An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

Abstract

Abstract. The unusual morphology of the Andromeda Spiral (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M 31 has a well-known outer ring 1−7 of star formation at a radius of 10 kpc whose center is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at both optical 8 and radio 9 wavelengths. The halo contains numerous loops and ripples. Here we report the discovery, based on analysis of previously-obtained data 10 , of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and offset by ∼ 0.5 kpc from the center of the galaxy. The two rings appear to bedensity waves propagating in the disk.Numerical simulations offer a completely new interpretation for the morphology of M 31: both rings result from a companion galaxyplunging head-on through the center of the disk of M 31. The most likely interloper is M 32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.Newly-acquired images 10 of M 31 secured by the Infrared Array Camera 11 (IRAC) onboard the Spitzer Space Telescope span the wavelength regime of 3.6 to 8.0 microns. These images offer unique probes of the morphologies of the stellar distribution and interstellar medium with no interference from extinction. Figure 1 shows the emission map of the interstellar medium at 8 microns, generated by subtracting a scaled 3.6 micron image (dominated by starlight) from the 8 micron image. The subtraction removes the contribution from stellar photospheres and leaves only the emission from dust grains 10 , which trace the interstellar medium of M 31. What is most striking in Figure 1 (and in the enlarged inset) is the presence of a complete though asymmetric inner ring of dust 6.9 by 4.4 arcmin in extent, translating to linear dimensions of about 1.5 by 1 kpc (assuming a distance 12 of 780 kpc). The inner ring lies between the two well known Baade spiral dust arms 13 , both of which are clearly seen in emission. The inner ring is elongated in a direction close to the minor axis and belongs to the central gas disk, which appears to be more face-on 14 . It is therefore not possible to know the inner rings precise ellipticity, but it is unlikely to be circular. IRAC imaging thus reveals two rings. The outer ring is offset by approximately 10 percent of its radius, while the inner ring is offset by about 40 percent or 0.5 kpc. The inner elliptical ring has been alluded to in earlier studies 15,16 , but all investigators have hitherto believed it to be a mini-spiral, related to a bar. Published Spitzer 24 micron images5 of M 31 show centrally-concentrated dust emission; the ring morphology is therefore disguised at these longer wavelengths. The IRAC images beautifully show the inner ring at high spatial resolution and furthermore confirm that this feature is a complete and continuous ring, even though offset and asymmetrical. Ther...

Molecular gas in the inner 0.7 kpc-radius ring of M 31

“…In Sect. 5.3, we remind the scenario of a head-on collision with M 32 as proposed by Block et al (2006) and show with a simple modelling that it can account for the various observables.…”

“…This cut has been made to subtract the large-scale HI emission seen in projection superposed to the centre, expected near the systemic velocity. The contours are the dust emission from Spitzer-IRAC (8 μm image where a scaled version of the 3.6 μm image has been subtracted to remove the stellar photospheres emission) (Block et al 2006). The HI cube is from Braun et al (2009). in the centre, as displayed in the right panel of Fig.…”

“…He proposes as the driver of the spiral structure the collision with M 32, which could also be responsible for the many tilts of the M 31 plane. The nearly head-on collision with M 32 has been developed in detail by Block et al (2006), which allows them to account for the two off-centred gas rings observed in M 31 at 0.7 kpc and 10 kpc radius. Indeed, these authors note that the gas morphology in M 31 does not show the usual signatures of the response to a bar.…”

“…Unlike the Cartwheel, where the companion is about 1/3rd of the mass of the target (major merger), in Andromeda, the collision can be called a minor merger, and produces much less contrasted rings in the main disc. Block et al (2006) propose that the collision partner was M 32, with about 1/10th of the mass (dark matter included) at the beginning. After stripping experienced in the collision, the A&A 536, A52 (2011) M 32 mass is now 1/23 that of the main target M 31.…”

Multi-scale radio-infrared correlations in M 31 and M 33: The role of magnetic fields and star formation

“…Possible origins of the strong Hα emission and high thermal fraction at R < 30 have been discussed by Devereux et al (1994) and Tabatabaei & Berkhuijsen (2010). As recent SF is scarce in this region, the gas must have been ionized by shocks, evolved stars (Groves et al 2012) and/or during the last encounter with a companion galaxy (Block et al 2006).…”