Problems in Structure Formation at Low Masses

The ΛCDM cosmological model is very successful at describing the universe on large scales. Tensions remain, however, at smaller scales. For example, the formation of globular clusters--ancient star clusters that seem to be very devoid in dark matter--remains mysterious given the typical ΛCDM paradigm of baryonic matter collapsing into much larger, existing DM halos.

Nine small galaxies of various sizes and shapes are depicted. In the background of each image you can see scattered stars.

The smallest, faintest galaxies that can be observed also pose a theoretical challenge. Anomalies in the rotation curves, distribution of dark matter, and other properties of these galaxies are difficult to explain using standard ΛCDM models and simulations (see Sales et al 2022 for a review).

The image to the left shows a selection of dwarf galaxies spanning six orders of magnitude in stellar mass (Bullock & Boylan-Kolchin 2017). All but the LMC, WLM, and Pegasus are devoid of gas, and no longer forming stars. The bar on the lower right hand corner shows a length scale corresponding to 200pc in each image. The faintest galaxies depicted in this figure represet the limit currently detectable in the local volume; future suveys have the potential to reveal lots of faint dwarf galaxies at much greater distances. Although small and faint, these galaxies have the potential to test the most modern theories of galaxy formation and the best, state-of-the-art cosmological simualtions in the field.

DM GHOSts and Halo Formation

An important correction to the standard ΛCDM paradigm--the stream velocity--is discussed here. Naoz and Narayan (2014) predicted that the "Supersonically Induced Gas Objects" (SIGOs) may be progenitors of globular clusters. However, the stream velocity also induces the formation of a second class of objects: Dark Matter + Gas Halos Offset by Streaming (DM GHOSts). These are very different from SIGOs. They are diffuse, highly dark matter enriched, and reach slightly higher masses.



The image above shows a region of space in our simulation 5 physical kpc on a side containing several DM GHOSts (A,B,C, and D) and a SIGO (Williams et al 2022). On the left, the projected density of gas in the region is shown, while on the left, the projected density of DM is plotted. Green circles show the boundaries of the DM halos, while the orange circles show the boundaries of the gas component of those same objects. The stream velocity has elongated both the gas and DM components and offset the gas and DM centers of mass. The SIGO appears as a gas object that had no associated DM halo. Meanwhile two dark-matter halos with no associated gas component are shown in light pink.

Predictions for JWST: the UV luminosity function and beyond

Recently, we carried out an investigation on how the stream velocity may impact the faint galaxies observed by JWST at high redshift (Williams et al 2024).

Click here to read a description of our study aimed at the general public.

  • We confirmed that the stream velocity causes an increase in the size of the DM halo for fixed stellar mass for low mass, early galaxies (z=12, galaxies with stellar mass less than a few million solar masses).
  • Star formation is delayed in galaxies formed in areas with high stream velocity.
  • Galaxies in regions with high stream velcocity greater than around 10 million solar masses in stars experience an initial delay in star formation, followed by a rapid burst of star formation caused by the accretion of gas streams that had been advected through this effect.
  • This burst of star formation corresponds to a significant boost in the UV luminosity function for JWST at z=12.

To the right, Fig. 5 of (Williams et al 2024) shows the predicted UVLF for JWST with (pink) and without (blue) the stream velocity. The colored points show observations with JWST at the time of publication. The grey lines show various theoretical predictions (not including the stream velocity). We suggest that a few magnitudes fainter than the current observational constraints, the stream velocity effect will enhance regions of the universe with a boosted UVLF as in the figure.

Rotational and Morphological Properties of DM GHOSts

My 2023 paper (Williams et al 2022) explores the rotational and morphological properties of these small, mysterious, ghostly structures, to investigate how they might be observed--either in the early universe, using JWST or lensing, or nearby, as some evolved object at the current redshift.

My major findings in this paper include:

  • Updated morphological investigation of supersonically-induced structures: I investigated their eccentricity and developed an analytical ellipsoid potential to contextualize these numerical results. Both the stream velocity and molecular cooling serve to elongate structures to more extreme axis ratios. Standard analytical models of collapse assume sphericity and isotropy (such as the NFW profile). These results indicate that in regions of streaming, these assumptions do not apply to the majority of systems.
  • The stream velocity also elongates the dark matter component of DM GHOSts, causing DM halos at low masses in regions of streaming to be more eccentric than their classical counterparts.
  • A study of rotation and angular momentum support of SIGOs and DM GHOSts in comparison with the ``classical" formation channel--these objects are more rotationally supported. I also investigated the alignment of these with respect to other parameters of the systems.
  • The first rotation curves of SIGOs and DM GHOSts. In the image to the right (Fig. 12 of Williams et al): Rotation curves of DM GHOSts (top panel) and classical halos (bottom panel). We found a bifurcation among the DM GHOSts: The lower mass systems seem to have a more slowly-rising rotation curve, indicated a core-type density profile, rather than a cusp, as is usually expected in standard structure formation.

In this paper, we speculate that anomalous properties of these DM GHOSts may be preserved to the current redshift and explain some observed anomalies in the population of faint dwarf galaxies, such as unusual rotation curves, scatter in the stellar-halo mass relation, and more.