Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDynamical masses of brightest cluster galaxies II: constraints on the stellar IMF
81
0
0.0
(
0
)
Added by
Susan Ilani Loubser
Publication date
2020
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We use stellar and dynamical mass profiles, combined with a stellar population analysis, of 32 brightest cluster galaxies (BCGs) at redshifts of 0.05 $leq z leq$ 0.30, to place constraints on their stellar Initial Mass Function (IMF). We measure the spatially-resolved stellar population properties of the BCGs, and use it to derive their stellar mass-to-light ratios ($Upsilon_{star rm POP}$). We find young stellar populations ($<$200 Myr) in the centres of 22 per cent of the sample, and constant $Upsilon_{star rm POP}$ within 15 kpc for 60 per cent of the sample. We further use the stellar mass-to-light ratio from the dynamical mass profiles of the BCGs ($Upsilon_{star rm DYN}$), modelled using a Multi-Gaussian Expansion (MGE) and Jeans Anisotropic Method (JAM), with the dark matter contribution explicitly constrained from weak gravitational lensing measurements. We directly compare the stellar mass-to-light ratios derived from the two independent methods, $Upsilon_{star rm POP}$ (assuming some IMF) to $Upsilon_{star rm DYN}$ for the subsample of BCGs with no young stellar populations and constant $Upsilon_{star rm POP}$. We find that for the majority of these BCGs, a Salpeter (or even more bottom-heavy) IMF is needed to reconcile the stellar population and dynamical modelling results although for a small number of BCGs, a Kroupa (or even lighter) IMF is preferred. For those BCGs better fit with a Salpeter IMF, we find that the mass-excess factor against velocity dispersion falls on an extrapolation (towards higher masses) of known literature correlations. We conclude that there is substantial scatter in the IMF amongst the highest-mass galaxies.
rate research
Read More
We investigate the stellar and dynamical mass profiles in the centres of 25 brightest cluster galaxies (BCGs) at redshifts of 0.05 $leq z leq$ 0.30. Our spectroscopy enables us to robustly measure the Gauss-Hermite higher order velocity moments $h_{3}$ and $h_{4}$, which we compare to measurements for massive early-type galaxies, and central group galaxies. We measure positive central values for $h_{4}$ for all the BCGs. We derive the stellar mass-to-light ratio ($Upsilon_{star rm DYN}$), and velocity anisotropy ($beta$) based on a Multi-Gaussian Expansion (MGE) and axisymmetric Jeans Anisotropic Methods (JAM, cylindrically- and spherically-aligned). We explicitly include a dark matter halo mass component, which is constrained by weak gravitational lensing measurements for these clusters. We find a strong correlation between anisotropy and velocity dispersion profile slope, with rising velocity dispersion profiles corresponding to tangential anisotropy and decreasing velocity dispersion profiles corresponding to radial anisotropy. The rising velocity dispersion profiles can also indicate a significant contribution from the intracluster light (ICL) to the total light (in projection) in the centre of the galaxy. For a small number of BCGs with rising velocity dispersion profiles, a variable stellar mass-to-light ratio can also account for the profile shape, instead of tangential anisotropy or a significant ICL contribution. We note that, for some BCGs, a variable $beta_{z}(r)$ (from radial to tangential anisotropy) can improve the model fit to the observed kinematic profiles. The observed diversity in these properties illustrates that BCGs are not the homogeneous class of objects they are often assumed to be.
Many recent works have attempted to constrain the stellar initial mass function (IMF) inside massive clusters by comparing their dynamical mass estimates to the measured light. These studies have come to different conclusions, with some claiming standard Kroupa-type IMFs, while others have claimed extreme non-standard IMFs. However, the results appear to be correlated with the age of the clusters, as older clusters (>80 Myr) all appear to be well fit by a Kroupa-type IMF whereas younger clusters display significant scatter in their best fitting IMF. Here we show that this is likely due to the fact that young clusters are out of virial equilibrium and therefore cannot be used for such studies. Hence only the older clusters are suitable for IMF studies. Using only these clusters we find that the IMF does not vary significantly. The youngest clusters can be used instead to constrain the star-formation efficiency (SFE) within clusters. We find that the SFE varies between 20 and 60% and we conclude that approximately 60% of young clusters are unbound and will not survive for more than a few 10s of Myr (i.e. infant mortality).
We measure the luminosity profiles of 16 brightest cluster galaxies (BCGs) at $0.4 < z < 0.8$ using high resolution F160W NICMOS and F814W WFPC2 HST imaging. The heterogeneous sample is drawn from a variety of surveys: seven from clusters in the Einstein Medium Sensitivity Survey, five from the Las Campanas Distant Cluster Survey and its northern hemisphere precursor, and the remaining four from traditional optical surveys. We find that the surface brightness profiles of all but three of these BCGs are well described by a standard de Vaucouleurs ($r^{1/4}$) profile out to at least $sim2r_{e}$ and that the biweight-estimated NICMOS effective radius of our high redshift BCGs ($r_{e} = 8.3pm 1.4$ kpc for $H_{0} = 80$ km s$^{-1}$ Mpc$^{-1}$, $Omega_{m} = 0.2, Omega_Lambda = 0.0$) is $sim 2$ times smaller than that measured for a local BCG sample. If high redshift BCGs are in dynamical equilibrium and satisfy the same scaling relations as low redshift ones, this change in size would correspond to a mass growth of a factor of 2 since $z sim 0.5$. However, the biweight-estimated WFPC2 effective radius of our sample is 18 $pm $ 5.1 kpc, which is fully consistent with the local sample. While we can rule out mass accretion rates higher than a factor of 2 in our sample, the discrepancy between our NICMOS and WFPC2 results, which after various tests we describe appears to be physical, does not yet allow us to place strong constraints on accretion rates below that level.
Understanding the formation history of brightest cluster galaxies is an important topic in galaxy formation. Utilizing the Planck Sunyaev-Zeldovich cluster catalog, and applying the Ansatz that the most massive halos at one redshift remain among the most massive ones at a slightly later cosmic epoch, we have constructed cluster samples at redshift z~0.4 and z~0.2 that can be statistically regarded as progenitor-descendant pairs. This allows us to study the stellar mass assembly history of BCGs in these massive clusters at late times, finding the degree of growth between the two epochs is likely at only few percent level, which is far lower compared to the prediction from a state-of-the-art semi-analytic galaxy formation model.
We present stellar kinematics and orbit superposition models for the central regions of four Brightest Cluster Galaxies (BCGs), based upon integral-field spectroscopy at Gemini, Keck, and McDonald Observatories. Our integral-field data span radii from < 100 pc to tens of kpc. We report black hole masses, M_BH, of 2.1 +/- 1.6 x 10^10 M_Sun for NGC 4889, 9.7 + 3.0 - 2.6 x 10^9 M_Sun for NGC 3842, and 1.3 + 0.5 - 0.4 x 10^9 M_Sun for NGC 7768. For NGC 2832 we report an upper limit of M_BH < 9 x 10^9 M_Sun. Stellar orbits near the center of each galaxy are tangentially biased, on comparable spatial scales to the galaxies photometric cores. We find possible photometric and kinematic evidence for an eccentric torus of stars in NGC 4889, with a radius of nearly 1 kpc. We compare our measurements of M_BH to the predicted black hole masses from various fits to the relations between M_BH and stellar velocity dispersion, luminosity, or stellar mass. The black holes in NGC 4889 and NGC 3842 are significantly more massive than all dispersion-based predictions and most luminosity-based predictions. The black hole in NGC 7768 is consistent with a broader range of predictions.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
