Astrophysics

Stellar tidal streams are sensitive tracers of the properties of the gravitational potential in which they orbit and detailed observations of their density structure can be used to place stringent constraints on fluctuations in the potential caused by, e.g., the expected populations of dark matter subhalos in the standard cold dark matter paradigm (CDM). Simulations of the evolution of stellar streams in live N -body halos without low-mass dark-matter subhalos, however, indicate that streams exhibit significant perturbations on small scales even in the absence of substructure. Here we demonstrate, using high-resolution N -body simulations combined with sophisticated semi-analytic and simple analytic models, that the mass resolutions of 10 4 -- 10 5 M ??commonly used to perform such simulations cause spurious stream density variations with a similar magnitude on large scales as those expected from a CDM-like subhalo population and an order of magnitude larger on small, yet observable, scales. We estimate that mass resolutions of ??00 M ??( ?? M ??) are necessary for spurious, numerical density variations to be well below the CDM subhalo expectation on large (small) scales. That streams are sensitive to a simulation's particle mass down to such small masses indicates that streams are sensitive to dark matter clustering down to these low masses if a significant fraction of the dark matter is clustered or concentrated in this way, for example, in MACHO models with masses of 10 -- 100 M ??.

Dark stars are compact massive objects, described by Einstein gravitational field equations with matter. The type we consider possesses no event horizon, instead, there is a deep gravitational well with a very strong redshift factor. Observationally, dark stars can be identified with black holes. Inside dark stars, Planck density of matter is reached, Planck cores are formed, where the equations are modified by quantum gravity. In the paper, several models of dark stars with Planck cores are considered, resulting in the following hypothesis on the composition of dark matter. The galaxies are flooded with low-energetic radiation from the dark stars. The particle type can be photons and gravitons from the Standard Model, can also be a new type of massless particles. The model estimations show that the extremely large redshift factor z??10 49 and the emission wavelength λ 0 ??10 14 m can be reached. The particles are not registered directly in the existing dark matter experiments. They come in a density sufficient to explain the observable rotation curves. The emission has a geometric dependence of density on radius ???r ?? , producing flat rotation curves. The distribution of sources also describes the deviations from the flat shape. The model provides a good fit of experimental rotation curves. Outbreaks caused by a fall of an external object on a dark star lead to emission wavelength shifted towards smaller values. The model estimations give the outbreak wavelength λ?? m compatible with fast radio bursts. The paper raises several principal questions. White holes with Planck core appear to be stable. Galactic rotation curves in the considered setup do not depend on the matter type. Inside the galaxy, dark matter can be of hot radial type. At cosmological distances, it can behave like the cold uniform type.

In this manuscript, we report strong linear correlation between shifted velocity and line width of the broad blue-shifted [OIII] components in SDSS quasars. Broad blue-shifted [OIII] components are commonly treated as indicators of outflows related to central engine, however, it is still an open question whether the outflows are related to central accretion properties or related to local physical properties of NLRs (narrow emission line regions). Here, the reported strong linear correlation with the Spearman Rank correlation coefficient 0.75 can be expected under the assumption of AGN (active galactic nuclei) feedback driven outflows, through a large sample of 535 SDSS quasars with reliable blue-shifted broad [OIII] components. Moreover, there are much different detection rates for broad blue-shifted and broad red-shifted [OIII] components in quasars, and no positive correlation can be found between shifted velocity and line width of the broad red-shifted [OIII] components, which provide further and strong evidence to reject possibility of local outflows in NLRs leading to the broad blue-shifted [OIII] components in quasars. Thus, the strong linear correlation can be treated as strong evidence for the broad blue-shifted [OIII] components as better indicators of outflows related to central engine in AGN. Furthermore, rather than central BH masses, Eddington ratios and continuum luminosities have key roles on properties of the broad blue-shifted [OIII] components in quasars.

