Dex In Mass Research Articles

ALMA resolves giant molecular clouds in a tidal dwarf galaxy

Tidal dwarf galaxies (TDGs) are gravitationally bound condensations of gas and stars that formed during galaxy interactions. Here we present multi-configuration ALMA observations of J1023+1952, a TDG in the interacting system Arp 94, where we resolved CO(2–1) emission down to giant molecular clouds (GMCs) at 0.64″∼45 pc resolution. We find a remarkably high fraction of extended molecular emission (∼80−90%), which is filtered out by the interferometer and likely traces diffuse gas. We detect 111 GMCs that give a similar mass spectrum as those in the Milky Way and other nearby galaxies (a truncated power law with a slope of −1.76 ± 0.13). We also study Larson’s laws over the available dynamic range of GMC properties (∼2 dex in mass and ∼1 dex in size): GMCs follow the size-mass relation of the Milky Way, but their velocity dispersion is higher such that the size-linewidth and virial relations appear super-linear, deviating from the canonical values. The global molecular-to-atomic gas ratio is very high (∼1) while the CO(2–1)/CO(1–0) ratio is quite low (∼0.5), and both quantities vary from north to south. Star formation predominantly takes place in the south of the TDG, where we observe projected offsets between GMCs and young stellar clusters ranging from ∼50 pc to ∼200 pc; the largest offsets correspond to the oldest knots, as seen in other galaxies. In the quiescent north, we find more molecular clouds and a higher molecular-to-atomic gas ratio (∼1.5); atomic and diffuse molecular gas also have a higher velocity dispersion there. Overall, the organisation of the molecular interstellar medium in this TDG is quite different from other types of galaxies on large scales, but the properties of GMCs seem fairly similar, pointing to near universality of the star-formation process on small scales.

The dichotomy of dark matter fraction and total mass density slope of galaxies over five dex in mass

AbstractWe analyse the mass density distribution in the centres of galaxies across five orders of magnitude in mass range. Using high-quality spiral galaxy rotation curves and infrared photometry from SPARC, we conduct a systematic study of their central dark matter (DM) fraction (fDM) and their mass density slope (α), within their effective radius. We show that lower mass spiral galaxies are more DM dominated and have more shallow mass density slopes when compared with more massive galaxies, which have density profiles closer to isothermal. Low-mass (${M_{*}}\lesssim 10^{10}\, {\mathrm{M}_\odot}$) gas-rich spirals span a wide range of fDM values, but systematically lower than in gas-poor systems of similar mass. With increasing galaxy mass, the values of fDM decrease and the density profiles steepen. In the most massive late-type gas-poor galaxies, a possible flattening of these trends is observed. When comparing these results to massive (${M_{*}}\gtrsim 10^{10}\, {\mathrm{M}_\odot}$) elliptical galaxies from SPIDER and to dwarf ellipticals (dEs) from SMACKED, these trends result to be inverted. Hence, the values of both fDM and α, as a function of M*, exhibit a U-shape trend. At a fixed stellar mass, the mass density profiles in dEs are steeper than in spirals. These trends can be understood by stellar feedback from a more prolonged star formation period in spirals, causing a transformation of the initial steep density cusp to a more shallow profile via differential feedback efficiency by supernovae, and by galaxy mergers or AGN feedback in higher mass galaxies.

How to Measure Galaxy Star Formation Histories. II. Nonparametric Models

Abstract Nonparametric star formation histories (SFHs) have long promised to be the “gold standard” for galaxy spectral energy distribution (SED) modeling as they are flexible enough to describe the full diversity of SFH shapes, whereas parametric models rule out a significant fraction of these shapes a priori. However, this flexibility is not fully constrained even with high-quality observations, making it critical to choose a well-motivated prior. Here, we use the SED-fitting code Prospector to explore the effect of different nonparametric priors by fitting SFHs to mock UV–IR photometry generated from a diverse set of input SFHs. First, we confirm that nonparametric SFHs recover input SFHs with less bias and return more accurate errors than do parametric SFHs. We further find that, while nonparametric SFHs robustly recover the overall shape of the input SFH, the primary determinant of the size and shape of the posterior star formation rate as a function of time (SFR(t)) is the choice of prior, rather than the photometric noise. As a practical demonstration, we fit the UV–IR photometry of ∼6000 galaxies from the Galaxy and Mass Assembly survey and measure scatters between priors to be 0.1 dex in mass, 0.8 dex in SFR100 Myr, and 0.2 dex in mass-weighted ages, with the bluest star-forming galaxies showing the most sensitivity. An important distinguishing characteristic for nonparametric models is the characteristic timescale for changes in SFR(t). This difference controls whether galaxies are assembled in bursts or in steady-state star formation, corresponding respectively to (feedback-dominated/accretion-dominated) models of galaxy formation and to (larger/smaller) confidence intervals derived from SED fitting. High-quality spectroscopy has the potential to further distinguish between these proposed models of SFR(t).

