Connection between dynamically derived IMF normalisation and stellar populations

In 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.Comment: 4 pages, 2 figures. To appear in Proceedings of IAU Symposium 311, Galaxy Masses as Constraints of Formation Models, M,. Cappellari & S. Courteau, ed

The stellar populations of massive galaxies in the local Universe

I present a brief review of the stellar population properties of massive galaxies, focusing on early-type galaxies in particular, with emphasis on recent results from the ATLAS3D Survey. I discuss the occurrence of young stellar ages, cold gas, and ongoing star formation in early-type galaxies, the presence of which gives important clues to the evolutionary path of these galaxies. Consideration of empirical star formation histories gives a meaningful picture of galaxy stellar population properties, and allows accurate comparison of mass estimates from populations and dynamics. This has recently provided strong evidence of a non-universal IMF, as supported by other recent evidences. Spatially-resolved studies of stellar populations are also crucial to connect distinct components within galaxies to spatial structures seen in other wavelengths or parameters. Stellar populations in the faint outer envelopes of early-type galaxies are a formidable frontier for observers, but promise to put constraints on the ratio of accreted stellar mass versus that formed 'in situ' - a key feature of recent galaxy formation models. Galaxy environment appears to play a key role in controlling the stellar population properties of low mass galaxies. Simulations remind us, however, that current day galaxies are the product of a complex assembly and environment history, which gives rise to the trends we see. This has strong implications for our interpretation of environmental trends.Comment: 10 pages, 4 figures, LaTeX. Invited talk for the IAU Symposium 295 "The Intriguing Life of Massive Galaxies". To appear in Proc. of the XXVIII IAU General Assembly, Beijing, China, August 2012, eds. D. Thomas, A. Pasquali; I. Ferreras. Cambridge University Pres

Keeping partners together: algorithmic results for the hospitals/residents problem with couples

The Hospitals/Residents problem with Couples (HRC) is a generalisation of the classical Hospitals/Residents problem (HR) that is important in practical applications because it models the case where couples submit joint preference lists over pairs of hospitals (h i ,h j ). We consider a natural restriction of HRC in which the members of a couple have individual preference lists over hospitals, and the joint preference list of the couple is consistent with these individual lists in a precise sense. We give an appropriate stability definition and show that, in this context, the problem of deciding whether a stable matching exists is NP-complete, even if each resident’s preference list has length at most 3 and each hospital has capacity at most 2. However, with respect to classical (Gale-Shapley) stability, we give a linear-time algorithm to find a stable matching or report that none exists, regardless of the preference list lengths or the hospital capacities. Finally, for an alternative formulation of our restriction of HRC, which we call the Hospitals/Residents problem with Sizes (HRS), we give a linear-time algorithm that always finds a stable matching for the case that hospital preference lists are of length at most 2, and where hospital capacities can be arbitrary

Popular matchings: structure and algorithms

An instance of the popular matching problem (POP-M) consists of a set of applicants and a set of posts. Each applicant has a preference list that strictly ranks a subset of the posts. A matching M of applicants to posts is popular if there is no other matching M' such that more applicants prefer M' to M than prefer M to M'. This paper provides a characterization of the set of popular matchings for an arbitrary POP-M instance in terms of a structure called the switching graph, a directed graph computable in linear time from the preference lists. We show that the switching graph can be exploited to yield efficient algorithms for a range of associated problems, including the counting and enumeration of the set of popular matchings and computing popular matchings that satisfy various additional optimality criteria. Our algorithms for computing such optimal popular matchings improve those described in a recent paper by Kavitha and Nasre

Two channels of supermassive black hole growth as seen on the galaxies mass-size plane

