scholarly journals Dynamical masses of brightest cluster galaxies – II. Constraints on the stellar IMF

2020 ◽  
Vol 500 (3) ◽  
pp. 4153-4165
Author(s):  
S I Loubser ◽  
H Hoekstra ◽  
A Babul ◽  
Y M Bahé ◽  
M Donahue

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 ≤$z$ ≤ 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 and Jeans Anisotropic Method, 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.

2019 ◽  
Vol 14 (S353) ◽  
pp. 257-258
Author(s):  
S. I. Loubser

AbstractWe study the stellar and dynamical masses, as well as the stellar populations, of brightest cluster galaxies (BCGs) located in 32 massive clusters, and for a sub-sample of these use the results to place constraints on the Initial Mass Function (IMF). We measure the spatially-resolved stellar population properties of the BCGs, such as recent star formation episodes, and use it to predict their stellar mass-to-light ratios (ϒ*POP). We find that ∼60 per cent of the BCGs have constant ϒ*POP over the radial range (<15 kpc). We also use the stellar and dynamical mass profiles to derive the stellar mass-to-light ratio from dynamics (ϒ*DYN, see Loubser, these proceedings). We directly compare ϒ*POP with ϒ*DYN, and find that for most BCGs, a Salpeter IMF is needed to explain their properties, but we also find a small subset of BCGs for which a Kroupa-like IMF is needed to explain their properties.


2020 ◽  
Vol 496 (2) ◽  
pp. 1857-1880 ◽  
Author(s):  
S I Loubser ◽  
A Babul ◽  
H Hoekstra ◽  
Y M Bahé ◽  
E O’Sullivan ◽  
...  

ABSTRACT We investigate the stellar and dynamical mass profiles in the centres of 25 brightest cluster galaxies (BCGs) at redshifts of 0.05 ≤ z ≤ 0.30. Our spectroscopy enables us to robustly measure the Gauss–Hermite higher order velocity moments h3 and h4, which we compare to measurements for massive early-type galaxies, and central group galaxies. We measure positive central values for h4 for all the BCGs. We derive the stellar mass-to-light ratio ($\Upsilon _{\star \rm DYN}$), and velocity anisotropy (β) based on a multi-Gaussian expansion (MGE) and axisymmetric Jeans Anisotropic Methods (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 βz(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.


2019 ◽  
Vol 14 (S353) ◽  
pp. 255-256
Author(s):  
S. I. Loubser

AbstractWe investigate the stellar and dynamical mass profiles of 32 brightest cluster galaxies (BCGs, MK = −25.7 to −27.8 mag) in massive clusters (0.05 < z < 0.30), and in particular the rising velocity dispersion profiles of 23 of these BCGs found in Loubser et al. (2018). We make comprehensive measurements of the Gauss-Hermite higher order velocity moments h3 and h4, and find positive central values for h4 for all the BCGs. We model the stellar and dynamical mass profiles of 25 of the BCGs using the Multi-Gaussian Expansion (MGE) and Jeans Anisotropic Method (JAM) for an axisymmetric case, deriving the stellar mass-to-light ratio (ϒ*DYN), and anisotropy (βz). We further explicitly add a dark matter halo mass component (MDM within r200) which we constrain from weak lensing results. In this proceedings, we summarise the study and show an example of the results.


2016 ◽  
Vol 465 (1) ◽  
pp. 192-212 ◽  
Author(s):  
Simon Zieleniewski ◽  
Ryan C. W. Houghton ◽  
Niranjan Thatte ◽  
Roger L. Davies ◽  
Sam P. Vaughan

2012 ◽  
Vol 8 (S295) ◽  
pp. 200-203
Author(s):  
Chiara Tonini

AbstractWe investigate the evolution of Brightest Cluster Galaxies (BCGs) from redshift z ~ 1.6 to z = 0. We upgrade the hierarchical semi-analytic model of Croton et al. (2006) with a new spectro-photometric model that produces realistic galaxy spectra, making use of the Maraston (2005) stellar populations and a new recipe for the dust extinction. We compare the model predictions of the K-band luminosity evolution and the J-K, V-I and I-K colour evolution with a series of datasets, including Collins et al. (Nature, 2009) who argued that semi-analytic models based on the Millennium simulation cannot reproduce the red colours and high luminosity of BCGs at z > 1. We show instead that the model is well in range of the observed luminosity and correctly reproduces the colour evolution of BCGs in the whole redshift range up to z ~ 1.6. We argue that the success of the semi-analytic model is in large part due to the implementation of a more sophisticated spectro-photometric model. An analysis of the model BCGs shows an increase in mass by a factor 2-3 since z ~ 1, and star formation activity down to low redshifts. While the consensus regarding BCGs is that they are passively evolving, we argue that this conclusion is affected by the degeneracy between star formation history and stellar population models used in SED-fitting, and by the inefficacy of toy-models of passive evolution to capture the complexity of real galaxies, especially those with rich merger histories like BCGs. Following this argument, we also show that in the semi-analytic model, the BCGs show a realistic mix of stellar populations, and that these stellar populations are mostly old. In addition, the age-redshift relation of the model BCGs follows that of the Universe, meaning that given their merger history and star formation history, the ageing of BCGs is always dominated by the ageing of their stellar populations. In a ΛCDM Universe, we define such evolution as ‘passive in the hierarchical sense’.


