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Register a new userHigh M/L ratios of UCDs: a variation of the IMF?
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Various studies have established that the dynamical M/L ratios of ultra-compact dwarf galaxies (UCDs) tend to be at the limit or beyond the range explicable by standard stellar populations with canonical IMF. We discuss how IMF variations may account for these high M/L ratios and how observational approaches may in the future allow to discriminate between those possibilities. We also briefly discuss the possibility of dark matter in UCDs.
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A new type of compact stellar systems, labelled ``ultra-compact dwarf galaxies (UCDs), was discovered in the last decade. Recent studies show that their dynamical mass-to-light ratios (M/L) tend to be too high to be explained by canonical stellar populations, being on average about twice as large as those of Galactic globular clusters of comparable metallicity. If this offset is caused by dark matter in UCDs, it would imply dark matter densities as expected for the centers of cuspy dark matter halos, incompatible with cored dark matter profiles. Investigating the nature of the high M/L ratios in UCDs therefore offers important constraints on the phase space properties of dark matter particles. Here we describe an observational method to test whether a bottom-heavy IMF may cause the high M/L ratios of UCDs. We propose to use the CO index at 2.3mu -- which is sensitive to the presence of low-mass stars -- to test for a bottom heavy IMF. In the case that the high M/L ratios are caused by a bottom-heavy IMF, we show that the equivalent width of the CO index will be up to 30% weaker in UCDs compared to sources with similar metallicity that have canonical IMFs. We find that these effects are well detectable with current astronomical facilities in a reasonable amount of time (a few hours to nights). Measuring the CO index of UCDs hence appears a promising tool to investigate the origin of their high M/L ratios.
Under Newtonian gravity total masses for dSph galaxies will scale as $M_{T} propto R_{e} sigma^{2}$, with $R_{e}$ the effective radius and $sigma$ their velocity dispersion. When both of the above quantities are available, the resulting masses are compared to observed stellar luminosities to derive Newtonian mass to light ratios, given a physically motivated proportionality constant in the above expression. For local dSphs and the growing sample of ultrafaint such systems, the above results in the largest mass to light ratios of any galactic systems known, with values in the hundreds and even thousands being common. The standard interpretation is for a dominant presence of an as yet undetected dark matter component. If however, reality is closer to a MONDian theory at the extremely low accelerations relevant to such systems, $sigma$ will scale with { stellar mass} $M_{*}^{1/4}$. This yields an expression for the mass to light ratio which will be obtained under Newtonian assumptions of $(M/L)_{N}=120 R_{e}(Upsilon_{*}/L)^{1/2}$. Here we compare $(M/L)_{N}$ values from this expression to Newtonian inferences for this ratios for the actual $(R_{e}, sigma, L)$ observed values for a sample of recently observed ultrafaint dSphs, obtaining good agreement. Then, for systems where no $sigma$ values have been reported, we give predictions for the $(M/L)_{N}$ values which under a MONDian scheme are expected once kinematical observations become available. For the recently studied Dragonfly 44 { and Crater II systems}, reported $(M/L)_{N}$ values are also in good agreement with MONDian expectations.
We show that the discrepancy between the Tully-Fisher relation and the luminosity function predicted by most phenomenological galaxy formation models is mainly due to overmerging of galaxy haloes. We have circumvented this overmerging problem, which is inherent in both the Press-Schechter formalism and dissipationless N-body simulations, by including a specific galaxy halo formation recipe into an otherwise standard N-body code. This numerical technique provides the merger trees which, together with simplified gas dynamics and star formation physics, constitute our implementation of a phenomenological galaxy formation model. Resolving the overmerging problem provides us with the means to match both the I-band Tully-Fisher relation and the B and K band luminosity functions within an EdS sCDM structure formation scenario. It also allows us to include models for chemical evolution and starbursts, which improves the match to observational data and renders the modelling more realistic. We show that the inclusion of chemical evolution into the modelling requires a significant fraction of stars to be formed in short bursts triggered by merging events.
Within a galaxy the stellar mass-to-light ratio $Upsilon_*$ is not constant. Spatially resolved kinematics of nearby early-type galaxies suggest that allowing for a variable initial mass function (IMF) returns significantly larger $Upsilon_*$ gradients than if the IMF is held fixed. If $Upsilon_*$ is greater in the central regions, then ignoring the IMF-driven gradient can overestimate $M_*^{rm dyn}$ by as much as a factor of two for the most massive galaxies, though stellar population estimates $M_*^{rm SP}$ are also affected. Large $Upsilon_*$-gradients have four main consequences: First, $M_*^{rm dyn}$ cannot be estimated independently of stellar population synthesis models. Second, if there is a lower limit to $Upsilon_*$ and gradients are unknown, then requiring $M_*^{rm dyn}=M_*^{rm SP}$ constrains them. Third, if gradients are stronger in more massive galaxies, then $M_*^{rm dyn}$ and $M_*^{rm SP}$ can be brought into agreement, not by shifting $M_*^{rm SP}$ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising $M_*^{rm dyn}$ estimates in the literature downwards. Fourth, accounting for $Upsilon_*$ gradients changes the high-mass slope of the stellar mass function $phi(M_*^{rm dyn})$, and reduces the associated stellar mass density. These conclusions potentially impact estimates of the need for feedback and adiabatic contraction, so our results highlight the importance of measuring $Upsilon_*$ gradients in larger samples.
Evolutionary synthesis models are the prime method to construct models of stellar populations, and to derive physical parameters from observations. One of the assumptions for such models so far has been the time-independence of the stellar mass function. However, dynamical simulations of star clusters in tidal fields have shown the mass function to change due to the preferential removal of low-mass stars from clusters. Here we combine the results from dynamical simulations of star clusters in tidal fields with our evolutionary synthesis code GALEV to extend the models by a new dimension: the total cluster disruption time. We reanalyse the mass function evolution found in N-body simulations of star clusters in tidal fields, parametrise it as a function of age and total cluster disruption time and use this parametrisation to compute GALEV models as a function of age, metallicity and the total cluster disruption time. We study the impact of cluster dissolution on the colour (generally, they become redder) and magnitude (they become fainter) evolution of star clusters, their mass-to-light ratios (off by a factor of ~2 -- 4 from standard predictions), and quantify the effect on the cluster age determination from integrated photometry (in most cases, clusters appear to be older than they are, between 20 and 200%). By comparing our model results with observed M/L ratios for old compact objects in the mass range 10^4.5 -- 10^8 Msun, we find a strong discrepancy for objects more massive than 10^7 Msun (higher M/L). This could be either caused by differences in the underlying stellar mass function or be an indication for the presence of dark matter in these objects. Less massive objects are well represented by the models. The models for a range of total cluster disruption times are available online. (shortened)
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