Spectacular tails of ionised gas in the Virgo cluster galaxy NGC 4569 (1601.04978v2)

Abstract: We obtained using MegaCam at the CFHT a deep narrow band Halpha+[NII] wide field image of NGC 4569, the brightest late-type galaxy in the Virgo cluster. The image reveals the presence of long tails of diffuse ionised gas without any associated stellar component extending from the disc of the galaxy up to ~ 80 kpc (projected distance) with a typical surface brightness of a few 10^-18 erg s-1 cm-2 arcsec-2. These features provide direct evidence that NGC 4569 is undergoing a ram presure stripping event. The image also shows a prominent 8 kpc spur of ionised gas associated to the nucleus that spectroscopic data identify as an outflow. With some assumptions on the 3D distribution of the gas, we use the Halpha surface brightness of these extended low surface brightness features to derive the density and the mass of the gas stripped during the interaction of the galaxy with the ICM. The comparison with ad-hoc chemo-spectrophotometric models of galaxy evolution indicates that the mass of the Halpha emitting gas in the tail is comparable to that of the cold phase stripped from the disc, suggesting that the gas is ionised within the tail during the stripping process. The lack of star forming regions suggests that mechanisms other than photoionisation are responsible for the excitation of the gas (shocks, heat conduction, magneto hydrodynamic waves). This analysis indicates that ram pressure stripping is efficient in massive (M_star ~ 10^10.5 Mo) galaxies located in intermediate mass (~ 10^14 Mo) clusters under formation. It also shows that the mass of gas expelled by the nuclear outflow is ~ 1 % than that removed during the ram pressure stripping event. All together these results indicate that ram pressure stripping, rather than starvation through nuclear feedback, can be the dominant mechanism responsible for the quenching of the star formation activity of galaxies in high density environments.

Analysis of Ionized Gas Tails in the Virgo Cluster Galaxy NGC 4569

The paper of the ionized gas tails in NGC 4569, the brightest late-type galaxy in the Virgo cluster, offers significant advancements in our understanding of ram pressure stripping—a process wherein the interstellar medium (ISM) of a galaxy is removed due to pressure exerted by the intracluster medium as the galaxy travels through the cluster. Using the MegaCam instrument at the CFHT, deep narrow-band Hα+[NII] imaging has unveiled extensive ionized gas tails stretching up to approximately 80 kpc from the galaxy's disc. The surface brightness of these structures is on the order of a few 10^-18 erg s^-1 cm^-2 arcsec^-2. This discovery substantiates ongoing ram pressure stripping in NGC 4569.

Numerical and Observational Insights

1. Ionized Gas Mass and Density Estimation: The mass of the ionized gas tails is estimated to be around 3.2 × 10^9 M_☉. These structures provide concrete indications that the galaxy's ISM is being efficiently stripped and ionized, corroborated by the comparably significant estimated mass loss of the cold gas phase from the disc.
2. Spectral Properties and Ionization Sources: Spectroscopy has revealed that the gas within these tails is not hosting star-forming regions, implying that ionization mechanisms beyond stellar photoionization contribute to the observed features. Potential sources include shocks, heat conduction, and magneto-hydrodynamic (MHD) waves.
3. Kinematic Analysis: Long-slit spectroscopic data highlights the detachment of the tails from the galaxy's rotational dynamics, displaying high velocities and turbulent dispersions indicative of outflows and complex motion inherent to the stripping process.
4. Environmental Context: The findings align with the environmental quenching mechanisms relevant to cluster dynamics. Given the moderately massive Virgo cluster (≈10^14 M_☉), ram pressure appears to significantly impact even the most massive cluster galaxies (≈10^10.5 M_☉).

Implications for Galaxy Evolution and Cluster Dynamics

The results underscore the essential role of ram pressure stripping in altering the structural integrity and star formation capabilities of galaxies in dense environments. This mechanism is demonstrated to be more effective than nuclear feedback in expelling mass in NGC 4569. The dynamics observed in the Virgo cluster highlight how intermediate-mass clusters under formation can still influence large galaxies, suggesting a broader applicability to galaxy evolution models.

Prospects for Future Research

This paper sets the stage for future investigations into the precise mechanisms responsible for ionizing gas within ram pressure-stripped tails. Further, the lack of star formation in the tails of NGC 4569, juxtaposed against star-forming regions in similar environments (e.g., ESO 137-001), prompts questions about the variable influences of the intracluster medium density and stripping duration. Advanced spectral and imaging studies, particularly employing integral field spectroscopy like MUSE, will refine our knowledge of these interactions.

Overall, the comprehensive examination of NGC 4569 not only enriches our comprehension of galaxy-cluster interactions but also provides crucial data to refine large-scale simulations of galaxy evolution. This is instrumental in bridging the gap between observational data and theoretical models for galaxy transformation in cluster environments.