Impact of an AGN featureless continuum on estimation of stellar population properties

The effect of the featureless power-law (PL) continuum of an active galactic nucleus (AGN) on the estimation of physical properties of galaxies with optical population spectral synthesis (PSS) remains largely unknown. With this in mind, we fit synthetic galaxy spectra representing a wide range of galaxy star formation histories (SFHs) and including distinct PL contributions of the form $F_{\nu} \propto \nu^{-\alpha}$ with the PSS code STARLIGHT to study to which extent various inferred quantities (e.g. stellar mass, mean age, and mean metallicity) match the input. The synthetic spectral energy distributions (SEDs) computed with our evolutionary spectral synthesis code include an AGN PL component with $0.5 \leq \alpha \leq 2$ and a fractional contribution $0.2 \leq x_{\mathrm{AGN}} \leq 0.8$ to the monochromatic flux at 4020 \AA. At the empirical AGN detection threshold $x_{\mathrm{AGN}}\simeq 0.26$ that we previously inferred in a pilot study on this subject, our results show that the neglect of a PL component in spectral fitting can lead to an overestimation by $\sim$2 dex in stellar mass and by up to $\sim$1 and $\sim$4 dex in the light- and mass-weighted mean stellar age, respectively, whereas the light- and mass-weighted mean stellar metallicity are underestimated by up to $\sim$0.3 and $\sim$0.6 dex, respectively. Other fitting set-ups including either a single PL or multiple PLs in the base reveal, on average, much lower unsystematic uncertainties of the order of those typically found when fitting purely stellar SEDs with stellar templates, however, reaching locally up to $\sim$1, 3 and 0.4 dex in mass, age and metallicity, respectively. Our results underscore the importance of an accurate modelling of the AGN spectral contribution in PSS fits as a minimum requirement for the recovery of the physical and evolutionary properties of stellar populations in active galaxies.

AChandraX-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

NGC6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Our main aim is to clarify global properties of NGC6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. We use X-ray data, complemented by optical/IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modeling of group-stacked X-ray source spectra. We find a large cluster population down to ~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be ~5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong Halpha emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over ~2 dex in mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.

Making the observational parsimonious richness a working mass proxy

Richness, i.e., the number of bright cluster galaxies, is known to correlate with the cluster mass, however, to exploit it as mass proxy we need a way to estimate the aperture in which galaxies should be counted that minimizes the scatter between mass and richness. In this work, using a sample of 39 clusters with accurate caustic masses at 0.1<z<0.22, we first show that the scatter between mass and richness derived from survey data is negligibly small, as small as best mass proxies. The scatter turns out to be smaller than in some previous works and has a 90% upper limit of 0.05 dex in mass. The current sample, adjoining 76 additional clusters analyzed in previous works, establishes an almost scatterless, minimally evolving (if at all), mass-richness scaling in the redshift range 0.03<z<0.55. We then exploit this negligible scatter to derive the reference aperture to be used to compute richness and to predict the mass of cluster samples. These predicted masses have a total 0.16 dex scatter with caustic mass, about half of which is not intrinsic to the proxy, but related to the noisiness of the caustic masses used for test proxy performances. These results make richness-based masses of best quality and available for large samples at a low observational cost.

