Southern Galactic Hemisphere Research Articles

Mapping the Universe with gamma-ray bursts

Abstract We explore large-scale cosmic structure using the spatial distribution of 542 gamma-ray bursts (GRBs) having accurately-measured positions and spectroscopic redshifts. Prominent cosmological clusters are identified in both the northern and southern galactic hemispheres (avoiding extinction effects in the plane of the Milky Way) using the Bootstrap Point-Radius method. The Northern Galactic hemisphere contains a significant group of four GRBs in the redshift range 0.59 ≤ z ≤ 0.62 (with a Bootstrap probability of p = 0.012) along with the previously-identified Hercules–Corona Borealis Great Wall (in the revised redshift range 0.9 ≤ z ≤ 2.1; p = 0.017). The Southern Galactic hemisphere contains the previously-identified Giant GRB Ring (p = 0.022) along with another possible cluster of 7 − 9 GRBs at 1.17 ≤ z ≤ 1.444 (p = 0.031). Additionally, both the Hercules–Corona Borealis Great Wall and the Giant GRB Ring have become more prominent as the GRB sample size has grown. The approach used here underscores the potential value of GRB clustering as a probe of large-scale cosmic structure, complementary to galaxy and quasar clustering. Because of the vast scale on which GRB clustering provides valuable insights, it is important that optical GRB monitoring continue so that additional spectroscopic redshift measurements should be obtained.

Constraining a Model of the Radio Sky below 6 MHz Using the Parker Solar Probe/FIELDS Instrument in Preparation for Upcoming Lunar-based Experiments

We present a Bayesian analysis of data from the FIELDS instrument on board the Parker Solar Probe (PSP) spacecraft with the aim of constraining low-frequency (≲6 MHz) sky in preparation for several upcoming lunar-based experiments. We utilize data recorded during PSP’s coning roll maneuvers, in which the axis of the spacecraft is pointed 45° off of the Sun. The spacecraft then rotates about a line between the Sun and the spacecraft with a period of 24 minutes. We reduce the data into two formats: roll-averaged, in which the spectra are averaged over the roll, and phase-binned, in which the spectra are binned according to the phase of the roll. We construct a forward model of the FIELDS observations that includes numerical simulations of the antenna beam, an analytic emissivity function of the galaxy, and estimates of the absorption due to free electrons. Fitting 5 parameters, we find that the roll-averaged data can be fit well by this model, and we obtain posterior parameter constraints that are in general agreement with previous estimates. The model is not, however, able to fit the phase-binned data well, likely due to limitations such as the lack of nonsmooth emission structure at both small and large scales, enforced symmetry between the northern and southern galactic hemispheres, and large uncertainties in the free electron density. This suggests that significant improvement in the low-frequency sky model is needed in order to fully and accurately represent the sky at frequencies below 6 MHz.

Gaussian Decomposition of λ21 cm H i profiles, the Critical Ionization Velocity, and the Interstellar Helium Abundance

Following an established protocol of science—that results must be reproducible—we examine the Gaussian fits to Galactic λ21 cm (H i) emission profiles obtained by two seemingly complementary methods using data from the Leiden–Argentine–Bonn all-sky survey. One is based on the method used by Verschuur, the other by Nidever et al. (2008). The comparisons led to the identification of four problems that might arise when an algorithm is applied to huge databases without close monitoring: (1) different methods of calculating measuring the goodness of fit; (2) an ultra-broad component found to imperfectly bridge the gap between low- and intermediate-velocity gas; (3) the lack of an imposed spatial coherence allowing different components to appear and disappear in profiles separated by a fraction of a beamwidth; and (4) multiple, fundamentally different solutions for profiles at both the north and south Galactic poles. Confirming evidence emerges from this study of an underlying component with a line width of an order 34 km s−1. If this feature is the result of the critical ionization velocity effect acting on interstellar helium, it can be used to calculate its interstellar abundance. Analysis of H i profiles in an area in the southern Galactic hemisphere using multitelescope data gives a helium abundance of 0.094 ± 0.035, in excellent agreement with the accepted cosmic abundance of 0.085.

