scholarly journals What does (not) drive the variation of the low-mass end of the stellar initial mass function of early-type galaxies

2021 ◽  
Vol 645 ◽  
pp. L1
Author(s):  
C. E. Barbosa ◽  
C. Spiniello ◽  
M. Arnaboldi ◽  
L. Coccato ◽  
M. Hilker ◽  
...  
Keyword(s):  
Stellar Population ◽  
Velocity Dispersion ◽  
Mass Function ◽  
Initial Mass Function ◽  
Population Parameters ◽  
Early Type ◽  
Stellar Velocity ◽  
Stellar Initial Mass Function ◽  
Low Mass ◽  
The Imf

Context. The stellar initial mass function (IMF) seems to be variable and not universal, contrary to what has been argued in the literature over the last three decades. Several relations of the low-mass end of the IMF slope with other stellar population, photometrical, and kinematical parameters of massive early-type galaxies (ETGs) have been proposed, but consensus on the factual cause of the observed variations has not yet been reached. Aims. We investigate the relationship between the IMF and other stellar population parameters in NGC 3311, the central galaxy of the Hydra I cluster. NGC 3311 is a unique laboratory, characterized by old and metal-rich stars, that is similar to other massive ETGs for which the IMF slope has been measured to be bottom-heavy (i.e., dwarf-rich); however, it has unusual stellar velocity dispersion and [α/Fe] profiles, both of which increase with radius. Methods. We use the spatially resolved stellar population parameters (age, total metallicity, and [α/Fe]) that were derived in a forthcoming paper (Barbosa et al. 2020) – via the full-spectrum fitting of high signal-to-noise MUSE observations – to compare the IMF slope in the central part of NGC 3311 (R ≲ 16 kpc) against other stellar parameters, with the goal of assessing their relations and dependencies. Results. For NGC 3311, we unambiguously invalidate the previously observed direct correlation between the IMF slope and the local stellar velocity dispersion, confirming some doubts that had been raised in the literature. This relation may simply arise as a spatial coincidence between the region with the largest stellar velocity dispersion and the region where the oldest in situ population is found and dominates the light. We also show robust evidence that the proposed IMF−metallicity relation is contaminated by the degeneracy between these two parameters. We do confirm that the stellar content in the innermost region of NGC 3311 follows a bottom-heavy IMF, in line with other literature results. The tightest correlations we found are those between stellar age and the IMF and between the galactocentric radius and the IMF. Conclusions. The variation of the IMF at its low-mass end is not due to kinematical, dynamical, or global properties in NGC 3311. We speculate instead that the IMF might be dwarf-dominated in the “red nuggets” that formed through a very short and intense star formation episode at high redshifts (z >  2) when the Universe was denser and richer in gas, and which then ended up being the central cores of today’s giant ellipticals.

scholarly journals Variation in the Stellar Initial Mass Function from the Chromospheric Activity of M Dwarfs in Early-type Galaxies

2021 ◽  
Vol 923 (1) ◽  
pp. 43
Author(s):  
Pieter van Dokkum ◽  
Charlie Conroy
Keyword(s):  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Early Type ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Chromospheric Activity ◽  
Stellar Initial Mass Function ◽  
Low Mass ◽  
The Imf

Abstract Mass measurements and absorption-line studies indicate that the stellar initial mass function (IMF) is bottom-heavy in the central regions of many early-type galaxies, with an excess of low-mass stars compared to the IMF of the Milky Way. Here we test this hypothesis using a method that is independent of previous techniques. Low-mass stars have strong chromospheric activity characterized by nonthermal emission at short wavelengths. Approximately half of the UV flux of M dwarfs is contained in the λ1215.7 Lyα line, and we show that the total Lyα emission of an early-type galaxy is a sensitive probe of the IMF with a factor of ∼2 flux variation in response to plausible variations in the number of low-mass stars. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure the Lyα line in the centers of the massive early-type galaxies NGC 1407 and NGC 2695. We detect Lyα emission in both galaxies and demonstrate that it originates in stars. We find that the Lyα to i-band flux ratio is a factor of 2.0 ± 0.4 higher in NGC 1407 than in NGC 2695, in agreement with the difference in their IMFs as previously determined from gravity-sensitive optical absorption lines. Although a larger sample of galaxies is required for definitive answers, these initial results support the hypothesis that the IMF is not universal but varies with environment.