OPTIMAL SURVEY STRATEGIES AND PREDICTED PLANET YIELDS FOR THE KOREAN MICROLENSING TELESCOPE NETWORK

The Korean Microlensing Telescope Network (KMTNet) will consist of three 1.6m telescopes each with a 4 deg^{2} field of view (FoV) and will be dedicated to monitoring the Galactic Bulge to detect exoplanets via gravitational microlensing. KMTNet's combination of aperture size, FoV, cadence, and longitudinal coverage will provide a unique opportunity to probe exoplanet demographics in an unbiased way. Here we present simulations that optimize the observing strategy for, and predict the planetary yields of, KMTNet. We find preferences for four target fields located in the central Bulge and an exposure time of t_{exp} = 120s, leading to the detection of ~2,200 microlensing events per year. We estimate the planet detection rates for planets with mass and separation across the ranges 0.1 <= M_{p}/M_{Earth} <= 1000 and 0.4 <= a/AU <= 16, respectively. Normalizing these rates to the cool-planet mass function of Cassan (2012), we predict KMTNet will be approximately uniformly sensitive to planets with mass 5 <= M_{p}/M_{Earth} <= 1000 and will detect ~20 planets per year per dex in mass across that range. For lower-mass planets with mass 0.1 <= M_{p}/M_{Earth} < 5, we predict KMTNet will detect ~10 planets per year. We also compute the yields KMTNet will obtain for free-floating planets (FFPs) and predict KMTNet will detect ~1 Earth-mass FFP per year, assuming an underlying population of one such planet per star in the Galaxy. Lastly, we investigate the dependence of these detection rates on the number of observatories, the photometric precision limit, and optimistic assumptions regarding seeing, throughput, and flux measurement uncertainties.

DYNAMICAL VERSUS STELLAR MASSES IN COMPACT EARLY-TYPE GALAXIES: FURTHER EVIDENCE FOR SYSTEMATIC VARIATION IN THE STELLAR INITIAL MASS FUNCTION

Several independent lines of evidence suggest that the stellar initial mass function (IMF) in early-type galaxies becomes increasingly `bottom-heavy' with increasing galaxy mass and/or velocity dispersion, sigma. Here we consider evidence for IMF variation in a sample of relatively compact early-type galaxies drawn from the Sloan Digital Sky Survey. These galaxies are of sufficiently high stellar density that a dark halo likely makes a minor contribution to the total dynamical mass, Mdyn, within one effective radius. We fit our detailed stellar population synthesis models to the stacked absorption line spectra of these galaxies in bins of sigma and find evidence from IMF-sensitive spectral features for a bottom-heavy IMF at high sigma. We also apply simple `mass-follows-light' dynamical models to the same data and find that Mdyn is significantly higher than what would be expected if these galaxies were stellar dominated and had a universal Milky Way IMF. Adopting Mdyn M_* therefore implies that the IMF is `heavier' at high sigma. Most importantly, the quantitative amount of inferred IMF variation is very similar between the two techniques, agreeing to within 0.1 dex in mass. The agreement between two independent techniques, when applied to the same data, provides compelling evidence for systematic variation in the IMF as a function of early-type galaxy velocity dispersion. Any alternative explanations must reproduce both the results from dynamical and stellar population-based techniques.

FREQUENCY OF SOLAR-LIKE SYSTEMS AND OF ICE AND GAS GIANTS BEYOND THE SNOW LINE FROM HIGH-MAGNIFICATION MICROLENSING EVENTS IN 2005-2008

We present the first measurement of planet frequency beyond the "snow line" for planet/star mass-ratios[-4.5<log q<-2]: d^2 N/dlog q/dlog s=(0.36+-0.15)/dex^2 at mean mass ratio q=5e-4, and consistent with being flat in log projected separation, s. Our result is based on a sample of 6 planets detected from intensive follow-up of high-mag (A>200) microlensing events during 2005-8. The sample host stars have typical mass M_host 0.5 Msun, and detection is sensitive to planets over a range of projected separations (R_E/s_max,R_E*s_max), where R_E 3.5 AU sqrt(M_host/Msun) is the Einstein radius and s_max (q/5e-5)^{2/3}, corresponding to deprojected separations ~3 times the "snow line". Though frenetic, the observations constitute a "controlled experiment", which permits measurement of absolute planet frequency. High-mag events are rare, but the high-mag channel is efficient: half of high-mag events were successfully monitored and half of these yielded planet detections. The planet frequency derived from microlensing is a factor 7 larger than from RV studies at factor ~25 smaller separations [2<P<2000 days]. However, this difference is basically consistent with the gradient derived from RV studies (when extrapolated well beyond the separations from which it is measured). This suggests a universal separation distribution across 2 dex in semi-major axis, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all planetary systems were "analogs" of the Solar System, our sample would have yielded 18.2 planets (11.4 "Jupiters", 6.4 "Saturns", 0.3 "Uranuses", 0.2 "Neptunes") including 6.1 systems with 2 or more planet detections. This compares to 6 planets including one 2-planet system in the actual sample, implying a first estimate of 1/6 for the frequency of solar-like systems.