An Optical Spectrum of the Diffuse Galactic Light from BOSS and IRIS

We present a spectrum of the diffuse Galactic light (DGL) between 3700 and 10,000 Å, obtained by correlating optical sky intensity with far-infrared dust emission. We use nearly 250,000 blank-sky spectra from BOSS/SDSS-III together with IRIS-reprocessed maps from the IRAS satellite. The larger sample size compared to SDSS-II results in a factor-of-2 increase in signal to noise. We combine these data sets with a model for the optical/far-infrared correlation that accounts for self-absorption by dust. The spectral features of the DGL agree remarkably well with the features present in stellar spectra. There is evidence for a difference in the DGL continuum between the regions covered by BOSS in the northern and southern Galactic hemispheres. We interpret the difference at red wavelengths as the result of a difference in stellar populations, with mainly old stars in both regions, but a higher fraction of young stars in the south. There is also a broad excess in the southern DGL spectrum over the prediction of a simple radiative transfer model, without a clear counterpart in the north. We interpret this excess, centered at ∼6500 Å, as evidence for luminescence in the form of extended red emission. The observed strength of the 4000 Å break indicates that at most ∼7% of the dust-correlated light at 4000 Å can be due to blue luminescence. Our DGL spectrum provides constraints on dust scattering and luminescence, independent of measurements of extinction.

Tidally stripped halo stars from the Large Magellanic Cloud in the Galactic North

ABSTRACT We examine whether the Large Magellanic Cloud (LMC) is currently losing its stellar halo to Milky Way (MW) tides. We present a live N-body model for the ongoing MW–LMC interaction that predicts a prominent stream of stars tidally stripped from the progenitor LMC. We use this model to define a strategy to search for stripped material in kinematic space. Of the available stellar tracers, we conclude that samples of RR Lyrae stars provide the highest density of kinematic tracers at present. Using a sample of RR Lyrae stars with Gaia EDR3 astrometry, we show that the LMC stellar halo in the Southern Galactic hemisphere extends at least out to ∼30○ from the galaxy centre. In addition, several leading arm candidates are found in the Northern hemisphere as far above the disc plane as b = +34○ (at 68○ from the LMC).

SIBELIUS-DARK: a galaxy catalogue of the local volume from a constrained realization simulation

ABSTRACT We present sibelius-dark, a constrained realization simulation of the local volume to a distance of 200 Mpc from the Milky Way. sibelius-dark is the first study of the ‘Simulations Beyond The Local Universe’ (sibelius) project, which has the goal of embedding a model Local Group-like system within the correct cosmic environment. The simulation is dark-matter-only, with the galaxy population calculated using the semi-analytic model of galaxy formation, galform. We demonstrate that the large-scale structure that emerges from the sibelius constrained initial conditions matches well the observational data. The inferred galaxy population of sibelius-dark also match well the observational data, both statistically for the whole volume and on an object-by-object basis for the most massive clusters. For example, the K-band number counts across the whole sky, and when divided between the northern and southern Galactic hemispheres, are well reproduced by sibelius-dark. We find that the local volume is somewhat unusual in the wider context of ΛCDM: it contains an abnormally high number of supermassive clusters, as well as an overall large-scale underdensity at the level of ≈5 per cent relative to the cosmic mean. However, whilst rare, the extent of these peculiarities does not significantly challenge the ΛCDM model. sibelius-dark is the most comprehensive constrained realization simulation of the local volume to date, and with this paper we publicly release the halo and galaxy catalogues at z = 0, which we hope will be useful to the wider astronomy community.

Measuring the Mass of the Large Magellanic Cloud with Stellar Streams Observed by S 5

Abstract Stellar streams are excellent probes of the underlying gravitational potential in which they evolve. In this work, we fit dynamical models to five streams in the Southern Galactic hemisphere, combining observations from the Southern Stellar Stream Spectroscopic Survey (S 5), Gaia EDR3, and the Dark Energy Survey, to measure the mass of the Large Magellanic Cloud (LMC). With an ensemble of streams, we find a mass of the LMC ranging from ∼14–19 × 1010 M ⊙, probed over a range of closest approach times and distances. With the most constraining stream (Orphan–Chenab), we measure an LMC mass of 18.8 − 4.0 + 3.5 × 10 10 M ⊙ , probed at a closest approach time of 310 Myr and a closest approach distance of 25.4 kpc. This mass is compatible with previous measurements, showing that a consistent picture is emerging of the LMC’s influence on structures in the Milky Way. Using this sample of streams, we find that the LMC’s effect depends on the relative orientation of the stream and LMC at their point of closest approach. To better understand this, we present a simple model based on the impulse approximation and we show that the LMC’s effect depends both on the magnitude of the velocity kick imparted to the stream and the direction of this kick.