scholarly journals IMF radial gradients in most massive early-type galaxies

2019 ◽  
Vol 489 (3) ◽  
pp. 4090-4110 ◽  
Author(s):  
F La Barbera ◽  
A Vazdekis ◽  
I Ferreras ◽  
A Pasquali ◽  
C Allende Prieto ◽  
...  
Keyword(s):  
Stellar Population ◽  
Velocity Dispersion ◽  
Mass Density ◽  
Initial Mass Function ◽  
Early Type ◽  
Low Mass Stars ◽  
Surface Mass ◽  
Low Mass ◽  
Surface Mass Density ◽  
The Imf

ABSTRACT Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and near-infrared initial mass function (IMF)-sensitive features in a representative sample of galaxies at the very high mass end of the galaxy population. The sample consists of seven early-type galaxies (ETGs) at z ∼ 0.05, with central velocity dispersion in the range 300 ≲ σ ≲ 350 km s−1. Using state-of-the-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiO and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to ∼4 kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $\rm \sim 10^{10}\, M_\odot \, kpc^{-2}$, or, alternatively, within a characteristic radius of ∼2 kpc. Radial distance, in physical units, and surface mass density are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter that fully explains variations in the stellar IMF, but IMF radial profiles at z ∼ 0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.

scholarly journals Stellar velocity dispersion and initial mass function gradients in dissipationless galaxy mergers

2020 ◽  
Vol 499 (1) ◽  
pp. 559-572
Author(s):  
Carlo Nipoti ◽  
Carlo Cannarozzo ◽  
Francesco Calura ◽  
Alessandro Sonnenfeld ◽  
Tommaso Treu
Keyword(s):  
Velocity Dispersion ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Galaxy Mergers ◽  
Orbital Parameters ◽  
Spatially Resolved ◽  
Stellar Velocity ◽  
Stellar Initial Mass Function ◽  
The Imf

ABSTRACT The stellar initial mass function (IMF) is believed to be non-universal among early-type galaxies (ETGs). Parametrizing the IMF with the so-called IMF mismatch parameter αIMF, which is a measure of the stellar mass-to-light ratio of an ensemble of stars and thus of the ‘heaviness’ of its IMF, one finds that for ETGs αe (i.e. αIMF integrated within the effective radius Re) increases with σe (the line-of-sight velocity dispersion σlos integrated within Re) and that, within the same ETG, αIMF tends to decrease outwards. We study the effect of dissipationless (dry) mergers on the distribution of the IMF mismatch parameter αIMF in ETGs using the results of binary major and minor merging simulations. We find that dry mergers tend to make the αIMF profiles of ETGs shallower, but do not alter significantly the shape of the distributions in the spatially resolved σlos–αIMF space. Individual galaxies undergoing dry mergers tend to decrease their αe, due to erosion of αIMF gradients and mixing with stellar populations with lighter IMF. Their σe can either decrease or increase, depending on the merging orbital parameters and mass ratio, but tends to decrease for cosmologically motivated merging histories. The αe–σe relation can vary with redshift as a consequence of the evolution of individual ETGs: based on a simple dry-merging model, ETGs of given σe are expected to have higher αe at higher redshift, unless the accreted satellites are so diffuse that they contribute negligibly to the inner stellar distribution of the merger remnant.

scholarly journals Connection between dynamically derived IMF normalisation and stellar populations

2014 ◽  
Vol 10 (S311) ◽  
pp. 49-52
Author(s):  
Richard M. McDermid
Keyword(s):  
Stellar Population ◽  
Stellar Dynamics ◽  
Mass Function ◽  
Abundance Ratio ◽  
Initial Mass Function ◽  
Low Mass Stars ◽  
Stellar Initial Mass Function ◽  
Low Mass ◽  
Future Work ◽  
The Imf

AbstractIn this contributed talk I present recent results on the connection between stellar population properties and the normalisation of the stellar initial mass function (IMF) measured using stellar dynamics, based on a large sample of 260 early-type galaxies observed as part of the ATLAS3D project. This measure of the IMF normalisation is found to vary non-uniformly with age- and metallicity-sensitive absorption line strengths. Applying single stellar population models, there are weak but measurable trends of the IMF with age and abundance ratio. Accounting for the dependence of stellar population parameters on velocity dispersion effectively removes these trends, but subsequently introduces a trend with metallicity, such that ‘heavy’ IMFs favour lower metallicities. The correlations are weaker than those found from previous studies directly detecting low-mass stars, suggesting some degree of tension between the different approaches of measuring the IMF. Resolving these discrepancies will be the focus of future work.

