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Are We Really Missing Small Galaxies?

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dwarf galaxy
Theory predicts the formation of many dwarf galaxies like the one imaged here, LEDA 677373. A new study explores why we observe so many fewer small galaxies than predicted. [ESA/Hubble]

One long-standing astrophysical puzzle is that of so-called “missing” dwarf galaxies: the number of small dwarf galaxies that we observe is far fewer than that predicted by theory. New simulations, however, suggest that perhaps there’s no mystery after all.

Missing Dwarfs

Via Lactea

Dark-matter cosmological simulations predict many small galaxy halos for every large halo that forms. [The Via Lactea project]

Models of a lambda-cold-dark-matter (ΛCDM) universe predict the distribution of galaxy halo sizes throughout the universe, suggesting there should be many more small galaxies than large ones. In what has become known as “the missing dwarf problem”, however, we find that while we observe the expected numbers of galaxies at the larger end of the scale, we don’t see nearly enough small galaxies to match the predictions.

Are these galaxies actually missing? Are our predictions wrong? Or are the galaxies there and we’re just not spotting them? A recent study led by Alyson Brooks (Rutgers University) uses new simulations to explore what’s causing the difference between theory and observation.

detectable fraction of galaxies

The fraction of detectable halos as a function of velocity, according to the authors’ simulations. Below ~35 km/s, the detectability of the galaxies drops precipitously. [Brooks et al. 2017]

Simulating Galactic Velocities

Because we can’t weigh a galaxy directly, one proxy used for galaxy mass is its circular velocity; the more massive a galaxy, the faster gas and stars rotate around its center. The discrepancy between models and observations lies in what’s known as the “galaxy velocity function”, which describes the number density of galaxies for a given circular velocity. While theory and observations agree for galaxies with circular velocities above ~100 km/s, theory predicts far more dwarfs below this velocity than we observe.

To investigate this problem, Brooks and collaborators ran a series of cosmological simulations based on our understanding of a ΛCDM universe. Instead of exploring the result using only dark matter, however, the team included baryons in their simulations. They then produced mock observations of the resulting galaxy velocities to see what an observed velocity function would look like for their simulated galaxies.

No Problem After All?

galaxy velocity function

Comparison of theoretical velocity functions to observations. The black dashed line shows the original, dark-matter-only model predictions; the black solid line includes the effects of detectability. Blue lines show the authors’ new model, including the effects of detectability and inclusion of baryons. The red and teal data points from observations match this corrected model well. [Brooks et al. 2017]

Based on their baryon-inclusive simulations, Brooks and collaborators argue that there are two main factors that have contributed to the seeming theory/observation mismatch of the missing dwarf problem:

  1. Galaxies with low velocities aren’t detectable by our current surveys.
    The authors found that the detectable fraction of their simulated galaxies plunges as soon as galaxy velocity drops below ~35 km/s. They conclude that we’re probably unable to see a large fraction of the smallest galaxies.
  2. We’re not correctly inferring the circular velocity of the galaxies.
    Circular velocity is usually measured by looking at the line width of a gas tracer like HI. The authors find that this doesn’t trace the full potential wells of the dwarf galaxies, however, resulting in an incorrect interpretation of their velocities.

The authors show that the inclusion of these effects in the theoretical model significantly changes the predicted shape of the galaxy velocity function. This new function beautifully matches observations, neatly eliminating the missing dwarf problem. Perhaps this long-standing mystery has been a problem of interpretation all along!

Citation

Alyson M. Brooks et al 2017 ApJ 850 97. doi:10.3847/1538-4357/aa9576