λ21-cm Interstellar HI Profiles, Critical Ionization Velocities, and Derived Electron Densities

Analysis of neutral hydrogen λ21-cm profiles at high northern and southern Galactic latitudes reveals unexpected properties, connects unanticipated associations, and confirms the fundamental parameters of the interstellar medium. Gaussian decomposition of profiles from the second data release of the Galactic Arecibo L-Band Feed Array (GALFA) confirms earlier results of a pervasive component with a linewidth of order 34 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , consistent with the critical ionization velocity (CIV) of helium. The ratio of the column density of this component to the entire profile is proportional to the interstellar helium abundance. The results are comparable to the accepted value of the cosmic abundance, 0.085, in the southern Galactic hemisphere, but different for directions in the northern Galactic hemisphere. Normalizing to the cosmic abundance allows us to estimate how much HI is “missing” from the λ21-cm observations in the northern sky, confirming an asymmetry that has been known for decades. These CIV results also predict the volume density of interstellar electrons along different lines of site, which match values derived from pulsar dispersion measures and Hα observations. These results provide strong support for the hypothesis that interstellar λ21-cm HI profile linewidths are modulated by the CIV plasma phenomenon.

The tidal stream generated by the globular cluster NGC 3201

ABSTRACT We detect a tidal stream generated by the globular cluster NGC 3201 extending over ∼140 deg on the sky, using the Gaia DR2 data, with the maximum-likelihood method we presented previously to study the M68 tidal stream. Most of the detected stream is the trailing one, which stretches in the southern Galactic hemisphere and passes within a close distance of 3.2 kpc from the Sun, therefore making the stream highly favourable for discovering relatively bright member stars, while the leading arm is further from us and behind a disc foreground that is harder to separate from. The cluster has just crossed the Galactic disc and is now in the northern Galactic hemisphere, moderately obscured by dust, and the part of the trailing tail closest to the cluster is highly obscured behind the plane. We obtain a best-fitting model of the stream which is consistent with the measured proper motion, radial velocity, and distance to NGC 3201, and show it to be the same as the previously detected Gjöll stream by Ibata et al. We identify ∼200 stars with the highest likelihood of being stream members using only their Gaia kinematic data. Most of these stars (170) are photometrically consistent with being members of NGC 3201 when they are compared to the cluster H–R diagram, only once a correction for dust absorption and reddening by the Galaxy is applied. The remaining stars are consistent with being random foreground objects according to simulated data sets. We list these 170 highly likely stream member stars.

Hemispheric Handedness in the Galactic Synchrotron Polarization Foreground

Abstract The large-scale magnetic field of the Milky Way is thought to be created by an αΩ dynamo, which implies that it should have opposite handedness north and south of the Galactic midplane. Here we attempt to detect a variation in handedness using polarization data from the Wilkinson Microwave Anisotropy Probe. Previous analyzes of the parity-even and parity-odd parts of linear polarization of the global dust and synchrotron emission have focused on quadratic correlations in spectral space of, and between, these two components. Here, by contrast, we analyze the parity-odd polarization itself and show that it has, on average, opposite signs in northern and southern Galactic hemispheres. Comparison with a Galactic mean-field dynamo model shows broad qualitative agreement and reveals that the sign of the observed hemispheric dependence of the azimuthally averaged parity-odd polarization is not determined by the sign of α, but by the sense of differential rotation.