scholarly journals Fornax 3D project: a two-dimensional view of the stellar initial mass function in the massive lenticular galaxy FCC 167

2019 ◽  
Vol 626 ◽  
pp. A124 ◽  
Author(s):  
I. Martín-Navarro ◽  
M. Lyubenova ◽  
G. van de Ven ◽  
J. Falcón-Barroso ◽  
L. Coccato ◽  
...  
Keyword(s):  
Stellar Population ◽  
Population Analysis ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Dynamical Structure ◽  
Two Dimensional ◽  
Early Type ◽  
Stellar Initial Mass Function ◽  
The Imf

The stellar initial mass function (IMF) regulates the baryonic cycle within galaxies, and is a key ingredient for translating observations into physical quantities. Although it was assumed to be universal for decades, there is now growing observational evidence showing that the center of massive early-type galaxies hosts a larger population of low-mass stars than is expected based on observations from the Milky Way. Moreover, these variations in the IMF have been found to be related to radial metallicity variations in massive galaxies. We present here a two-dimensional stellar population analysis of the massive lenticular galaxy FCC 167 (NGC 1380) as part of the Fornax3D project. Using a newly developed stellar population fitting scheme, we derive a full two-dimensional IMF map of an early-type galaxy. This two-dimensional analysis allows us go further than a radial analysis, showing how the metallicity changes along a disk-like structure while the IMF follows a distinct, less disky distribution. Thus, our findings indicate that metallicity cannot be the sole driver of the observed radial IMF variations. In addition, a comparison with the orbital decomposition shows suggestive evidence of a coupling between stellar population properties and the internal dynamical structure of FCC 167, where metallicity and IMF maps seem to track the distribution of cold and warm orbits, respectively.

The IMF-SFH connection in massive early-type galaxies

2015 ◽  
Vol 11 (S315) ◽  
Author(s):  
I. Ferreras ◽  
C. Weidner ◽  
A. Vazdekis ◽  
F. La Barbera
Keyword(s):  
Age Distribution ◽  
Stellar Populations ◽  
Mass Function ◽  
Initial Mass Function ◽  
Population Synthesis ◽  
Early Type ◽  
Massive Galaxies ◽  
Stellar Initial Mass Function ◽  
The Many ◽  
The Imf

The stellar initial mass function (IMF) is one of the fundamental pillars in studies of stellar populations. It is the mass distribution of stars at birth, and it is traditionally assumed to be universal, adopting generic functions constrained by resolved (i.e. nearby) stellar populations (e.g., Salpeter 1955; Kroupa 2001; Chabrier 2003). However, for the vast majority of cases, stars are not resolved in galaxies. Therefore, the interpretation of the photo-spectroscopic observables is complicated by the many degeneracies present between the properties of the unresolved stellar populations, including IMF, age distribution, and chemical composition. The overall good match of the photometric and spectroscopic observations of galaxies with population synthesis models, adopting standard IMF choices, made this issue a relatively unimportant one for a number of years. However, improved models and observations have opened the door to constraints on the IMF in unresolved stellar populations via gravity-sensitive spectral features. At present, there is significant evidence of a non-universal IMF in early-type galaxies (ETGs), with a trend towards a dwarf-enriched distribution in the most massive systems (see, e.g., van Dokkum & Conroy 2010; Ferreras et al. 2013; La Barbera et al. 2013). Dynamical and strong-lensing constraints of the stellar M/L in similar systems give similar results, with heavier M/L in the most massive ETGs (see, e.g., Cappellari et al. 2012; Posacki et al. 2015). Although the interpretation of the results is still open to discussion (e.g., Smith 2014; La Barbera 2015), one should consider the consequences of such a bottom-heavy IMF in massive galaxies.

scholarly journals The star formation timescale of elliptical galaxies

2019 ◽  
Vol 632 ◽  
pp. A110 ◽  
Author(s):  
Zhiqiang Yan ◽  
Tereza Jerabkova ◽  
Pavel Kroupa
Keyword(s):  
Star Formation ◽  
Stellar Population ◽  
Mass Function ◽  
Elliptical Galaxies ◽  
Initial Mass Function ◽  
Different Types ◽  
The Galaxy ◽  
Stellar Initial Mass Function ◽  
Low Mass ◽  
Natural Solution

