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Register a new userThe role of tidal interactions in driving galaxy evolution
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We carry out a statistical analysis of galaxy pairs selected from chemical hydrodynamical simulations with the aim at assessing the capability of hierarchical scenarios to reproduce recent observational results for galaxies in pairs. Particularly, we analyse the effects of mergers and interactions on the star formation (SF) activity, the global mean chemical properties and the colour distribution of interacting galaxies. We also assess the effects of spurious pairs.
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Dwarf galaxies (M*<10^9 Msun) are key drivers of mass assembly in high mass galaxies, but relatively little is understood about the assembly of dwarf galaxies themselves. Using the textsc{NewHorizon} cosmological simulation (40 pc spatial resolution), we investigate how mergers and fly-bys drive the mass assembly and structural evolution of around 1000 field and group dwarfs up to z=0.5. We find that, while dwarf galaxies often exhibit disturbed morphologies (5 and 20 per cent are disturbed at z=1 and z=3 respectively), only a small proportion of the morphological disturbances seen in dwarf galaxies are driven by mergers at any redshift (for 10^9 Msun, mergers drive only 20 per cent morphological disturbances). They are instead primarily the result of interactions that do not end in a merger (e.g. fly-bys). Given the large fraction of apparently morphologically disturbed dwarf galaxies which are not, in fact, merging, this finding is particularly important to future studies identifying dwarf mergers and post-mergers morphologically at intermediate and high redshifts. Dwarfs typically undergo one major and one minor merger between z=5 and z=0.5, accounting for 10 per cent of their total stellar mass. Mergers can also drive moderate star formation enhancements at lower redshifts (3 or 4 times at z=1), but this accounts for only a few per cent of stellar mass in the dwarf regime given their infrequency. Non-merger interactions drive significantly smaller star formation enhancements (around two times), but their preponderance relative to mergers means they account for around 10 per cent of stellar mass formed in the dwarf regime.
We employ the earlier published proper motions of the newly discovered Antlia 2 dwarf galaxy derived from Gaia data to calculate its orbital distribution in the cosmologically recent past. Using these observationally motivated orbits, we calculate the effect of the Antlia 2 dwarf galaxy on the outer HI disk of the Milky Way, using both test particle and Smoothed Particle Hydrodynamics simulations. We find that orbits with low pericenters, $sim$ 10 kpc, produce disturbances that match the observed outer HI disk perturbations. We have independently recalculated the proper motion of the Antlia 2 dwarf from Gaia data and found a proper motion of $(mu_{alpha}cosdelta, mu_{delta}) = (-0.068,0.032) pm (0.023,-0.031)~rm mas/yr$, which agrees with results from Torrealba et al. (2019) within the errors, but gives lower mean pericenters, e.g., $sim$ 15 kpc for our fiducial model of the Milky Way. We also show that the Sagittarius dwarf galaxy interaction does not match the observed perturbations in the outer gas disk. Thus, Antlia 2 may be the driver of the observed large perturbations in the outer gas disk of the Galaxy. The current location of the Antlia 2 dwarf galaxy closely matches that predicted by an earlier dynamical analysis (Chakrabarti & Blitz 2009) of the dwarf that drove ripples in the outer Galaxy, and, in particular, its orbit is nearly coplanar to the Galactic disk. If the Antlia 2 dwarf galaxy is responsible for the perturbations in the outer Galactic disk, it would have a specific range of proper motions that we predict here; this can be tested soon with Gaia DR-3 and Gaia DR-4 data.
A key unresolved question is the role that galaxy mergers play in driving stellar mass growth over cosmic time. Recent observational work hints at the possibility that the overall contribution of `major mergers (mass ratios $gtrsim$1:4) to cosmic stellar mass growth may be small, because they enhance star formation rates by relatively small amounts at high redshift, when much of todays stellar mass was assembled. However, the heterogeneity and relatively small size of todays datasets, coupled with the difficulty in identifying genuine mergers, makes it challenging to $textit{empirically}$ quantify the merger contribution to stellar mass growth. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation, to comprehensively quantify the contribution of mergers to the star formation budget over the lifetime of the Universe. We show that: (1) both major and minor mergers enhance star formation to similar amounts, (2) the fraction of star formation directly attributable to merging is small at all redshifts (e.g. $sim$35 and $sim$20 per cent at z$sim$3 and z$sim$1 respectively) and (3) only $sim$25 per cent of todays stellar mass is directly attributable to galaxy mergers over cosmic time. Our results suggest that smooth accretion, not merging, is the dominant driver of stellar mass growth over the lifetime of the Universe.
Using deep B band imaging down to mu_{B} = 26 mag arcsec^{-2}, we present evidence for tidal tails in the Antlia Dwarf galaxy, one of the most distant members of the Local Group. This elongation is in the direction of Antlias nearest neighbor, the Magellanic-type NGC 3109. The tail is offset by less than 10 degrees from a vector linking the centers of the two galaxies, indicative of interactions between the pair. Combined with the warped disc previously identified in NGC 3109, Antlia and NGC 3109 must be at a small separation for tidal features to be present in Antlia. We calculate that Antlia cannot be completely disrupted by NGC 3109 in a single interaction unless its orbit pericenter is less than 6 kpc, however multiple interactions could significantly alter its morphology. Therefore despite being located right at the edge of the Local Group, environmental effects are playing an important role in Antlias evolution.
We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1<z<4). The fraction of massive galaxies in major mergers (mass ratio R<4:1) is around 3%, a factor of ~2.5 lower than minor mergers (4:1<R <10:1) at these epochs, with no trend with redshift. At z~1, around a third of massive galaxies have undergone a major merger, while all such systems have undergone either a major or minor merger. While almost all major mergers at z>3 are blue (i.e. have significant associated star formation), the proportion of red mergers increases rapidly at z<2, with most merging systems at z~1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (~20-40%) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10:1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard LCDM paradigm, mergers are minority players in driving star formation at the epochs where the bulk of todays stellar mass was formed.
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