Kinematics of main-sequence stars from the Gaia DR2 and PMA proper motions

ABSTRACTWe aim at analysis of kinematics of main-sequence stars from the GaiaDR2 and PMA catalogues as well as comparison of kinematic parameters derived from their proper motions. We decompose the stellar velocity field on to a set of vector spherical harmonics (VSH), and, using the relations between the decomposition coefficients and the Ogorodnikov–Milne (O–M) model parameters, calculate the latter. The method of VSH allowed to detect all systematic constituents present in the GaiaDR2 and PMA stellar velocity fields. We notice incompleteness of the O–M model, discuss the kinematic parameters derived within its framework, as well as significant decomposition coefficients that do not have analogues within this model. For separate analysis of the kinematic parameters in the Northern and Southern Galactic hemispheres, we apply the decomposition on to a set of zonal VSH (ZVSH). Modelling the stellar velocity field allowed to confirm the causes of some significant beyond-the-model harmonics. Based on stellar proper motions from the Gaia DR2 and PMA catalogues, we confirm the previous conclusion that the values of ω1 and $M^+_{23}$ O–M model parameters derived in the Northern and Southern Galactic hemispheres have opposite signs. This fact takes place due to a vertical gradient of the linear rotation velocity about the Galactic centre. We estimate influence of the vertical gradient on the value of solar velocity component Y⊙. We confirm that the beyond-the-model coefficients t211 and s310 are caused by the vertical gradient as well.

Searching for H i imprints in cosmic web filaments with 21-cm intensity mapping

ABSTRACT We investigate the possible presence of neutral hydrogen (H i) in intergalactic filaments at very low redshift (z ∼ 0.08), by stacking a set of 274 712 2dFGRS galaxy pairs over 21-cm maps obtained with dedicated observations conducted with the Parkes radio telescope, over a total sky area of approximately 1300 deg2 covering two patches in the northern and in the southern Galactic hemispheres. The stacking is performed by combining local maps in which each pair is brought to a common reference frame; the resulting signal from the edge galaxies is then removed to extract the filament residual emission. We repeat the analysis on maps cleaned removing either 10 or 20 foreground modes in a principal component analysis. Our study does not reveal any clear H i excess in the considered filaments in either case; we determine upper limits on the total filament H i brightness temperature at $T_{\rm b} \lesssim 10.3 \, \mu \text{K}$ for the 10-mode and at $T_{\rm b} \lesssim 4.8 \, \mu \text{K}$ for the 20-mode removed maps at the 95 per cent confidence level. These estimates translate into upper limits for the local filament H i density parameter, $\Omega _{\rm HI}^{\rm (f)} \lesssim 7.0\times 10^{-5}$ and $\Omega _{\rm HI}^{\rm (f)} \lesssim 3.2\times 10^{-5}$, respectively, and for the H i column density, $N_{\rm HI} \lesssim 4.6\times 10^{15}\, \text{cm}^{-2}$ and $N_{\rm HI} \lesssim 2.1\times 10^{15}\, \text{cm}^{-2}$, respectively. These column density constraints are consistent with previous detections of H i in the warm-hot intergalactic medium obtained observing broad Ly α absorption systems. This work shows for the first time how such constraints can be achieved using the stacking of galaxy pairs on 21-cm maps.

Identifying Galactic Halo Substructure in 6D Phase Space Using ∼13,000 LAMOST K Giants

Abstract We construct a large halo K-giant sample by combining the positions, distances, radial velocities, and metallicities of over 13,000 LAMOST DR5 halo K giants with the Gaia DR2 proper motions, which covers a Galactocentric distance range of 5–120 kpc. Using a position–velocity clustering estimator (the 6Distance), we statistically quantify the presence of position–velocity substructure at high significance: K giants have more close pairs in position–velocity space than a smooth stellar halo. We find that the amount of substructure in the halo increases with increasing distance and metallicity. With a percolation algorithm named friends-of-friends to identify groups, we identify members belonging to Sagittarius (Sgr) Streams, Monoceros Ring, Virgo Overdensity, Hercules–Aquila Cloud, Orphan Streams, and other unknown substructures and find that the Sgr streams account for a large part of grouped stars beyond 20 kpc and enhance the increase of substructure with distance and metallicity. For the first time, we identify spectroscopic members of Monoceros Ring in the southern and northern Galactic hemispheres, which presents a rotation of about 185 km s−1 and a mean metallicity of −0.66 dex.