Imaging surveys of CO and other molecular transition lines are fundamental to measuring the large-scale distribution of molecular gas in the Milky Way. Due to finite angular resolution and sensitivity, however, observational effects are inevitable in the surveys, but few studies are available on the extent of uncertainties involved. The purpose of this work is to investigate the dependence of observations on angular resolution (beam sizes), sensitivity (noise levels), distances, and molecular tracers. To this end, we use high-quality CO images of a large-scale region (25.8 <l< 49.7 deg and |b|<5 deg) mapped by the Milky Way Imaging Scroll Painting (MWISP) survey as a benchmark to simulate observations with larger beam sizes and higher noise levels, deriving corresponding beam filling and sensitivity clip factors. The sensitivity clip factor is defined to be the completeness of observed flux. Taking the entire image as a whole object, we found that 12CO has the largest beam filling and sensitivity clip factors and C18O has the lowest. For molecular cloud samples extracted from images, the beam filling factor can be described by a characteristic size, l 1/4 =0.762 (in beam size), at which the beam filling factor is approximately 1/4. The sensitivity clip factor shows a similar relationship but is more correlated with the mean voxel signal-to-noise ratio of molecular clouds. This result may serve as a practical reference on beam filling and sensitivity clip factors in further analyses of the MWISP data and other observations.

Primordial supermassive stars (SMSs) formed in atomic-cooling halos at z ~ 15 - 20 are leading candidates for the seeds of the first quasars. Past numerical studies of the evolution of SMSs have typically assumed constant accretion rates rather than the highly variable flows in which they form. We model the evolution of SMSs in the cosmological flows that create them using the Kepler stellar evolution and implicit hydrodynamics code. We find that they reach masses of 1 - 2 x 10 5 M ??before undergoing direct-collapse to black holes (DCBHs) during or at the end of their main-sequence hydrogen burning, at 1 - 1.5 Myr, regardless of halo mass, spin, or merger history. We also find that realistic, highly-variable accretion histories allow for a much greater diversity of supermassive stellar structures, including in some cases largely thermally relaxed objects, which may provide a significant source of radiative feedback. Our models indicate that the accretion histories predicted for purely atomic-cooling halos may impose a narrow spectrum of masses on the seeds of the first massive quasars, however further studies incorporating realistic feedback will be essential in order to confirm whether or not this holds true in all cases. Our results also indicate that multiple SMSs at disparate stages of evolution can form in these halos, raising the possibility of SMS binaries and supermassive X-ray binaries (SMXBs), as well as DCBH mergers which could be detected by LISA.

We present a systematic X-ray and multiwavelength study of a sample of 47 active galactic nuclei (AGNs) with reverberation-mapping measurements. This sample includes 21 super-Eddington accreting AGNs and 26 sub-Eddington accreting AGNs. Using high-state observations with simultaneous X-ray and UV/optical measurements, we investigate whether super-Eddington accreting AGNs exhibit different accretion disk-corona connections compared to sub-Eddington accreting AGNs. We find tight correlations between the X-ray-to-UV/optical spectral slope parameter ( α OX ) and the monochromatic luminosity at 2500 ? ( L 2500 ? ) for both the super- and sub-Eddington subsamples. The best-fit α OX ??L 2500 ? relations are consistent overall, indicating that super-Eddington accreting AGNs are not particularly X-ray weak in general compared to sub-Eddington accreting AGNs. We find dependences of α OX on both the Eddington ratio ( L Bol / L Edd ) and black hole mass ( M BH ) parameters for our full sample. A multi-variate linear regression analysis yields α OX =??.13log( L Bol / L Edd )??.10log M BH ??.69 , with a scatter similar to that of the α OX ??L 2500 ? relation. The hard (rest-frame >2 keV ) X-ray photon index ( ? ) is strongly correlated with L Bol / L Edd for the full sample and the super-Eddington subsample, but these two parameters are not significantly correlated for the sub-Eddington subsample. A fraction of super-Eddington accreting AGNs show strong X-ray variability, probably due to small-scale gas absorption, and we highlight the importance of employing high-state (intrinsic) X-ray radiation to study the accretion disk-corona connections in AGNs.

In addition to the well-known gas phase mass-metallicity relation (MZR), recent spatially-resolved observations have shown that local galaxies also obey a mass-metallicity gradient relation (MZGR) whereby metallicity gradients can vary systematically with galaxy mass. In this work, we use our recently-developed analytic model for metallicity distributions in galactic discs, which includes a wide range of physical processes -- radial advection, metal diffusion, cosmological accretion, and metal-enriched outflows -- to simultaneously analyse the MZR and MZGR. We show that the same physical principles govern the shape of both: centrally-peaked metal production favours steeper gradients, and this steepening is diluted by the addition of metal-poor gas, which is supplied by inward advection for low-mass galaxies and by cosmological accretion for massive galaxies. The MZR and the MZGR both bend at galaxy stellar mass ??10 10 ??10 10.5 M ??, and we show that this feature corresponds to the transition of galaxies from the advection-dominated to the accretion-dominated regime. We also find that both the MZR and MZGR strongly suggest that low-mass galaxies preferentially lose metals entrained in their galactic winds. While this metal-enrichment of the galactic outflows is crucial for reproducing both the MZR and the MZGR at the low-mass end, we show that the flattening of gradients in massive galaxies is expected regardless of the nature of their winds.

