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

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

The nature and origin of disky elliptical galaxies

The observational trend that disky elliptical galaxies exhibit younger luminosity- weighted ages than boxy ellipticals is investigated. The presence of a possible young stellar disk embedded in these galaxies is explored by comparing kinematics derived from the near-infrared Calcium II triplet (around 8600 Å) and Hβ (4863 Å) Balmer line, thought to be sensitive to older and younger stars respectively. Using synthetic stellar population spectra of these two wavelength regions, it is found that a young disk component produces observable differences in the kinematics derived from the two wavelength regions. Specifically, very young disks produce differences in the Gauss-Hermite coefficients, h(_3) and h(_4). Disks with an intermediate age produce offsets in the rotation velocities. Older disks produce clear two-component structure in the derived LOSVDs. Thus, diagnostic indicators are established which can be applied to observations. A comparison is presented of the major- and minor-axis kinematics derived from the Calcium II triplet and Hβ absorption features for a small sample of disky elliptical galaxies with enhanced Hβ absorption strength, indicative of a young component. For two galaxies in the sample, NGC 584 and NGC 821, Hβ gives a rotation velocity higher than that from the Calcium II triplet. These offsets are not consistent with the spectral models, since the offsets in velocity are not accompanied by the expected offsets in the other LOSVD parameters. This implies that the disks have either formed over time with a modest star- formation rate; or that the young stars in these systems are present in both the disk and spheroid components. From dynamical modelling of ground-based integral-field spectroscopy combined with HST STIS data, the disky elliptical NGC 821 is found to have a mass-to-light ratio of 4.12± 0.06 in I-band solar units, and harbours a central black hole of mass (3.41 ± 0.68) x 10(^7) M(_ʘ). This black hole mass is consistent with Gebhardt et al. (2002), who use the same STIS data with their independent modelling code. The phase-space distribution of the orbits in the model shows evidence for a two-component structure, which corresponds to a slowly rotating spheroidal component superimposed with a flattened, strongly rotating component. This second component, which has properties similar to a disk, accounts for 15% of the total system mass. Applying the two-component stellar population models, a disk of this mass would have formed 6 Gyr ago to produce the observed Hβ absorption. Such a disk is not consistent with the long-slit observations, as no offset was found between the Calcium II triplet and Hβ velocity dispersions. However, this cannot be strongly excluded due to the effects of metallicity and disk velocity dispersion on the spectral modelling

Measuring the low mass end of the Mbh - sigma relation

We show that high quality laser guide star (LGS) adaptive optics (AO) observations of nearby early-type galaxies are possible when the tip-tilt correction is done by guiding on nuclei while the focus compensation due to the changing distance to the sodium layer is made 'open loop'. We achieve corrections such that 40% of flux comes from R<0.2 arcsec. To measure a black hole mass (Mbh) one needs integral field observations of both high spatial resolution and large field of view. With these data it is possible to determine the lower limit to Mbh even if the spatial resolution of the observations are up to a few times larger than the sphere of influence of the black hole.Comment: 4 pages, 2 figures, LaTeX. To appear in "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista and C.C. Popescu, AIP Conf. Ser., in pres

A way to deal with the fringe-like pattern in VIMOS-IFU data

The use of integral field units is now commonplace at all major observatories offering efficient means of obtaining spectral as well as imaging information at the same time. IFU instrument designs are complex and spectral images have typically highly condensed formats, therefore presenting challenges for the IFU data reduction pipelines. In the case of the VLT VIMOS-IFU, a fringe-like pattern affecting the spectra well into the optical and blue wavelength regime as well as artificial intensity variations, require additional reduction steps beyond standard pipeline processing. In this research note we propose an empirical method for the removal of the fringe-like pattern in the spectral domain and the intensity variations in the imaging domain. We also demonstrate the potential consequences for data analysis if the effects are not corrected. Here we use the example of deriving stellar velocity, velocity dispersion and absorption line-strength maps for early-type galaxies. We derive for each spectrum, reduced by the ESO standard VIMOS pipeline, a correction-spectrum by using the median of the eight surrounding spectra as a proxy for the unaffected, underlying spectrum. This method relies on the fact that our science targets (nearby ETGs) cover the complete FoV of the VIMOS-IFU with slowly varying spectral properties and that the exact shape of the fringe-like pattern is nearly independent and highly variable between neighboring spatial positions. We find that the proposed correction methods for the removal of the fringe-like pattern and the intensity variations in VIMOS-IFU data-cubes are suitable to allow for meaningful data analysis in our sample of nearby early-type galaxies. Since the method relies on the scientific target properties it is not suitable for general implementation in the pipeline software for VIMOS.Comment: 7 pages, 10 figures, accepted for publication in A&A, new version after comments from language edito