Dark molecular gas in Pegasus–Pisces

Abstract We examine the molecular content of a large region (∼2200 square degrees) in Pegasus–Pisces with an estimated dark molecular gas fraction of 59 per cent. Using the extensive CO(1-0) Southern Galactic hemisphere, high-latitude survey by Magnani et al. (2000), we re-examined the CO-detectable mass estimates for the region. By averaging all the CO spectra in subsections ranging in size from 3° × 3° to 15° × 15°, we decreased the rms of the averaged CO spectra by factors of 3–10, effectively trading spatial resolution for sensitivity. With the new spectra, we are able to make estimates of the CO-detectable mass as a function of sensitivity. Using the optimal estimate, the CO-detectable mass increases from 2200 to 4000 M⊙, thereby decreasing the dark molecular gas fraction in the region to 0.24. CO(1–0) observations with rms values in the 20–30 mK range can nearly double the molecular mass in regions with diffuse and translucent molecular clouds.

Gaia-DR2 extended kinematical maps

Context. The Gaia Collaboration has used Gaia-DR2 sources with six-dimensional (6D) phase space information to derive kinematical maps within 5 kpc of the Sun, which is a reachable range for stars with relative error in distance lower than 20%. Aims. Here we aim to extend the range of distances by a factor of two to three, thus adding the range of Galactocentric distances between 13 kpc and 20 kpc to the previous maps, with their corresponding error and root mean square values. Methods. We make use of the whole sample of stars of Gaia-DR2 including radial velocity measurements, which consists in more than seven million sources, and we apply a statistical deconvolution of the parallax errors based on the Lucy’s inversion method of the Fredholm integral equations of the first kind, without assuming any prior. Results. The new extended maps provide lots of new and corroborated information about the disk kinematics: significant departures of circularity in the mean orbits with radial Galactocentric velocities between −20 and +20 km s−1 and vertical velocities between −10 and +10 km s−1; variations of the azimuthal velocity with position; asymmetries between the northern and the southern Galactic hemispheres, especially towards the anticenter that includes a larger azimuthal velocity in the south; and others. Conclusions. These extended kinematical maps can be used to investigate the different dynamical models of our Galaxy, and we will present our own analyses in the forthcoming second part of this paper. At present, it is evident that the Milky Way is far from a simple stationary configuration in rotational equilibrium, but is characterized by streaming motions in all velocity components with conspicuous velocity gradients.

Possible connection between the asymmetry of the North Polar Spur and Loop I and Fermi bubbles

The origin of North Polar Spur (NPS) and Loop-I has been debated over almost half a century and is still unresolved. Most of the confusion is caused by the absence of any prominent counterparts of these structures in the southern Galactic hemisphere (SGH). This has also led to doubts over the claimed connection between the NPS and Fermi Bubbles (FBs). I show in this paper, that such asymmetries of NPS and Loop-I in both X-rays and $\gamma$-rays can be easily produced if the circumgalactic medium (CGM) density in the southern hemisphere is only smaller by $\approx 20\%$ than the northern counterpart in case of a star formation driven wind scenario. The required mechanical luminosity, $\mathcal{L} \approx 4-5\times 10^{40} $ erg s$^{-1}$ (reduces to $\approx 0.3$ M$_\odot$ yr$^{-1}$ including the non-thermal pressure) and the age of the FBs, $t_{\rm age} \approx 28$ Myr, are consistent with previous estimations in case of a star formation driven wind scenario. One of the main reasons for the asymmetry is the projection effects at the Solar location. Such a proposition is also consistent with the fact that the southern FB is $\approx 5^\circ$ bigger than the northern one. The results, therefore, indicate towards a possibility for a common origin of the NPS, Loop-I and FBs from the Galactic centre (GC). I also estimate the average sky brightness in X-ray towards the south Galactic pole and North Galactic pole in the ROSAT-R67 band and find that the error in average brightness is far too large to have any estimation of the deficiency in the southern hemisphere.

