NGC4438 is a disturbed spiral galaxy located in the Virgo cluster, which has been the subject of numerous studies due to its peculiar characteristics. This galaxy shows obvious signs of interaction with its environment, including possible collisions with other galaxies and effects of the intracluster medium. In this paper, a detailed analysis of NGC4438 is presented, with special emphasis on the observed tidal streams and evidence of a past collision with the elliptical galaxy M86. The information gathered comes from several studies at different wavelengths, from radio to X-ray observations, providing a comprehensive view of the physical processes that have shaped this galaxy.
Characteristics of NGC4438
NGC4438 is a disturbed spiral galaxy located in the Virgo cluster, classified as a LINER (Low Ionization Emission Line Nuclear Region) galaxy. Observations at different wavelengths reveal a complex structure, with an active nucleus and evidence of disturbance in both its stellar disk and interstellar medium. X-ray emission in NGC4438 is observed in three main regions: a nuclear region of approximately 700 pc, a spherical bulge of 2.3 kpc, and a network of filaments extending between 4 and 10 kpc to the west and southwest of the galaxy. These features suggest that NGC4438 has experienced significant interactions with its environment, altering its original morphology and physical properties.
EEvidence Collision with M86
One of the most significant findings in the study of NGC4438 is evidence for a past collision with the giant elliptical galaxy M86. Deep Hα+[NII] observations by Kenney et al. (2008) revealed a set of filaments that clearly connect M86 with NGC4438, providing direct evidence of a previously unrecognized interaction between the two galaxies. Spectroscopy of selected regions shows a rather smooth velocity gradient between M86 and NGC4438, consistent with the collision scenario.
According to Machacek et al. (2004), the X-ray properties of the gas outside the nuclear region in NGC4438 and NGC4435 favor a high-velocity, off-center collision between these galaxies that occurred approximately 100 million years ago. This collision would have imparted significant energy to the interstellar medium of M86, probably heating the gas and preventing it from cooling to form stars. In addition, some Hα filaments are associated with the well-known ridge of bright X-ray emission northwest of M86’s core, suggesting that the collision is responsible for peculiarities of M86 previously attributed to other effects.
Tidal streams in NGC4438
Tidal streams are structures of stellar material, gas, and dust that form when the gravitational forces of a nearby galaxy stretch and deform another galaxy during a close encounter or collision. In the case of NGC4438, several studies have identified structures that can be interpreted as tidal streams resulting from its interaction with M86 and possibly other galaxies in the Virgo cluster. Hota et al. (2007) detected a faint HI tail to the north of NGC4438, close to a stellar tail seen in deep optical observations. This HI tail has a total extent of about 50 kpc and a mass of 1.4×10^8 solar masses if at the distance of NGC4438. The velocity of this HI tail is approximately -10 km/s, similar to the HI emission from IC 3355. In addition, HI observations show an elongated structure offset approximately 4.1 kpc on the western side of NGC4438, with a size of approximately 9.8 kpc and a mass of 1.8×10^8 solar masses. The velocity field suggests systematic rotation, indicating that this structure could be material torn from the disk of NGC4438 during the collision.
Herschel-SPIRE observations by Cortese et al. (2010) revealed the presence of extra-planar dust up to about 4-5 kpc away from the galaxy disk. This dust closely follows the distribution of the stripped atomic and molecular hydrogen, supporting the idea that gas and dust are similarly perturbed by the cluster environment. Interestingly, the extra-planar dust lacks a warm-temperature component compared to the material still present in the disk, explaining why it was overlooked in previous far-infrared investigations. These observations provide additional evidence that NGC4438 has undergone material stripping processes due to gravitational interactions and possibly also to the sweeping pressure of the intracluster medium.