Spatially resolved variations of the IMF mass normalization in early-type galaxies as probed by molecular gas kinematics

We here present the first spatially-resolved study of the IMF in external galaxies derived using a dynamical tracer of the mass-to-light ratio. We use the kinematics of relaxed molecular gas discs in seven early-type galaxies (ETGs) selected from the ATLAS3D survey to dynamically determine mass-to-light ratio (M/L) gradients. These M/L gradients are not very strong in the inner parts of these objects, and galaxies that do show variations are those with the highest specific star formation rates. Stellar population parameters derived from star formation histories are then used in order to estimate the stellar initial mass function function (IMF) mismatch parameter, and shed light on its variation within ETGs. Some of our target objects require a light IMF, otherwise their stellar population masses would be greater than their dynamical masses. In contrast, other systems seem to require heavier IMFs to explain their gas kinematics. Our analysis again confirms that IMF variation seems to be occurring within massive ETGs. We find good agreement between our IMF normalisations derived using molecular gas kinematics and those derived using other techniques. Despite this, we do not see find any correlation between the IMF normalisation and galaxy dynamical properties or stellar population parameters, either locally or globally. In the future larger studies which use molecules as tracers of galaxy dynamics can be used to help us disentangle the root cause of IMF variation

Enhancing Covid-19 Decision-Making by Creating an Assurance Case for Simulation Models

Simulation models have been informing the COVID-19 policy-making process. These models, therefore, have significant influence on risk of societal harms. But how clearly are the underlying modelling assumptions and limitations communicated so that decision-makers can readily understand them? When making claims about risk in safety-critical systems, it is common practice to produce an assurance case, which is a structured argument supported by evidence with the aim to assess how confident we should be in our risk-based decisions. We argue that any COVID-19 simulation model that is used to guide critical policy decisions would benefit from being supported with such a case to explain how, and to what extent, the evidence from the simulation can be relied on to substantiate policy conclusions. This would enable a critical review of the implicit assumptions and inherent uncertainty in modelling, and would give the overall decision-making process greater transparency and accountability.Comment: 6 pages and 2 figure

Detection of Enhanced Central Mass-to-Light Ratios in Low-Mass Early-Type Galaxies: Evidence for Black Holes?

We present dynamical measurements of the central mass-to-light ratio ($M/L$) of a sample of 27 low-mass early-type ATLAS$^{3D}$ galaxies. We consider all ATLAS$^{3D}$ galaxies with 9.7$<$log(M$_\star/$M$_\odot)$$<$10.5 in our analysis, selecting out galaxies with available high-resolution Hubble Space Telescope (HST) data, and eliminating galaxies with significant central color gradients or obvious dust features. We use the HST images to derive mass models for these galaxies and combine these with the central velocity dispersion values from ATLAS$^{3D}$ data to obtain a central dynamical $M/L$ estimate. These central dynamical $M/L$s are higher than dynamical $M/L$s derived at larger radii and stellar population estimates of the galaxy centers in $\sim$80\% of galaxies, with a median enhancement of $\sim$14\% and a statistical significance of 3.3$\sigma$. We show that the enhancement in the central $M/L$ is best described either by the presence of black holes in these galaxies or by radial IMF variations. Assuming a black hole model, we derive black hole masses for the sample of galaxies. In two galaxies, NGC 4458 and NGC 4660, the data suggests significantly over-massive BHs, while in most others only upper limits are obtained. We also show that the level of $M/L$ enhancements we see in these early-type galaxy nuclei are consistent with the larger enhancements seen in ultracompact dwarf galaxies (UCDs), supporting the scenario where massive UCDs are created by stripping galaxies of these masses.Comment: Accepted to ApJ, 17 pages, 13 figure