The alpha element to iron peak element ratio, for example [Mg/Fe], is a commonly applied indicator of the galaxy star formation timescale (SFT) since the two groups of elements are mainly produced by different types of supernovae that explode over different timescales. However, it is insufficient to consider only [Mg/Fe] when estimating the SFT. The [Mg/Fe] yield of a stellar population depends on its metallicity. Therefore, it is possible for galaxies with different SFTs and at the same time different total metallicity to have the same [Mg/Fe]. This effect has not been properly taken into consideration in previous studies. In this study, we assume the galaxy-wide stellar initial mass function (gwIMF) to be canonical and invariant. We demonstrate that our computation code reproduces the SFT estimations of previous studies, where only the [Mg/Fe] observational constraint is applied. We then demonstrate that once both metallicity and [Mg/Fe] observations are considered, a more severe “downsizing relation” is required. This means that either low-mass ellipticals have longer SFTs (> 4 Gyr for galaxies with mass below 1010 M⊙) or massive ellipticals have shorter SFTs (≈200 Myr for galaxies more massive than 1011 M⊙) than previously thought. This modification increases the difficulty in reconciling such SFTs with other observational constraints. We show that applying different stellar yield modifications does not relieve this formation timescale problem. The quite unrealistically short SFT required by [Mg/Fe] and total metallicity would be prolonged if a variable stellar gwIMF were assumed. Since a systematically varying gwIMF has been suggested by various observations this could present a natural solution to this problem.

scholarly journals Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance

2020 ◽  
Vol 498 (3) ◽  
pp. 4051-4059 ◽  
Author(s):  
Timothy A Davis ◽  
Freeke van de Voort
Keyword(s):  
Mass Loss ◽  
Cosmic Time ◽  
Mass Function ◽  
Stellar Mass ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Early Type ◽  
Root Cause ◽  
Stellar Initial Mass Function ◽  
Low Mass

ABSTRACT The observed stellar initial mass function (IMF) appears to vary, becoming bottom-heavy in the centres of the most massive, metal-rich early-type galaxies. It is still unclear what physical processes might cause this IMF variation. In this paper, we demonstrate that the abundance of deuterium in the birth clouds of forming stars may be important in setting the IMF. We use models of disc accretion on to low-mass protostars to show that those forming from deuterium-poor gas are expected to have zero-age main-sequence masses significantly lower than those forming from primordial (high deuterium fraction) material. This deuterium abundance effect depends on stellar mass in our simple models, such that the resulting IMF would become bottom-heavy – as seen in observations. Stellar mass loss is entirely deuterium free and is important in fuelling star formation across cosmic time. Using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation we show that stellar mass-loss-induced deuterium variations are strongest in the same regions where IMF variations are observed: at the centres of the most massive, metal-rich, passive galaxies. While our analysis cannot prove that the deuterium abundance is the root cause of the observed IMF variation, it sets the stage for future theoretical and observational attempts to study this possibility.

scholarly journals Comparing IMF-sensitive indices of intermediate-mass quiescent galaxies in various environments

2019 ◽  
Vol 486 (3) ◽  
pp. 3788-3804 ◽  
Author(s):  
Elham Eftekhari ◽  
Moein Mosleh ◽  
Alexandre Vazdekis ◽  
Saeed Tavasoli
Keyword(s):  
Large Scale ◽  
Velocity Dispersion ◽  
Mass Function ◽  
Initial Mass Function ◽  
Sloan Digital Sky Survey ◽  
Intermediate Mass ◽  
Composite Spectra ◽  
Stellar Initial Mass Function ◽  
Varying Environments ◽  
The Imf

ABSTRACT Using samples drawn from the Sloan Digital Sky Survey, we study for the first time the relation between large-scale environments (clusters, groups, and voids) and the stellar initial mass function (IMF). We perform an observational approach based on the comparison of IMF-sensitive indices of quiescent galaxies with similar mass in varying environments. These galaxies are selected within a narrow redshift interval (0.020 < z < 0.055) and spanning a range in velocity dispersion from 100 to 200 km s−1. The results of this paper are based upon analysis of composite spectra created by stacking the spectra of galaxies, binned by their velocity dispersion and redshift. The trends of spectral indices as measured from the stacked spectra, with respect to velocity dispersion, are compared in different environments. We find a lack of dependence of the IMF on the environment for intermediate-mass galaxy regime. We verify this finding by providing a more quantitative measurement of the IMF variations among galactic environments using MILES stellar population models with a precision of ΔΓb ∼ 0.2.

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