We investigate the variation of black hole masses (Mbh) as a function of their host galaxy stellar mass (Mstar) and half-light radius (Re). We confirm that the scatter in Mbh within this plane is essentially the same as that in the Mbh - sigma relation, as expected from the negligible scatter reported in the virial mass estimator sigma_v^2=GxMstar/(5xRe). All variation in Mbh happens along lines of constant sigma_v on the (Mstar, Re) plane, or Mstar $\propto$ Re for Mstar <2x10^11 Msun. This trend is qualitatively the same as those previously reported for galaxy properties related to stellar populations, like age, metallicity, alpha enhancement, mass-to-light ratio and gas content. We find evidence for a change in the Mbh variation above the critical mass of Mcrit ~ 2x10^11 Msun. This behaviour can be explained assuming that Mbh in galaxies less massive than Mcrit can be predicted by the Mbh - sigma relation, while Mbh in more massive galaxies follow a modified relation which is also dependent on Mstar once Mstar >Mcrit. This is consistent with the scenario where the majority of galaxies grow through star formation, while the most massive galaxies undergo a sequence of dissipation-less mergers. In both channels black holes and galaxies grow synchronously, giving rise to the black hole - host galaxy scaling relations, but there is no underlying single relation that is universal across the full range of galaxy masses.Comment: 11 pages, 5 figures; MNRAS accepted (minor text changes

Systematic trends in total-mass profiles from dynamical models of early-type galaxies

We study trends in the slope of the total mass profiles and dark matter fractions within the central half-light radius of 258 early-type galaxies, using data from the volume-limited ATLAS$^{\mathrm{3D}}$ survey. We use three distinct sets of dynamical models, which vary in their assumptions and also allow for spatial variations in the stellar mass-to-light ratio, to test the robustness of our results. We confirm that the slopes of the total mass profiles are approximately isothermal, and investigate how the total-mass slope depends on various galactic properties. The most statistically-significant correlations we find are a function of either surface density, $\Sigma_e$, or velocity dispersion, $\sigma_e$. However there is evidence for a break in the latter relation, with a nearly universal logarithmic slope above \log_{10}[\sigma_e/(\si{km~s^{-1}})]\sim 2.1 and a steeper trend below this value. For the 142 galaxies above that critical $\sigma_e$ value, the total mass-density logarithmic slopes have a mean value $\left\langle\gamma^\prime\right\rangle = -2.192 \pm 0.016$ ($1\sigma$ error) with an observed rms scatter of only $\sigma_{\gamma^\prime}=0.167 \pm 0.016$. Considering the observational errors, we estimate an intrinsic scatter of $\sigma_{\gamma^\prime}^\mathrm{intr} \approx 0.15$. These values are broadly consistent with those found by strong lensing studies at similar radii and agree, within the tight errors, with values recently found at much larger radii via stellar dynamics or HI rotation curves (using significantly smaller samples than this work).Comment: 17 pages, 11 figures, 3 tables. Published in MNRA

Integral Field Spectrographs: a user's view

We easily tend to think of Integral-Field Spectrographs (IFS) along two opposing trends: as either the beautiful combination between photometry and spectroscopy, or as our worst nightmare including the dark side of both worlds. I favour a view where each IFS is considered individually, as one instrument with specific performances which can be used optimally for a certain range of scientific programs. It is indeed true that data-wise, IFS do sometime merge the characteristics of classic (e.g., long-slit) spectrographs with annoying issues associated with Imagers. This is in fact the price to pay to access a drastically different perspective of our favourite targets. The challenge is then to provide the necessary tools to properly handle the corresponding data. However, this should certainly not be thought as something specific to IFS: such a challenge should be accepted for any instrument, and most importantly solved prior to its delivery at the telescope.Comment: 6 pages, 2 figures. Invited talk, to appear in the Proceedings of "The 2007 ESO Instrument Calibration Workshop", ESO Astrophysics Symposia, Springe

Remote detection of OH

This is a remote measurement technique utilizing a XeCl excimer laser tuned to the Q sub 21 1 rotational transition of the 0-0, A-X band at 307.847 nm. A wavemeter is under development to monitor, on a pulse-to-pulse basis, both the laser lineshape and absolute wavelength. Fluorescence is detected with a multiple Fabry-Perot type filter with a spectral resolution on the order of 0.001 nm. This is tuned to the overlapping Q sub 2 2, Q sub 12 2, Q sub 2 3, and Q sub 12 3 rotational transitions at 308.986 nm. The fringe pattern from this filter is imaged using a discrete, multi-anode detector which has a photon gain of 10 to the 8th power. This permits the simultaneous monitoring of OH fluorescence, N2 and/or O2 rotational Raman scattering and broadband background levels. The use of three etalons in series provides sufficient rejection, approx. greater than 10 to the 10th power, against the laser radiation only 1.2 nm away