The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey [GA]

Galaxy formation entails the hierarchical assembly of mass, along with the condensation of baryons and the ensuing, self-regulating star formation. The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy’s formation history. The ordered-rotation dominated orbits with near maximum circularity $\lambda_z \simeq1$ and the random-motion dominated orbits with low circularity $\lambda_z \simeq0$ are called kinematically cold and kinematically hot, respectively. The fraction of stars on cold' orbits, compared to the fraction of stars onhot’ orbits, speaks directly to the quiescence or violence of the galaxies’ formation histories. Here we present such orbit distributions, derived from stellar kinematic maps via orbit-based modelling for a well defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey, includes the main morphological galaxy types and spans the total stellar mass range from $10^{8.7}$ to $10^{11.9}$ solar masses. Our analysis derives the orbit-circularity distribution as a function of galaxy mass, $p(\lambda_z~|~M_\star)$, and its volume-averaged total distribution, $p(\lambda_z)$. We find that across most of the considered mass range and across morphological types, there are more stars on warm' orbits defined as $0.25\le \lambda_z \le 0.8$ than on eithercold’ or `hot’ orbits. This orbit-based “Hubble diagram” provides a benchmark for galaxy formation simulations in a cosmological context.

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L. Zhu, G. Ven, R. Bosch, et. al.
Tue, 21 Nov 17

Comments: Nature Astronomy accepted, 22 pages, 8 figures

X-ray radiative transfer in protoplanetary disks – The role of dust and X-ray background fields [SSA]

The X-ray luminosities of T Tauri stars are about two to four orders of magnitude higher than the luminosity of the contemporary Sun. As these stars are born in clusters, their disks are not only irradiated by their parent star but also by an X-ray background field produced by the cluster members. We aim to quantify the impact of X-ray background fields produced by young embedded clusters on the chemical structure of disks. Further, we want to investigate the importance of the dust for X-ray radiative transfer in disks.
We present a new X-ray radiative transfer module for the radiation thermo-chemical disk code ProDiMo, which includes X-ray scattering and absorption by both the gas and dust component. For the X-ray radiative transfer, we consider irradiation by the star and by X-ray background fields. To study the impact of X-rays on the chemical structure of disks we use the well-established disk ionization tracers N2H+ and HCO+.
For evolved dust populations, X-ray opacities are mostly dominated by the gas; only for photon energies $E\gtrsim5-10\,$keV, dust opacities become relevant. Consequently, the local disk X-ray radiation field is only affected in dense regions close to the disk midplane. X-ray background fields can dominate the local X-ray disk ionization rate for disk radii $r\gtrsim20\,$au. However, the N2H+ and HCO+ column densities are only significantly affected in case of low cosmic-ray ionization rates, or if the background flux is at least a factor of ten higher than the flux level expected for clusters typical for the solar vicinity. Observable signatures of X-ray background fields in low-mass star-formation regions, like Taurus, are only expected for cluster members experiencing a strong X-ray background field. For the majority of the cluster members, the X-ray background field has only little impact on the disk chemical structure.

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C. Rab, M. Gudel, P. Woitke, et. al.
Tue, 21 Nov 17

Comments: 16 pages, 12 figures, accepted for publication in A&A

The high brightness temperature of B0529+483 revealed by RadioAstron and implications for interstellar scattering [HEAP]

The high brightness temperatures, $T_\mathrm{b}\gtrsim 10^{13}$ K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240,000 km we find two characteristic angular scales in the quasar core, about 100 $\mu$as and 10 $\mu$as. Some indications of scattering substructure are found. Very high brightness temperatures, $T_\mathrm{b}\geq 10^{13}$ K, are estimated at 4.8 GHz and 22 GHz even taking into account the refractive scattering. Our findings suggest a clear dominance of the particle energy density over the magnetic field energy density in the core of this quasar.

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S. Pilipenko, Y. Kovalev, A. Andrianov, et. al.
Tue, 21 Nov 17

Comments: 14 pages, 9 figures

Identifying the progenitors of present-day early-type galaxies in observational surveys: correcting `progenitor bias' using the Horizon-AGN simulation [GA]

As endpoints of the hierarchical mass-assembly process, the stellar populations of local early-type galaxies encode the assembly history of galaxies over cosmic time. We use Horizon-AGN, a cosmological hydrodynamical simulation, to study the merger histories of local early-type galaxies and track how the morphological mix of galaxies their progenitors evolves over time. We provide a framework for alleviating `progenitor bias’ — the bias that occurs if one uses only early-type galaxies to study the progenitor population. Early-types attain their final morphology at relatively early epochs — by $z\sim1$, around 60 per cent of today’s early-types have had their last significant merger. At all redshifts, the majority of mergers have one late-type progenitor, with late-late mergers dominating at $z>1.5$ and early-early mergers becoming significant only at $z<0.5$. Progenitor bias is severe at all but the lowest redshifts — e.g. at $z\sim0.6$, less than 50 per cent of the stellar mass in today’s early-types is actually in progenitors with early-type morphology, while, at $z\sim2$, studying only early-types misses almost all (80 per cent) of the stellar mass that eventually ends up in local early-type systems. At high redshift, almost all massive late-type galaxies, regardless of their local environment or star-formation rate, are progenitors of local early-type galaxies, as are lower-mass (M$\star$ $<$ 10$^{10.5}$ M${\odot}$) galaxies as long as they reside in high density environments. In this new era of large observational surveys (e.g. LSST, JWST), this study provides a framework for studying how today’s early-type galaxies have been built up over cosmic time.

