a mass outflow rate $\dot{M}\rm{{warm,out}}=2.0\pm0.8\;\rm{M}{\odot}\;\rm{yr}^{-1}$ and kinetic power $P\rm{{warm,out}}\;\sim\;4\times10^{41}\;\rm{erg}\;\rm{s}^{-1}$. Lastly, regions within our 3.0"x3.0" NIRSpec data where the outflowing gas reside show high [Fe II]/Pa$\beta$ and H$_2$/Br$\gamma$ line ratios, indicate enhanced mechanical heating caused by the outflows. The fluxes and ratios of Polycyclic Aromatic Hydrocarbons (PAH) in these regions are not significantly different compared to those elsewhere in the disk, suggesting the outflows may not significantly alter the PAH ionization state or grain size.

• The paper identifies high-velocity (up to 1000 km/s) warm H₂ outflows mapped to the southern nucleus of NGC 3256.
• The paper employs JWST’s NIRSpec and MIRI to analyze H₂ rotational lines, estimating a warm outflow mass of 1.4×10⁶ M☉.
• The paper measures a kinetic power of 4×10⁴¹ erg/s, indicating AGN-driven feedback that significantly impacts the local ISM.

An Analysis of Warm Molecular Outflows in the Merging Starburst Galaxy NGC 3256

The research paper "GOALS-JWST: The Warm Molecular Outflows of the Merging Starburst Galaxy NGC 3256" provides a comprehensive investigation of infrared characteristics of NGC 3256, a galaxy involved in a late-stage merging process. This paper utilizes the state-of-the-art capabilities of the James Webb Space Telescope (JWST), specifically using the Near-InfraRed Spectrograph (NIRSpec) and the Mid-InfraRed Instrument (MIRI), to explore the morphology and dynamics of warm molecular outflows.

Key Findings

1. Identification and Spatial Mapping of Outflows: The paper identifies collimated outflowing warm H$_2$ gas originating predominantly from the southern nucleus of NGC 3256. These outflows are characterized by velocities reaching up to approximately 1000 km s$^{-1}$ and extending nearly 0.7 kpc from the nucleus. The precise subarcsecond spatial resolution of JWST allows for the resolving of these complex structures with unprecedented clarity.
2. Mass and Temperature of Molecular Gas: By analyzing the full set of H$_2$ rotational lines (S(1) through S(8)), the research estimates the mass of the warm molecular gas in the outflows to be $M_{\rm warm, out} = 1.4 \times 10^6\ M_\odot$, which is about 6% of the cold H$_2$ mass derived from ALMA CO observations. The analysis indicates a larger fraction of warmer gas close to the southern nucleus, emphasizing this region as a significant source of H$_2$ heating.
3. Energetics and Kinetic Power: The kinetic energy calculated for these outflows gives insights into the feedback mechanisms at play. The kinetic power is estimated to be approximately $4 \times 10^{41}$ erg s$^{-1}$. These outflows, given their considerable velocity and mass loading factors, suggest mechanisms likely AGN-driven, affecting star formation rates by energizing the surrounding molecular gas.
4. Impacts on Local Interstellar Medium (ISM): The paper analyzes the effects of these outflows on ISM excitation using line ratios such as [Fe II]/Paβ and H$_2$/Brγ. The presence of elevated ratios in regions aligned with outflows supports the hypothesis of enhanced mechanical heating and possible shocks being induced by the outflows. However, the paper observes no significant perturbation in the local PAH ionization state or grain size, suggesting the outflows may not dramatically alter these aspects of the surrounding ISM.

Implications

The insights derived from the work on NGC 3256 enhance our understanding of the astrophysical processes occurring in late-stage galaxy mergers. The ability of JWST to resolve such features signifies a leap forward in studying galaxy mergers and their contribution to galaxy evolution. The research underscores the critical role of warm outflows in galaxy evolution, particularly in affecting star formation regulation via feedback processes.

Future Directions

This paper sets a precedent for future research using JWST to analyze the molecular gas dynamics in merging galaxies. Future research can expand on identifying the influence of such outflows across different merger stages and galaxy types, utilizing JWST's capabilities to unravel the myriad processes contributing to galactic evolution. Specifically, expanding these analyses to a broader range of nearby AGNs and ULIRGs would provide significant insights into the universality of these mechanisms.

In conclusion, the paper presents a critical step in leveraging JWST's capabilities to unravel the complexities of galaxy mergers, offering a window into the significant yet intricate processes governing galaxy formation and evolution.