Simulations of structure formation on galactic scales in the prevailing ΛCDM paradigm predict that Milky Way-sized host galaxies should accrete several massive satellites from z ≃ 1 to the present. We now consider the effects of these interactions on the disk of the parent galaxy. Given the significantly shorter dynamical timescales in the disk, the potential effects of a even a minor accretion event could be profound. In particular, such events could incite abnormal structures in dissipational HI disks, either creating features after a pass through or creating large scale, global distortions in the HI structure. In the stellar component of the disk, the effects could be both more subtle and more long lived.
One example of a burst of star formation in a Milky Way-like galaxy with an associated recent accretion event is seen in NGC 5387 (Beaton et al. 2014, note that NGC 5387 can be considered a Milky Way analog based on its properties, but contains about an order of magnitude less mass than the MW). This system shows a low surface-brightness feature in SDSS imaging, that was explored with deeper imaging by Beaton et al., who found a stellar stream extending over at least an entire orbital wrap, with median surface brightness of ∼ 24.5 mag arcsec−2 in R-band (see Figure 9). This stream has a redder color than the typical stellar populations of NGC 5387, and contains a total stellar mass of ∼ 6 × 108 M⊙. Coincident with the position of the stream's crossing of the NGC 5387 disk is a “blue overdensity” that is not only blue in optical colors, but is producing a large FUV flux as well. Evidence from the FUV flux as well as follow-up spectra obtained by Beaton et al. (2014) suggests a very recent (∼ 8 Myr ago) star formation event of total stellar mass 2.5 ± 1.3 × 107 M⊙. Beaton et al. conclude that this blue overdensity is a complex of multiple HII regions produced by star formation either induced in the disk by the minor merger event, or in the dwarf galaxy progenitor of the stream itself. Whichever scenario is shown to be true, it is clear that the blue overdensity and its associated tidal stream represent star formation induced by the accretion of a satellite about 1/50th the mass of its host.
Figure 9. Images of the stream and associated blue overdensity in NGC 5387. The left panel is the R-band image from the VATT, with the SDSS color image inlaid in the central regions, and clearly shows a full wrap of tidal debris about the disk of NGC 5387. The right panel shows a merged image composed of the SDSS, VATT, and GALEX FUV images, which more clearly highlights the blue overdensity near the northernmost edge of the disk of NGC 5387. The inset in the right panel is a schematic highlighting the disk in dark gray, the stream as a lighter gray loop, the blue overdensity as a blue circle, and two foreground stars. [Reproduced from Beaton et al. (2014).]
The presence of tidal streams encircling galaxies with warped disks (see, e.g., Martínez-Delgado et al. 2008) may suggest satellite galaxy perturbations as the origin of those disk features (e.g., Velazquez & White 1999, Weinberg & Blitz 2006). This suggests that promising galaxies to search for extragalactic tidal streams are those that display disk asymmetries in optical or HI images that may result from gravitational interaction with the tidally disrupting companions. One striking case of such a system is NGC 4013, an isolated spiral galaxy with a prominent HI warp (Bottema et al. 1987) that has been revealed by deep imaging (Martínez-Delgado et al. 2009) to contain a faint loop-like stellar tidal stream at fairly low inclination to the disk (see the upper panels of Figure 1). The sky-projected morphology of this structure displays a remarkable resemblance to an edge-on view of models of the Monoceros Ring feature (see Chapter 3) in the Milky Way as a tidal debris structure (Peñarrubia et al. 2005). This suggests that the progenitor system of the NGC 4013 stream may have been a low-mass satellite on a low-inclination, nearly-circular orbit that was accreted approximately ∼ 2.8 Gyr ago. Stellar streams have also been discovered in the warped spiral galaxies NGC 5907 (Martínez-Delgado et al. 2008, see also Figure 3) and M63 (Chonis et al. 2011, see also Figure 1), showing that disks that are apparently undisturbed as seen in the optical, but warped in HI maps, may reveal signatures of recent accretion events in deep imaging surveys.
It has been shown that such accretion events should lead to strong warping, flaring, and thickening of an initially cold stellar disk (Kazantzidis et al. 2008), and to the generation of bars and spiral structure (e.g., Toomre & Toomre 1972, Gauthier et al. 2006, Dubinski & Chakrabarty 2009, Purcell et al. 2011). Such perturbations may also lead to the formation of long-lived, ring like stellar features in the outer reaches of the disk that may extend several kiloparsecs from the disk plane, and have surface brightnesses in the range of 25-30 mag arcsec−2 (see Fig. 6 of Kazantzidis et al. 2008). Indeed, observations in the Milky Way are uncovering wave like perturbations in the disk in stellar densities (Widrow et al. 2012, Yanny & Gardner 2013, Xu et al. 2015) and velocities (Gómez et al. 2012a, Gómez et al. 2012b, Carlin et al. 2013, Williams et al. 2013), with an accompanying array of simulations predicting the formation of such features (e.g., Chakrabarti & Blitz 2009, Michel-Dansac et al. 2011, Purcell et al. 2011, Gómez et al. 2013, Faure et al. 2014, Widrow et al. 2014). Evidence for dynamically heated populations is not unique to the Milky Way – Dorman et al. (2013) have identified a kinematically cold population in the halo of M31.