A top-heavy stellar initial mass function in starbursts as an explanation for the high mass-to-light ratios of ultra compact dwarf galaxies

It has been shown recently that the dynamical V-band mass-to-light ratios of compact stellar systems with masses from 10^6 to 10^8 Solar masses are not consistent with the predictions from simple stellar population (SSP) models. Top-heavy stellar initial mass functions (IMFs) in these so-called ultra compact dwarf galaxies (UCDs) offer an attractive explanation for this finding, the stellar remnants and retained stellar envelopes providing the unseen mass. We therefore construct a model which quantifies by how much the IMFs of UCDs would have to deviate in the intermediate-mass and high-mass range from the canonical IMF in order to account for the enhanced M/L_V ratio of the UCDs. The deduced high-mass IMF in the UCDs depends on the age of the UCDs and the number of faint products of stellar evolution retained by them. Assuming that the IMF in the UCDs is a three-part power-law equal to the canonical IMF in the low-mass range and taking 20% as a plausible choice for the fraction of the remnants of high-mass stars retained by UCDs, the model suggests the exponent of the high-mass IMF to be approximately 1.6 if the UCDs are 13 Gyr old (i.e. almost as old as the Universe) or approximately 1.0 if the UCDs are 7 Gyr old, in contrast to 2.3 for the Salpeter-Massey IMF. If the IMF was as top-heavy as suggested here, the stability of the UCDs might have been threatened by heavy mass loss induced by the radiation and evolution of massive stars. The central densities of UCDs must have been in the range 10^6 to 10^7 Solar masses per cubic parsec when they formed with star formation rates of 10 to 100 Solar masses per year.

💡 Research Summary

The paper addresses a long‑standing discrepancy between the observed dynamical V‑band mass‑to‑light ratios (M/L_V) of ultra‑compact dwarf galaxies (UCDs) and the values predicted by simple stellar population (SSP) models that assume a canonical initial mass function (IMF). UCDs with masses between 10⁶ and 10⁸ M☉ systematically exhibit M/L_V ratios that are 30 %–100 % higher than SSP expectations, implying the presence of a substantial amount of unseen mass. The authors propose that this excess can be explained if the IMF in UCDs was “top‑heavy,” i.e., enriched in intermediate‑ and high‑mass stars relative to the standard Kroupa/Chabrier form.

To quantify the required deviation, they adopt a three‑segment power‑law IMF: the low‑mass (0.1–0.5 M☉) and intermediate‑mass (0.5–1 M☉) slopes are fixed to the canonical values, while the high‑mass slope (α for M > 1 M☉) is left free. They further assume that a fraction f_ret of the remnants of massive stars (white dwarfs, neutron stars, black holes) remains gravitationally bound to the UCD after supernova explosions. A plausible value of f_ret = 0.20 is adopted, reflecting the expectation that most remnants are ejected but a modest portion is retained.

Using observed velocity dispersions, radii, and luminosities, the authors compute dynamical masses for a sample of UCDs and compare these to SSP predictions for various ages and α values. The analysis shows that, for an old population (13 Gyr, comparable to the age of the Universe), a high‑mass slope of α ≈ 1.6 reproduces the observed M/L_V, whereas a younger age (7 Gyr) requires an even flatter slope, α ≈ 1.0. By contrast, the Salpeter‑Massey IMF has α = 2.35, indicating that the inferred IMF must be dramatically more top‑heavy.

A top‑heavy IMF has profound implications for the formation conditions of UCDs. The authors estimate that, to produce the required number of massive stars, the progenitor star‑forming clouds must have had central mass densities of 10⁶–10⁷ M☉ pc⁻³ and star‑formation rates of 10–100 M☉ yr⁻¹. Such extreme environments would generate intense radiation fields, stellar winds, and supernova feedback, leading to rapid mass loss. The paper discusses the stability of the nascent UCD under these conditions, concluding that if mass loss exceeds roughly 30 %–50 % of the initial mass, the system could become unbound. Therefore, while a top‑heavy IMF can explain the high M/L_V, it also raises the question of whether UCDs could survive the violent early phase.

The authors acknowledge several sources of uncertainty. The retained‑remnant fraction f_ret is poorly constrained; a lower value would demand an even flatter IMF, while a higher value would relax the required α. The true ages of individual UCDs are uncertain, and age directly influences the stellar mass‑to‑light conversion. Binary and higher‑order multiple systems could inflate the dynamical mass estimates, and a modest contribution from dark matter cannot be ruled out. Moreover, the dynamical mass estimates rely on assumptions of spherical symmetry and isotropic velocity distributions, which may not hold for all objects.

To test the top‑heavy IMF hypothesis, the paper proposes several observational strategies. High‑resolution spectroscopy in the near‑infrared and optical can constrain the low‑mass stellar content via gravity‑sensitive features, thereby directly measuring the IMF slope at the faint end. X‑ray and radio observations could reveal the presence of accreting black holes or pulsars, providing indirect evidence for a large population of massive‑star remnants. Finally, detailed N‑body simulations that incorporate realistic feedback, mass loss, and remnant retention are needed to assess whether a UCD formed with the inferred IMF can remain bound over a Hubble time.

In summary, the study presents a coherent framework in which a significantly top‑heavy IMF, combined with a modest retention of massive‑star remnants, can account for the anomalously high mass‑to‑light ratios observed in ultra‑compact dwarf galaxies. While the scenario is physically plausible and matches the data, it hinges on several poorly known parameters, and future observations and simulations are essential to confirm or refute this explanation.