It has been recently shown from observational data sets the variation of structural parameters and internal dynamical evolution of star clusters in the Milky Way and in the Large Magellanic Cloud (LMC), caused by the different gravitational field strengths that they experience. We report here some hints for such a differential tidal effects in structural parameters of star clusters in the Small Magellanic Cloud (SMC), which is nearly 10 times less massive than the LMC. A key contribution to this study is the consideration of the SMC as a triaxial spheroid, from which we estimate the deprojected distances to the SMC center of the statistically significant sample of star clusters analyzed. By adopting a 3D geometry of the SMC, we avoid the spurious effects caused by considering that a star cluster observed along the line-of-sight is close to the galaxy center. When inspecting the relationships between the star cluster sizes (represented by the 90% light radii), their eccentricities, masses and ages with the deprojected distances, we find: (i) the star cluster sizes are not visibly affected by tidal effects, because relatively small and large objects are spread through the SMC body. (ii) Star clusters with large eccentricities (> 0.4) are preferentially found located at deprojected distances smaller than ??7-8 kpc, although many star clusters with smaller eccentricities are also found occupying a similar volume. (iii) Star clusters more massive than log(M /Mo) ??4.0 are among the oldest star clusters, generally placed in the outermost SMC region and with a relative small level of flattening. These findings contrast with the more elongated, generally younger, less massive and innermost star clusters.

The ratio of α -elements to iron in galaxies holds valuable information about the star-formation history since their enrichment occurs on different timescales. The fossil record of stars in galaxies has mostly been excavated for passive galaxies, since the light of star-forming galaxies is dominated by young stars which have much weaker atmospheric absorption features. Here we use the cosmological EAGLE simulation to investigate the origin of variations in α -enhancement among star-forming galaxies at z=0 . The definition of α -enhancement in a composite stellar population is ambiguous. We elucidate two definitions - termed 'mean' and 'galactic' α -enhancement - in more detail. While a star-forming galaxy has a high 'mean' α -enhancement when its stars formed rapidly, a galaxy with a large 'galactic' α -enhancement generally had a delayed star formation history. We find that absorption-line strengths of Mg and Fe correlate with variations in α -enhancement. These correlations are strongest for the 'galactic' α -enhancement. However, we show that these are mostly caused by other effects which are cross-correlated with α -enhancement, such as variations in the light-weighted age. This severely complicates the retrieval of α -enhancements in star-forming galaxies. The ambiguity is not severe for passive galaxies and we confirm that spectral variations in these galaxies are caused by measurable variations in α -enhancements. We suggest that this more complex coupling between α -enhancement and star formation histories can guide the interpretation of new observations of star-forming galaxies.

Massive black holes (BHs) in dwarf galaxies can provide strong constraints on BH seeds, however reliably detecting them is notoriously difficult. High resolution radio observations were recently used to identify accreting massive BHs in nearby dwarf galaxies, with a significant fraction found to be non-nuclear. Here we present the first results of our optical follow-up of these radio-selected active galactic nuclei (AGNs) in dwarf galaxies using integral field unit (IFU) data from Gemini-North. We focus on the dwarf galaxy J1220+3020, which shows no clear optical AGN signatures in its nuclear SDSS spectrum covering the radio source. With our new IFU data, we confirm the presence of an active BH via the AGN coronal line [Fe X] and enhanced [O I] emission coincident with the radio source. Furthermore, we detect broad H α emission and estimate a BH mass of M BH = 10 4.9 M ??. We compare the narrow emission line ratios to standard BPT diagnostics and shock models. Spatially-resolved BPT diagrams show some AGN signatures, particularly in [O I]/H α , but overall do not unambiguously identify the AGN. A comparison of our data to shock models clearly indicates shocked emission surrounding the AGN. The physical model most consistent with the data is an active BH with a radiatively inefficient accretion flow (RIAF) that both photoionizes and shock-excites the surrounding gas. We conclude that feedback is important in radio-selected BHs in dwarf galaxies, and that radio surveys may probe a population of low accretion-rate BHs in dwarf galaxies that cannot be detected through optical surveys alone.