The scaling relation between richness and mass of galaxy clusters: a Bayesian approach

We use a sample of 53 galaxy clusters at 0.03 < z < 0.1 with available masses derived from the caustic technique and with velocity dispersions computed using 208 galaxies on average per cluster, in order to investigate the scaling between richness, mass and velocity dispersion. A tight scaling between richness and mass is found, with an intrinsic scatter of only 0.19 dex in mass and with a slope one, i.e. clusters that have twice as many galaxies are twice as massive. When richness is measured without any knowledge of the cluster mass or linked parameters (such as r200), it can predict mass with an uncertainty of 0.29 ± 0.01 dex. As a mass proxy, richness competes favourably with both direct measurements of mass given by the caustic method, which has typically 0.14 dex errors (versus 0.29) and X-ray luminosity, which offers a similar 0.30 dex uncertainty. The similar performances of X-ray luminosity and richness in predicting cluster masses has been confirmed using cluster masses derived from velocity dispersion fixed by numerical simulations. These results suggest that cluster masses can be reliably estimated from simple galaxy counts, at least at the redshift and masses explored in this work. This has important applications in the estimation of cosmological parameters from optical cluster surveys, because in current surveys clusters detected in the optical range outnumber, by at least one order of magnitude, those detected in X-ray. Our analysis is robust from an astrophysical perspective because the adopted masses are among the most hypothesis-parsimonious estimates of cluster mass and from a statistical perspective, because our Bayesian analysis accounts for terms usually neglected, such as the Poisson nature of galaxy counts, the intrinsic scatter and uncertain errors. The data and code used for the stochastic computation are provided in the paper.

Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs

In earlier papers in this series we determined the intrinsic stellar disc kinematics of 15 intermediate- to late-type edge-on spiral galaxies using a dynamical modelling technique. The sample covers a substantial range in maximum rotation velocity and deprojected face-on surface brightness, and contains seven spirals with either a boxy or peanut-shaped bulge. Here we discuss the structural, kinematical and dynamical properties. From the photometry we find that intrinsically more flattened discs tend to have a lower face-on central surface brightness and a larger dynamical mass-to-light ratio. This observation suggests that, at a constant maximum rotational velocity, lower surface brightness discs have smaller vertical stellar velocity dispersions. Although the individual uncertainties are large, we find from the dynamical modelling that at least 12 discs are submaximal. The average disc contributes 53 ± 4 per cent to the observed rotation at 2.2 disc scalelengths (hR), with a 1σ scatter of 15 per cent. This percentage becomes somewhat lower when effects of finite disc flattening and gravity by the dark halo and the gas are taken into account. Since boxy and peanut-shaped bulges are probably associated with bars, the result suggests that at 2.2hR the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal, as these discs are underrepresented in our sample. We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this σ versus vmax relation appears to correlate with the disc flattening, face-on central surface brightness and dynamical mass-to-light ratio. Low surface brightness discs tend to be more flattened and have smaller stellar velocity dispersions. The findings are consistent with the observed correlation between disc flattening and dynamical mass-to-light ratio and can generally be reproduced by the simple collapse theory for disc galaxy formation. Finally, the disc mass Tully–Fisher relation is offset from the maximum-disc scaled stellar mass Tully–Fisher relation of the Ursa Major cluster. This offset, −0.3 dex in mass, is naturally explained if the discs of the Ursa Major cluster spirals are submaximal.