1996 ◽  
Vol 175 ◽  
pp. 588-590
Author(s):  
D. Villani ◽  
S. Di Serego Alighieri

Stellar populations of high redshift radio galaxies (HzRG) (z up to 4.2) are the oldest stellar systems known, that is the ones formed at the earliest cosmological epochs. Therefore they are the best objects for providing us with information about the epoch of galaxy formation. The information on the stellar populations in HzRG are obtained from the study of their Integrated Spectral Energy Distribution (ISED) which are gathered both from spectra and integrated magnitudes. The most common approach for the interpretation of colors and spectral features of the energy distribution of galaxies is the Evolutionary Population Synthesis (EPS), which has been introduced for the first time by Tinsley in 1972. EPS models have often been used in the past to interpret the ISED of HzRG (Chambers & Charlot 1990; Lilly & Longair 1984; di Serego Alighieri et al. 1994) in order to draw conclusions on the age of the stellar populations and therefore on the epoch of galaxy formation. The results are sometimes conflicting and a number of very recent EPS models have become available (Bressan et al. 1995; Bruzual & Charlot 1993; Buzzoni 1989; Guiderdoni & Rocca-Volmerange 1987): we are therefore analysing the differences between the various EPS models with the aim of assessing their suitability to study the stellar population at early epochs. The EPS models assume for stars a given Initial Mass Function(IMF) as well as a Star Formation Rate (SFR). Then one can compute the number of stars with given mass present in the galaxy as a function of time. The position of each star in the HR diagram is determined by means of the isochrones, which are calculated from stellar evolutionary models. The ISED of a galaxy is obtained from the superposition of the spectra of single stars obtained from a stellar spectral library. Thus these models describe the galaxy ISED as a function of the time, giving a complete evolutionary picture.


1996 ◽  
Vol 171 ◽  
pp. 407-407 ◽  
Author(s):  
D. Lutz ◽  
F. Prada

Near-infrared longslit spectra of NGC 253 obtained with IRSPEC at the ESO NTT are presented. By analysis of the 12CO 2.29μm bandhead we find that the stellar population in the central starburst region (r ∼ 150 pc) rotates more slowly than the gas, but has a velocity dispersion of 128 km/s, about twice the value found for emission lines from the gas in this region. This implies an about five times higher dynamical mass than previously derived (Rieke et al. 1980), removing the need to invoke a lower mass cutoff in the starburst initial mass function. The peak of near-infrared emission is displaced from the dynamical center.


2012 ◽  
Vol 10 (H16) ◽  
pp. 127-127
Author(s):  
S. I. Loubser ◽  
P. Sánchez-Blázquez

AbstractThis study is part of a series devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their properties and the relationships between these and the properties of the host clusters. In this paper, we compare the stellar population properties derived from high signal-to-noise, optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements for 36 nearby BCGs to understand the diversity in the most rapidly evolving feature in old stellar systems, the UV-upturn. We investigate: (1) the possible differences between the UV-upturn of BCGs and those of a control sample of ordinary ellipticals in the same mass range, as well as possible correlations between the UV-upturn and other general properties of the galaxies; (2) possible correlations between the UV-upturn and the properties of the host clusters; (3) recently proposed scenarios where helium-sedimentation in the cluster centre can produce an enhanced UV-upturn. We find systematic differences between the UV-colours of BCGs and ordinary ellipticals, but we do not find correlations between these colours and the properties of the host clusters. Furthermore, the observations do not support the predictions made by the helium-sedimentation model as an enhancer of the UV-upturn.


2009 ◽  
Vol 5 (S262) ◽  
pp. 446-447
Author(s):  
Pieter Westera ◽  
François Cuisinier ◽  
Carlos R. Rabaça

AbstractWe examine the star forming phenomenon as it can be encountered in galaxies in the Sloan Digital Sky Survey, which possibly contains the largest homogeneous sample of star forming galaxy spectra to date.After eliminating all spectra with an insufficient signal-to-noise ratio, without strong emission lines, and without the [OII] λ3727 Å line, which is necessary for the determination of the gas metallicity (which excludes galaxies with redshift ≲ 0.024–0.025), our sample contains ~6000 spectra of star forming galaxies.Through a detailed stellar population analysis employing evolutionary synthesis methods we determined the stellar composition of these galaxies, that is, the masses, ages and metallicities of their partial stellar populations.We find that most, possibly all, galaxies of our sample contain, apart from the presently bursting, ionising young generation (≤ 107 yrs), old (≥ 109 yrs) and intermediate (between 107 and 109 yrs) populations, whereas the old population dominates the stellar mass (but not the light).We also find that high (stellar) mass galaxies have higher gas metallicities and lower present star formation rates relative to their total (stellar) masses, than low mass galaxies, indicating a higher chemical evolution degree for high mass galaxies.Furthermore, we find that gas enrichment mechanisms in star forming galaxies do not vary with galactic mass, being the same for low- and high-mass galaxies on average. Gas enrichment mechanisms seem to present a greater variety at the high-mass end, though, indicating a more complex assembly history for high-mass galaxies.


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