What is the Milky Way telling us about galaxy formation and evolution in general.
We are learning a lot about the Milky Way from Gaia and many imaging and spectroscopic surveys. Before long, we will have a fairly comprehensive observational description of the Milky Way. We will know the basic chemical and dynamical properties of its disk, bulge and halo, including some details of how these properties have evolved since the time the MW began to assemble.
Looking forward to a new generation of large telescopes and new surveys, how can we build on this detailed knowledge of the Milky Way, to see whether other large spirals share its properties? Here are a few possible items for the conference, concentrating on observational aspects. The DM section has nothing on direct detection.
- The evolution of stellar populations
The oldest stars of the MW disk, bulge and halo formed over a range of redshifts z > 2. In the era of JWST and the ELTs, how can we usefully relate the properties of these old MW stars to the properties of other galaxies observed at high redshifts? The chemical properties of these old stars have probably not evolved much since they formed, but dynamically we can expect a lot of evolution.
Comparing the MW with other large spirals at z = 0, did the MW and other spirals undergo a similar evolutionary path in their stellar populations? Did they undergo a similar rapid evolution in their age-metallicity relation (AMR) ? How different is the AMR for smaller spirals ? Can this be measured in other galaxies, from integrated light or individual stars?
- Disk assembly
The oldest disk stars in the MW are part of a thick disk that formed rapidly more than 10 Gyr ago. We know this directly, from the age and the a-enhancement of the thick disk stars. We also know that the thin disk formed more slowly over a much longer interval. It would be useful to know from direct observations of nearby large disk galaxies whether this was the usual path of disk assembly. The high turbulent velocities observed in starbursting disk galaxies at z > 2 suggest that it may be so.
In the Milky Way, the thick disk stars are usually identified chemically, in a compact thick a-enhanced structure. In other galaxies, the thick disk is identified structurally as an extended thick disk, probably partly made up of the flaring outer thin disk. The a-enhancement is important for our scenario of MW assembly, and it would be useful to know whether the thick disks of other galaxies have a-enhanced parts like the MW.
Getting adequate spectra of external thick disks to detect a-enhancement may be a bit difficult, but likely possible. Photometric method for detecting alpha-enhanced populations from their integrated light may also be possible.
- Radial migration, chemodynamic evolution, and the Galactic bar
Radial migration (RM) is believed to cause significant and continuing redistribution of stellar populations within the disk of the MW, so the current distribution of stellar populations does not reflect their distribution at the time of formation. RM is believed to be associated with transient asymmetric structures, like transient spiral arms and the formation of a bar. Is it possible to observe the effects of RM in other galaxies, such as the diffusion of chemical gradients with age, to evaluate its importance?
Recent simulations indicate that the bar of the MW is not just a structural feature, but is also important for the redistribution of stellar populations within the disk and bulge. Bars are common at z = 0 (more than 70% of galaxies have bar structures) but are rare for z > 1.5.
The dynamical parameters that regulate bar formation are not yet fully understood. The effect of bars on the chemical gradients in other spirals seems a well-defined observational question, and much work has gone into studying the problem, but it still remains uncertain. It seems worth continuing the search for these bar-related effects in other galaxies.
APOGEE spectra of bulge stars have revealed a flat metal-rich structure in the inner bulge, probably associated with the bar, and it maybe related to the migration of metal-rich stars from the inner galaxy to the solar neighborhood. It may be possible to detect such structures with IR photometry and IR spectra of integrated light in other nearby barred galaxies.
- Halo substructure
Halo substructure has been studied in detail in the MW and M31. M31 shows more and larger halo substructures, indicating the diversity between its satellite population and accretion history and those of the MW. Recent studies have also revealed the importance of the Magellanic Clouds in understanding the Galaxy’s halo, particularly through the infall of dwarf galaxies associated with the Clouds, through the influence of the Clouds on the dynamics of halo stellar streams, and through the distortion of the Galaxy’s dark matter potential by the MCs. The proximity of the Clouds also provides the opportunity to study in detail the effects of tidal interactions on the dynamics and star formation histories of this archetypal pair. This kind of work is currently possible out to distances of a few Mpc but only a few other galaxies have been studied in detail so far. This also relates to the next item.
- The out-of-equilibrium disk
Oscillations of the out-of-equilibrium MW disk are nicely seen in Gaia data. They are believed to be related to recent Galactic accretion or encounter events, and are potentially useful indicators of such events. Oscillations should then be common in other galaxies, with some indications from the HI kinematics of other spirals. M31 has had an active accretion history, and it would be interesting to see if oscillations of its disk can be detected kinematically.
- Dark matter
What do we know about the DM distributions in the MW and other large spirals, and how important is the DM in regulating the assembly and evolution of the galaxies, such as the formation of baryonic structures such as the bar, and their morphologies at z = 0?
DM halos are believed to keep growing to the present time. It would be interesting to know more observationally about this growth.
We have some statistical data on the scaling laws for dark halos of spirals, but only for those with minor bulges. Data on the relative contributions of baryons and dark matter to the gravitational field over the disks is gradually improving, for the MW and nearby spirals, and much more could be done with present technology. The ELTs offer the possibility of doing similar observations out to modest redshifts.
The DM content of the smallest dwarf galaxies remains interesting, in comparison with DM in the larger galaxies. Gaia is contributing to this field, but modeling these small systems is still difficult.
The nature of DM remains almost unconstrained. Recent microlensing programs are closing some of the options.