A comprehensive model for the formation and evolution of the faintest Milky Way dwarf satellites
Niusha AhvaziAndrew BensonLaura V. SalesEthan O. NadlerSachi WeerasooriyaXiaolong DuMia Sauda Bovill
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ABSTRACT In this study, we modify the semi-analytic model galacticus in order to accurately reproduce the observed properties of dwarf galaxies in the Milky Way. We find that reproducing observational determinations of the halo occupation fraction and mass–metallicity relation for dwarf galaxies requires us to include H2 cooling, an updated ultraviolet background radiation model, and to introduce a model for the metal content of the intergalactic medium. By fine-tuning various model parameters and incorporating empirical constraints, we have tailored the model to match the statistical properties of Milky Way dwarf galaxies, such as their luminosity function and size–mass relation. We have validated our modified semi-analytic framework by undertaking a comparative analysis of the resulting galaxy–halo connection. We predict a total of $300 ^{+75} _{-99}$ satellites with an absolute V-band magnitude (MV) less than 0 within 300 kpc from our Milky Way analogues. The fraction of subhaloes that host a galaxy at least this bright drops to 50 per cent by a halo peak mass of ∼8.9 × 107 M⊙, consistent with the occupation fraction inferred from the latest observations of Milky Way satellite population.We show that Smoothed Particle Hydrodynamics (SPH) simulations of dwarf galaxies interacting with a Milky Way-like disk produce moving groups in the simulated stellar disk. We analyze three different simulations: one that includes dwarf galaxies that mimic the Large Magellanic Cloud, Small Magellanic Cloud and the Sagittarius dwarf spheroidal; another with a dwarf galaxy that orbits nearly in the plane of the Milky Way disk; and a null case that does not include a dwarf galaxy interaction. We present a new algorithm to find large moving groups in the $V_R, V_ϕ$ plane in an automated fashion that allows us to compare velocity sub-structure in different simulations, at different locations, and at different times. We find that there are significantly more moving groups formed in the interacting simulations than in the isolated simulation. A number of dwarf galaxies are known to orbit the Milky Way, with at least one known to have had a close pericenter approach. Our analysis of simulations here indicates that dwarf galaxies like those orbiting our Galaxy produce large moving groups in the disk. Our analysis also suggests that some of the moving groups in the Milky Way may have formed due to dynamical interactions with perturbing dwarf satellites. The groups identified in the simulations by our algorithm have similar properties to those found in the Milky Way, including similar fractions of the total stellar population included in the groups, as well as similar average velocities and velocity dispersions.
Dwarf galaxy problem
Dwarf spheroidal galaxy
Local Group
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We report the discovery with the European Photon Imaging Camera CCDs on board XMM-Newton of a 54 mHz quasi-periodic oscillation (QPO) in the greater than 2 keV X-ray flux from an ultraluminous X-ray source (ULX) in the starburst galaxy M82. This is the first detection of a QPO in the X-ray flux from an extragalactic ULX and confirms that the source is a compact object. On the basis of the QPO strength and previous Chandra observations, it appears likely that the QPO is associated with the most luminous object in the central region of M82, CXO M82 J095550.2+694047; however, XMM imaging alone is not sufficient to unambiguously confirm this. The other plausible candidate is CXO M82 J095551.1+694045; however, the QPO luminosity is comparable to the peak luminosity of this object in Chandra observations, which argues against it being the source of the QPO. The QPO had a centroid frequency of 54.3 ± 0.9 mHz, a coherence Q ≡ ν0/Δνfwhm ≈ 5, and an amplitude (rms) in the 2-10 keV band of 8.5%. Below 0.2 Hz, the power spectrum can be fitted by a power law with index ≈1 and amplitude (rms) of 13.5%. The X-ray spectrum requires a curving continuum, with a disk blackbody at T = 3.1 keV providing an acceptable fit. A broad Fe line centered at 6.55 keV is required in all fits, but the equivalent width is sensitive to the continuum model. There is no evidence of a reflection component. The implied bolometric luminosity is ≈ × 1040 ergs s-1. Archival Rossi X-Ray Timing Explorer pointings at M82 also show evidence for QPOs in the 50-100 mHz frequency range. We discuss the implications of our findings for models of ULXs.
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Black-body radiation
BL Lac object
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We perform $N$-body simulations of star clusters in time-dependant galactic potentials. Since the Milky Way was built-up through mergers with dwarf galaxies, its globular cluster population is made up of clusters formed both during the initial collapse of the Galaxy and in dwarf galaxies that were later accreted. Throughout a dwarf-Milky Way merger, dwarf galaxy clusters are subject to a changing galactic potential. Building on our previous work, we investigate how this changing galactic potential affects the evolution of a cluster's half mass radius. In particular, we simulate clusters on circular orbits around a dwarf galaxy that either falls into the Milky Way or evaporates as it orbits the Milky Way. We find that the dynamical evolution of a star cluster is determined by whichever galaxy has the strongest tidal field at the position of the cluster. Thus, clusters entering the Milky Way undergo changes in size as the Milky Way tidal field becomes stronger and that of the dwarf diminishes. We find that ultimately accreted clusters quickly become the same size as a cluster born in the Milky Way on the same orbit. Assuming their initial sizes are similar, clusters born in the Galaxy and those that are accreted cannot be separated based on their current size alone.