Interstellar Matters: Neutral Hydrogen and the Galactic Magnetic Field

Abstract A very long and nearly straight H i filament at about −60 km s−1 in the southern Galactic hemisphere, seen nearly normal to the line of sight and well separated from low-velocity gas, has been studied in several ways in order to understand its physics, structure, and morphology. Gaussian analysis of 1800 profiles and examination of 140 declination–velocity cross sections shows that an underlying H i component, which is at least 15° long and about 1° wide, has a typical line width of 21 km s−1. It does not appear to be in thermal pressure equilibrium with its surroundings; rather, it may be confined by a magnetic field of 18 μG. Narrow, elongated features (threads), probably unresolved in the 4′ H i observations, have been identified within the boundaries of the filament. In general, each of these threads has two emission components, with line widths of the order of 8 and 3 km s−1, which may wind around each other. Analysis suggests that these cooler components have an average density of 29 cm−3 and may be confined by a magnetic field of 5 μG. These results, taken together, can be explained if this southern filament has magnetic substructure.

Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?* † ‡

Abstract Thanks to modern sky surveys, over 20 stellar streams and overdensity structures have been discovered in the halo of the Milky Way. In this paper, we present an analysis of spectroscopic observations of individual stars from one such structure, “A13,” first identified as an overdensity using the M giant catalog from the Two Micron All Sky Survey. Our spectroscopic observations show that stars identified with A13 have a velocity dispersion of ≲40 , implying that it is a genuine coherent structure rather than a chance superposition of random halo stars. From its position on the sky, distance (∼15 kpc heliocentric), and kinematical properties, A13 is likely to be an extension of another substructure at low Galactic latitude—the Galactic Anticenter Stellar Structure (also known as the Monoceros Ring)—toward smaller Galactic longitude and greater distance. Furthermore, the kinematics of A13 also connect it with another structure in the southern Galactic hemisphere—the Triangulum–Andromeda overdensity. We discuss these three connected structures within the context of a previously proposed scenario in which one or all of these features originate from the disk of the Milky Way.

Local properties of the large-scale peaks of the CMB temperature

In the present work, we study the largest structures of the CMB temperature measured by Planck in terms of the most prominent peaks on the sky, which, in particular, are located in the southern galactic hemisphere. Besides these large-scale features, the well-known Cold Spot anomaly is included in the analysis. All these peaks would contribute significantly to some of the CMB large-scale anomalies, as the parity and hemispherical asymmetries, the dipole modulation, the alignment between the quadrupole and the octopole, or in the case of the Cold Spot, to the non-Gaussianity of the field. The analysis of the peaks is performed by using their multipolar profiles, which characterize the local shape of the peaks in terms of the discrete Fourier transform of the azimuthal angle. In order to quantify the local anisotropy of the peaks, the distribution of the phases of the multipolar profiles is studied by using the Rayleigh random walk methodology. Finally, a direct analysis of the 2-dimensional field around the peaks is performed in order to take into account the effect of the galactic mask. The results of the analysis conclude that, once the peak amplitude and its first and second order derivatives at the centre are conditioned, the rest of the field is compatible with the standard model. In particular, it is observed that the Cold Spot anomaly is caused by the large value of curvature at the centre.

All-sky census of Galactic high-latitude molecular intermediate-velocity clouds

The HI halo clouds of the Milky Way, and in particular the intermediate-velocity clouds (IVCs), are thought to be connected to Galactic fountain processes. Observations of fountain clouds are important for understanding the role of matter recycling and accretion onto the Galactic disk and subsequent star formation. Here, we quantify the amount of molecular gas in the Galactic halo. We focus on the rare class of molecular IVCs (MIVCs) and search for new objects. The HI-FIR correlation was studied across the entire northern and southern Galactic hemispheres at Galactic latitudes $|b|>20^\circ$ to determine the amount and distribution of molecular gas in IVCs. We used the most recent large-scale HI and FIR data, the Effelsberg Bonn-HI Survey, the Parkes Galactic All-Sky Survey, and the Planck FIR surveys. We present a catalogue of 239 MIVC candidates on the northern and southern Galactic hemispheres. Among these candidates, all previously known MIVCs are recovered except for one single source. The frequency of candidates differs significantly between the northern and southern Galactic hemispheres and between negative and positive LSR velocities as well. In our approach we analyse the local Galactic environment. Extrapolating our results to the entire Galaxy, the global inflow of atomic and molecular IVC gas onto the Milky Way may account for the major fraction of the gaseous mass that is required to sustain the current Galactic star formation rate.