Active Core and Outflows
Observations at different wavelengths have also revealed the presence of an active core in NGC4438 and associated outflows. Perez et al. (2009) performed imaging and near-infrared spectroscopy of the nuclear region of NGC4438, finding evidence that shocks (possibly driven by a radio jet) contribute to a significant fraction of the [Fe II] excitation, while X-ray heating of a central AGN may be responsible for H2 excitation. Stellar kinematics measured from near-infrared CO lines show a strong peak in velocity dispersion of 178 km/s in the central 0.5 arcsec, which could be kinematic evidence for a central black hole. Using the standard M_bh-sigma relation, the authors suggest a central black hole mass of log(M_bh/Msun) ~ 7.0. Machacek et al. (2004) observed X-ray emission from the core and from two outflow bubbles extending 360(730) pc toward the NW(SE) of the core. The spectrum of the NO plus core outflow bubble fits well to an absorbed MEKAL plasma model of 0.58 keV plus a strongly absorbed power-law component. The electron density, cooling time, and X-ray gas mass in the NO outflow are ~0.5 cm^-3, 30 million years, and 3.5×10^6 solar masses, respectively.
High-resolution radio observations by Hota et al. (2007) with the VLA show the presence of a radio core with an inverted spectrum located between the highly asymmetric lobes of radio emission. This demonstrates that these lobes arise due to an AGN rather than a compact nuclear starburst, confirming the active nature of the core of NGC4438. Low-frequency radio continuum observations made with the GMRT detect extended emission on the western side of the galaxy, whose spectral index is flatter at higher frequencies and suggests that it is a mixture of thermal and non-thermal emission.
Implications for Galactic Evolution
The study of NGC4438 and its interaction with M86 has important implications for our understanding of galactic evolution in dense environments such as galaxy clusters. The collision between NGC4438 and M86 represents one of the closest examples of a recent high-speed collision between a large spiral and a large elliptical. This system provides a unique laboratory for studying how gravitational interactions contribute to the heating of the interstellar medium in elliptical galaxies, and how collisions in clusters transform galaxies. The Herschel-SPIRE observations by Cortese et al. (2010) illustrate the potential of Herschel data for our understanding of environmental effects on galactic evolution, particularly with respect to dust stripping in galaxy clusters. In addition, the presence of an active nucleus in NGC4438 raises questions about the role of galactic interactions in the activation and feeding of supermassive black holes.
Importantly, NGC4438 has been affected not only by the collision with M86, but also potentially by the sweeping pressure of the intracluster medium and possible interactions with other galaxies in the Virgo cluster. The combination of these processes has shaped the current morphology and properties of NGC4438, making it a fascinating example of how the environment can dramatically influence the evolution of galaxies. Future studies with higher sensitivity and higher resolution instruments may reveal additional details about the interaction history of NGC4438 and provide new insights into the physical processes involved in galactic interactions in cluster environments.
Conclusions
In this paper, we have presented a detailed analysis of the galaxy NGC4438 in the Virgo cluster, focusing on the observed tidal streams and evidence for a past collision with the elliptical galaxy M86. Observations at multiple wavelengths, from radio to X-ray, reveal a highly perturbed galaxy with complex structures resulting from gravitational interactions and possibly also from the sweeping pressure of the intracluster medium. The most compelling evidence for a collision between NGC4438 and M86 comes from the Hα filaments connecting the two galaxies, as well as the observed smooth velocity gradient between them. This collision, which occurred approximately 100 million years ago, has had a significant impact on both galaxies, altering their morphology and physical properties. The tidal streams observed in NGC4438, including HI’s tail to the north, HI’s elongated structure on the western side, and extra-planar dust, are testimony to the violent interaction history of this galaxy. In addition, the presence of an active core and associated outflows adds another dimension to the complexity of NGC4438.
The study of systems such as NGC4438 and M86 is fundamental to our understanding of how galactic interactions shape the evolution of galaxies in dense environments such as clusters. As astronomical instruments continue to improve in sensitivity and resolution, we can expect to discover even more fascinating details about the interaction history of NGC4438 and other similar galaxies, providing new insights into the physical processes that govern galactic evolution in the universe.