De Grijs Research Articles

An International Aging Research Collaboration During The COVID-19 Crisis: Mitigating Global Health Consequences

There are several reasons for forming an aging international research collaboration; however, creating a successful and productive research team during the global crisis may require extensive planning and efforts. Our team consists of ten scholars from five countries, including Hong Kong, Nepal, Singapore, Thailand, and the United States. To accomplish this initiative, we employ ten simple rules for establishing international research collaborations proposed by R. de Grijs (2015). We aim to examine impacts of the pandemic on physical activity, frailty, falls, depression and social networks in diverse older adults. We collect data by online survey and/or face-to –face survey using questionnaires including fear of the COVID, face mask use, Social Network, Rapid Assessment of Physical Activity, a simple frailty questionnaire, CDC fall risk checklist, short Fall-Efficacy Scale International and Patient Health Questionnaire-9. Topics of discussion included: research progression, lessons learned and barriers to international collaboration during the COVID-19 crisis.

The local vertical density distribution of ultracool dwarfs M7 to L2.5 and their luminosity function

We investigate the form of the local vertical density profile of the stars in the Galactic disk, close to the Galactic plane. We use a homogeneous sample of 34000 ultracool dwarfs M7 to L2.5 that all lie within 350 pc of the plane. We fit a profile of the form sech$^\alpha$, where $\alpha=2$ is the theoretically preferred isothermal profile and $\alpha=0$ is the exponential function. Larger values of $\alpha$ correspond to greater flattening of the profile towards the plane. We employ a likelihood analysis that accounts in a direct way for unresolved binaries in the sample, as well as for the spread in absolute magnitude $M_J$ within each spectral sub-type (Malmquist bias). We measure $\alpha=0.29^{+0.12}_{-0.13}$. The $\alpha=1$ (sech) and flatter profiles are ruled out at high confidence for this sample, while $\alpha=0$ (exponential) is included in the 95% credible interval. Any flattening relative to exponential is modest, and is confined to within 50 pc of the plane. The measured value of $\alpha$ is consistent with the results of the recent analysis by Xiang et al. Our value for $\alpha$ is also similar to that determined for nearby spiral galaxies by de Grijs et al., measured from photometry of galaxies viewed edge on. The measured profile allows an accurate determination of the local space density of ultracool dwarfs M7 to L2.5, and we use this to make a new determination of the luminosity function at the bottom of the main sequence. Our results for the luminosity function are a factor two to three lower than the recent measurement by Bardalez Gagliuffi et al., that uses stars in the local 25 pc radius bubble, but agree well with the older study by Cruz et al.

Non-Gaussian error distributions of galactic rotation speed measurements

We construct the error distributions for the galactic rotation speed ($\Theta_0$) using 137 data points from measurements compiled in De Grijs et al. (arXiv:1709.02501), with all observations normalized to the galactocentric distance of 8.3 kpc. We then checked (using the same procedures as in works by Ratra et al) if the errors constructed using the weighted mean and the median as the estimate, obey Gaussian statistics. We find using both these estimates that they have much wider tails than a Gaussian distribution. We also tried to fit the data to three other distributions: Cauchy, double-exponential, and Students-t. The best fit is obtained using the Students-$t$ distribution for $n=2$ using the median value as the central estimate, corresponding to a $p$-value of 0.1. We also calculate the median value of $\Theta_0$ using all the data as well as using the median of each set of measurements based on the tracer population used. Because of the non-gaussianity of the residuals, we point out that the subgroup median value, given by $\Theta_{med}=219.65$ km/sec should be used as the central estimate for $\Theta_0$.

Truncated disc surface brightness profiles produced by flares

AbstractPrevious studies have discarded that flares in galactic discs may explain the truncation that are frequently observed in highly-inclined galaxies (Kregel et al. 2002). However, no study has systematically analysed this hypothesis using realistic models for the disc, the flare and the bulge. We derive edge-on and face-on surface brightness profiles for a series of realistic galaxy models with flared discs that sample a wide range of structural and photometric parameters across the Hubble Sequence, accordingly to observations. The surface brightness profile for each galaxy model has been simulated for edge-on and face-on views to find out whether the flared disc produces a significant truncation in the disc in the edge-on view compared to the face-on view or not. In order to simulate realistic images of disc galaxies, we have considered the observational distribution of the photometric parameters as a function of the morphological type for three mass bins (10 < log10(M/M⊙) < 10.7, 10.7 < log10(M/M⊙) < 11 and log10(M/M⊙) > 11), and four morphological type bins (S0–Sa, Sb–Sbc, Sc–Scd and Sd–Sdm). For each mass bin, we have restricted the photometric and structural parameters of each modelled galaxy to their characteristic observational ranges (μ0, disc, μeff, bulge, B/T, Mabs, reff, nbulge, hR, disc) and the flare in the disc (hz, disc/hR, disc, ∂hz, disc/∂R, see de Grijs & Peletier 1997, Graham 2001, López-Corredoira et al. 2002, Yoachim & Dalcanton 2006, Bizyaev et al. 2014, Mosenkov et al. 2015).Contrary to previous claims, the simulations show that realistic flared disks can be responsible for the truncations observed in many edge-on systems, preserving the profile of the non-flared analogous model in face-on view. These breaks reproduce the properties of the weak-to-intermediate breaks observed in many real Type-II galaxies in the diagram relating the radial location of the break (RbrkII) in units of the inner disk scale-length with the break strength S (Laine et al. 2014). Radial variation of the scale-height of the disc (flaring) can explain the existence of many breaks in edge-on galaxies, especially of those with low break strengths $S=\log _{10}\frac{h_{o}}{h_{i}} \sim \ [-0.3,-0.1]$.