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G. Martin, S. Kaviraj, J. Devriendt, et. al.
Tue, 21 Nov 17

Comments: Accepted for publication in MNRAS

The Near-infrared Optimal Distances Method Applied to Galactic Classical Cepheids Tightly Constrains Mid-infrared Period–Luminosity Relations [SSA]

Classical Cepheids are well-known and widely used distance indicators. Since distance and extinction are usually degenerate, it is important to develop suitable methods to robustly anchor the distance scale. Here, we introduce a near-infrared (near-IR) optimal distance method to determine both the extinction values of and distances to a large sample of 289 Galactic classical Cepheids. The overall uncertainty in the derived distances is less than 4.9%. We compare our newly determined distances to the Cepheids in our sample with previously published distances to the same Cepheids with Hubble Space Telescope parallax measurements and distances based on the IR surface brightness method, Wesenheit functions, and the main-sequence fitting method. The systematic deviations in the distances determined here with respect to those of previous publications is less than 1-2%. We hence constructed Galactic mid-IR period-luminosity (PL) relations for classical Cepheids in the four Wide-Field Infrared Survey Explorer (WISE) bands (W1, W2, W3, and W4) and the four Spitzer Space Telescope IRAC bands ([3.6], [4.5], [5.8] and [8.0]). Based on our sample of hundreds of Cepheids, the WISE PL relations have been determined for the first time; their dispersion is approximately 0.10 mag. Using the currently most complete sample, our Spitzer PL relations represent a significant improvement in accuracy, especially in the [3.6] band which has the smallest dispersion (0.066 mag). In addition, the average mid-IR extinction curve for Cepheids has been obtained: A_W1/A_Ks=0.560, A_W2/A_Ks=0.479, A_W3/A_Ks=0.507, A_W4/A_Ks=0.406, A_[3.6]/A_Ks=0.481, A_[4.5]/A_Ks=0.469, A_[5.8]/A_Ks=0.427, and A_[8.0]/A_Ks=0.427 mag.

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S. Wang, X. Chen, R. Grijs, et. al.
Tue, 21 Nov 17

Comments: 19 pages, 5 figures, accepted for publication in The Astrophysical Journal

Formation of the first star cluster and massive star binaries by fragmentation of filamentary primordial gas clouds [GA]

We perform a set of cosmological simulations of early structure formation with incorporating baryonic streaming motions. We present a case where a significantly elongated gas cloud with $\sim!10^4\,$solar-masses (${\rm M_\odot}$) is formed in a pre-galactic ($\sim!10^7\,{\rm M_\odot}$) dark halo. The gas streaming into the halo compresses and heats the massive filamentary cloud to a temperature of $\sim!10000\,$Kelvin. The gas cloud cools rapidly by atomic hydrogen cooling and then by molecular hydrogen cooling down to $\sim!400\,$Kelvin. The rapid decrease of the temperature and hence of the Jeans mass triggers fragmentation of the filament to yield multiple gas clumps with a few hundred solar-masses. We estimate the mass of the primordial star formed in each fragment by adopting an analytic model based on a large set of radiation hydrodynamics simulations of protostellar evolution. The resulting stellar masses are in the range of $\sim!50$–$120\,{\rm M_\odot}$. The massive stars gravitationally attract each other and form a compact star cluster. We follow the dynamics of the star cluster using a hybrid $N$-body simulation. We show that massive star binaries are formed in a few million years through multi-body interactions at the cluster center. Eventual formation of the remnant black holes will leave a massive black hole binary, which can be a progenitor of strong gravitational wave sources similar to those recently detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).

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S. Hirano, N. Yoshida, Y. Sakurai, et. al.
Tue, 21 Nov 17

Comments: 13 pages, 11 figures, 2 tables

Mapping the Tidal Disruption of the Hercules Dwarf: A Wide-Field DECam Imaging Search for RR Lyrae Stars [GA]

We investigate the hypothesized tidal disruption of the Hercules ultra-faint dwarf galaxy (UFD). Previous tidal disruption studies of the Hercules UFD have been hindered by the high degree of foreground contamination in the direction of the dwarf. We bypass this issue by using RR Lyrae stars, which are standard candles with a very low field-volume density at the distance of Hercules. We use wide-field imaging from the Dark Energy Camera on CTIO to identify candidate RR Lyrae stars, supplemented with observations taken in coordination with the Beijing-Arizona Sky Survey on the Bok Telescope. Combining color, magnitude, and light-curve information, we identify three new RR Lyrae stars associated with Hercules. All three of these new RR Lyrae stars lie outside its published tidal radius. When considered with the nine RR Lyrae stars already known within the tidal radius, these results suggest that a substantial fraction of Hercules’ stellar content has been stripped. With this degree of tidal disruption, Hercules is an interesting case between a visibly disrupted dwarf (such as the Sagittarius dwarf spheroidal galaxy) and one in dynamic equilibrium. The degree of disruption also shows that we must be more careful with the ways we determine object membership when estimating dwarf masses in the future. One of the three discovered RR Lyrae stars sits along the minor axis of Hercules, but over two tidal radii away. This type of debris is consistent with recent models that suggest Hercules’ orbit is aligned with its minor axis.

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C. Garling, B. Willman, D. Sand, et. al.
Tue, 21 Nov 17

Comments: 12 pages, 5 figures, 2 tables, accepted for publication in ApJ