Day 1574

Monday.

1905.09839
A fundamental test for stellar feedback recipes in galaxy simulations
Fujimoto, et al

Direct comparisons between galaxy simulations and observations that both reach scales < 100 pc are strong tools to investigate the cloud-scale physics of star formation and feedback in nearby galaxies. Here we carry out such a comparison for hydrodynamical simulations of a Milky Way-like galaxy, including stochastic star formation, HII region and supernova feedback, and chemical post-processing at 8 pc resolution. Our simulation shows excellent agreement with almost all kpc-scale and larger observables, including total star formation rates, radial profiles of CO, HI, and star formation through the galactic disc, mass ratios of the ISM components, both whole-galaxy and resolved Kennicutt-Schmidt relations, and giant molecular cloud properties. However, we find that our simulation does not reproduce the observed de-correlation between tracers of gas and star formation on < 100 pc scales, known as the star formation 'uncertainty principle', which indicates that observed clouds undergo rapid evolutionary lifecycles. We conclude that the discrepancy is driven by insufficiently-strong pre-supernova feedback in our simulation, which does not disperse the surrounding gas completely, leaving star formation tracer emission too strongly associated with molecular gas tracer emission, inconsistent with observations. This result implies that the cloud-scale de-correlation of gas and star formation is a fundamental test for feedback prescriptions in galaxy simulations, one that can fail even in simulations that reproduce all other macroscopic properties of star-forming galaxies.

1905.09908

Kinematics of multiple stellar populations in globular clusters with Gaia

Cordoni, et al

The internal dynamics of multiple stellar populations in Globular Clusters (GCs) provide unique constraints on the physical processes responsible for their formation. Specifically, the present-day kinematics of cluster stars, such as rotation and velocity dispersion, seem to be related to the initial configuration of the system. In a recent work, we analyzed for the first time the kinematics of the different stellar populations in NGC 0104 over a large field of view, exploiting the Gaia Data Release 2 proper motions combined with multi-band ground-based photometry. In this paper, we extend this analysis to six GCs, namely NGC 0288, NGC 5904, NGC 6121, NGC 6752 and NGC 6838 and further explore NGC 0104. Among the analyzed clusters only NGC 0104 and NGC 5904 show significant rotation in the plane of the sky. By separating our sample in 1G and 2G stars we find that overall these two populations exhibit a similar rotation pattern in NGC 0104. However, some hints of different rotations between 1G and 2G stars are observed in the external regions of this cluster. Interestingly, 1G and 2G stars in NGC 5904 exhibit different rotation curves, with distinct phases. The radial components of the motion of 1G and 2G stars show different radial trends, in contrast with what is observed in most of the other clusters. There is no evidence for rotation among the selected 1G and 2G stars of the remaining clusters. The analysis of the velocity-dispersion profiles of multiple populations confirms that 2G stars of NGC 0104 show stronger anisotropy than the 1G.

1905.09920

Multiwavelength cluster mass estimates and machine learning

Cohn, Battaglia

One emerging application of machine learning methods is the inference of galaxy cluster masses. Often cluster mass predictions are made from observables by fitting or deriving scaling relations; if multiwavelength measurements are available, these scaling relation based estimates are combined into a likelihood. Here, machine learning is used in a simulation to instead directly combine five multiwavelength measurements to obtain cluster masses. Comparisons of the contributions of each observable to the accuracy of the resulting mass measurement are made using model-agnostic Importance Permutation values, as well as by brute force comparison of different combinations of observables. As machine learning relies upon the accuracy of the training set in capturing the observables, their correlations, and the observational selection function, and the training set originates from simulations, a few ways of testing whether a simulation and observations are consistent are explored as well.

1905.10312

Evidence for disc regulation in the lowest-mass stars of the young stellar cluster NGC 2264

Orcajo, et al

In the pre-main-sequence stage, star-disc interactions have been shown to remove stellar angular momentum and regulate the rotation periods of stars with M2 and earlier spectral types. Whether disc regulation also extends to stars with later spectral types still remains a matter of debate. Here we present a star-disc interaction study in a sample of over 180 stars with spectral types M3 and later (corresponding to stellar masses $\leq 0.3 M_\odot$) in young stellar cluster NGC 2264. Combining rotation periods from the literature, new and literature spectral types, and newly presented deep Spitzer observations, we show that stars with masses below 0.3 $M_\odot$ with discs also rotate slower than stars without a disc in the same mass regime. Our results demonstrate that disc-regulation still operates in these low-mass stars, although the efficiency of this process might be lower than in higher-mass objects. We confirm that stars with spectral types earlier and later than M2 have distinct period distributions and that stars with spectral types M5 and later rotate even faster M3 and M4-type stars.

1905.10359

Stellar systems following the $R^{1/M}$ luminosity law.  III.  Photometric, intrinsic and dynamical properties for all S\'sic indices

Baes, Ciotti

The S\'ersic or $R^{1/m}$ model has become the de facto standard model to describe the surface brightness profiles of early-type galaxies and the bulges of spiral galaxies. The photometric, intrinsic and dynamical properties of this model have been investigated, but mainly for fairly large S\'ersic indices $m$. For small values of $m$, appropriate for low-mass and dwarf ellipticals, a detailed investigation of these properties is still lacking. In this study, we use a combination of numerical and analytical techniques to investigate the S\'ersic model over the entire range of S\'ersic parameters, focusing on the small $m$ regime, where a number of interesting and surprising properties are found. For all values $m<1$, the model is characterised by a finite central luminosity density, and for $m<\tfrac12$, even a central depression in the luminosity density profile. This behaviour translates to the dynamical properties: we show that all S\'ersic models with $m \geqslant\tfrac12$ can be supported by an isotropic velocity dispersion tensor, and that these isotropic models are stable to both radial and non-radial perturbations. The models with $m < \tfrac12$, on the other hand, cannot be supported by an isotropic velocity dispersion tensor.