Galactic tide
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We perform $N$-body simulations of star clusters in time-dependant galactic potentials. Since the Milky Way was built-up through mergers with dwarf galaxies, its globular cluster population is made up of clusters formed both during the initial collapse of the Galaxy and in dwarf galaxies that were later accreted. Throughout a dwarf-Milky Way merger, dwarf galaxy clusters are subject to a changing galactic potential. Building on our previous work, we investigate how this changing galactic potential affects the evolution of a cluster's half mass radius. In particular, we simulate clusters on circular orbits around a dwarf galaxy that either falls into the Milky Way or evaporates as it orbits the Milky Way. We find that the dynamical evolution of a star cluster is determined by whichever galaxy has the strongest tidal field at the position of the cluster. Thus, clusters entering the Milky Way undergo changes in size as the Milky Way tidal field becomes stronger and that of the dwarf diminishes. We find that ultimately accreted clusters quickly become the same size as a cluster born in the Milky Way on the same orbit. Assuming their initial sizes are similar, clusters born in the Galaxy and those that are accreted cannot be separated based on their current size alone.
Galactic tide
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The high redhsift blazars powered by supermassive black holes with masses exceeding $10^9\:M_\odot$ have the highest jet power and luminosity and are important probes to test the physics of relativistic jets at the early epochs of the Universe. We present a multi-frequency spectral and temporal study of high redshift blazar PKS 0537-286 by analyzing data from Fermi-LAT, NuSTAR Swift XRT and UVOT. Although the time averaged $\gamma$-ray spectrum of the source is relatively soft (indicating the high-energy emission peak is below the GeV range), several prominent flares were observed when the spectrum hardened and the luminosity increased above $10^{49}\:{\rm erg\:s^{-1}}$. The X-ray emission of the source varies in different observations and is characterised by a hard spectrum $\leq1.38$ with a luminosity of $>10^{47}\:{\rm erg\:s^{-1}}$. The broadband spectral energy distribution in the quiescent and flaring periods was modeled within a one-zone leptonic scenario assuming different locations of the emission region and considering both internal (synchrotron radiation) and external (from the disk, broad-line region and dusty torus) photon fields for the inverse Compton scattering. The modeling shows that the most optimistic scenario, from the energy requirement point of view, is when the jet energy dissipation occurs within the broad-line region. The comparison of the model parameters obtained for the quiescent and flaring periods suggests that the flaring activities are most likely caused by the hardening of the emitting electron spectral index and shifting of the cut-off energy to higher values.
Spectral energy distribution
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Sagittarius
Dwarf spheroidal galaxy
Galactic plane
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We discuss the detection limits and current status of a uniform survey of SDSS I for ultra-faint Milky Way dwarf galaxies. We present the properties of two new, low surface brightness Milky Way companions discovered as a result of this survey. One of these companions is the Ursa Major dwarf, the newest dwarf spheroidal companion to the Milky Way and the lowest luminosity galaxy yet known. Ursa Major is about 100 kpc away and is similar to Sextans, but with roughly an order of magnitude fewer stars. The other companion, SDSSJ1049+5103, lies $\sim$ 50 kpc away. Its stellar distribution suggests that it may be undergoing tidal stripping. This companion is extremely faint (M$_V$ $\sim$ -3) but has a large half-light size for its luminosity. It is therefore unclear whether it is a globular cluster or a dwarf galaxy.
Dwarf spheroidal galaxy
Dwarf galaxy problem
Local Group
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Local Group
Galactic astronomy
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We perform N-body simulations of star clusters in time-dependant galactic potentials. Since the Milky Way was built up through mergers with dwarf galaxies, its globular cluster population is made up of clusters formed both during the initial collapse of the Galaxy and in dwarf galaxies that were later accreted. Throughout a dwarf Milky Way merger, dwarf galaxy clusters are subject to a changing galactic potential. Building on our previous work, we investigate how this changing galactic potential affects the evolution of a cluster's half-mass radius. In particular, we simulate clusters on circular orbits around a dwarf galaxy that either falls into the Milky Way or evaporates as it orbits the Milky Way. We find that the dynamical evolution of a star cluster is determined by whichever galaxy has the strongest tidal field at the position of the cluster. Thus, clusters entering the Milky Way undergo changes in size as the Milky Way tidal field becomes stronger and that of the dwarf diminishes. We find that ultimately accreted clusters quickly become the same size as a cluster born in the Milky Way on the same orbit. Assuming their initial sizes are similar, clusters born in the Galaxy and those that are accreted cannot be separated based on their current size alone.
Galactic tide
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We discuss the detection limits and current status of a uniform survey of SDSS I for ultra-faint Milky Way dwarf galaxies. We present the properties of two new, low surface brightness Milky Way companions discovered as a result of this survey. One of these companions is the Ursa Major dwarf, the newest dwarf spheroidal companion to the Milky Way and the lowest luminosity galaxy yet known. Ursa Major is about 100 kpc away and is similar to Sextans, but with roughly an order of magnitude fewer stars. The other companion, SDSSJ1049+5103, lies $\sim$ 50 kpc away. Its stellar distribution suggests that it may be undergoing tidal stripping. This companion is extremely faint (M$_V$ $\sim$ -3) but has a large half-light size for its luminosity. It is therefore unclear whether it is a globular cluster or a dwarf galaxy.
Dwarf spheroidal galaxy
Dwarf galaxy problem
Local Group
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