How West met East in Chinese astronomy

The development of early modern astronomy in China is filled with fascinating characters and strange tales. Richard de Grijs reveals details of research into the people behind this tumultuous period that has, until now, been known to only a handful of dedicated scholars.

DIVISION VII: COMMISSION 37: STAR CLUSTERS AND ASSOCIATIONS

The business session for Commission 37 was held on 24 August 2012 at the IAU General Assembly in Beijing. The meeting was attended by about a dozen members of our Comission, including President Carraro, VP de Grijs and several committee members. We introduced ourselves and then went through a powerpoint presentation first prepared by outgoing President Elmegreen and revised by incoming President Carraro. The contents of the powerpoint presentation are given in this summary.

Radial truncations in stellar discs in galaxies

We discuss the possible origin of the radial truncations in stellar discs, using measurements that we presented in an earlier paper (Kregel, van der Kruit & de Grijs, 2002). A tentative correlation is found with the de-projected face-on central surface brightness; lower surface brightness discs tend to have a smaller truncation radius in units of scalelength. This and our earlier finding that in smaller spirals the truncation tends to occur at a larger number of scalelengths are best reproduced when the truncation is caused by a constant gas density threshhold on star formation.

Chemical evolution of the M82 B fossil starburst

M82 B is an old starburst site located in the eastern part of the M82 disc. We derive the distributions of age and metallicity of the star clusters located in this region of M82 by using theoretical evolutionary population synthesis models. Our analysis is based on the comparison of the $BVIJ$ photometry obtained by de Grijs et al. (2001) with the colours of single-generation stellar populations. We show that M82 B went through a chemical enrichment phase up to super-solar metallicities around the time of the last close encounter between M82 and its large neighbour galaxy M81. We date and confirm the event triggering the enhanced cluster formation at about 1 Gyr ago. At almost the same time an additional, distinct subpopulation of metal-poor clusters formed in the part of M82 B nearest to the galactic centre. The formation of these peculiar clusters may be related to infall of circumgalactic gas onto M82 B.

M 82: starburst Rosetta Stone

We present evolutionary synthesis modeling of the nuclear regions of the starburst galaxy M 82, based on near-infrared integral field spectroscopy and mid-infrared ISO spectroscopy. Our data indicate the occurrence of two distinct starburst episodes in the central 500 pc about 8 – 15 Myr and 5 Myr ago, each lasting a few million years only. The first burst was most intense within 50 pc of the nucleus while the second burst took place in a circumnuclear ring of radius ~ 85 pc and along the stellar bar at larger radii. These recent episodes succeeded earlier starburst activity ~ 1 kpc away from the nucleus and peaking 650 Myr ago, as traced by the luminous star clusters studied by de Grijs et al. (2001). The combined results of both studies reveal complex evolution of starburst activity in M 82 which is consistent with a tidally-induced bar-driven scenario.

Environmental effects on the star formation mode in M82

The last tidal encounter between M82 and M81 about 500 Myr ago had a major impact on what was probably an otherwise normal, quiescent, disk galaxy. It caused a concentrated burst of star formation activity, as evidenced by the peak in the age distribution of the cluster sample in M82’s fossil starburst region ‘B’ (de Grijs et al. 2001; cf. Figure 1), which decreased rapidly within a few hundred Myr.

Star formation timescales in M82

Richard de Grijs reviews the picture of starbirth presented in this nearby, prototype starburst galaxy, a well-studied example of processes that must be widespread in the early universe. The last tidal encounter between M82 and M81, some 500 Myr ago, had a major impact on what was probably an otherwise normal, quiescent disk galaxy. It caused a concentrated burst of star formation in the form of massive star clusters that decreased rapidly, within a few 100 Myr. The current starburst in the centre of the galaxy is likely to arise from large-scale propagating star formation. Alternatively, it may be related to late infall of tidally disrupted debris from M82 itself. Star formation here overall may, in fact, come from a combination of these two mechanisms, in the sense that the star formation in the active core is actively propagating, whilst the overall evolution of the starburst depends on tidal debris raining back onto the disk of the galaxy, giving rise to the present-day starburst.