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Spatial, kinematic, and orbital properties, along with ages and chemical compositions of the thin disc, thick disc, and various stellar substructures in the halo, are studied based on data from the LAMOST and Gaia surveys. The star formation in the Galactic thin and thick disc, with peak metallicities of − 0.20 and − 0.45 dex, is found to have peaked about 5.5 and 12.5 Gyr ago, respectively. The thin disc is also found to have experienced an initial star formation burst about 12.5 Gyr ago. The pro-grade population Splash and hot-disc (HD), with peak metallicity of about − 0.60 and − 0.43, are found to be about 13.03 and 12.21 Gyr old, respectively, with peak eccentricity of 0.70 and 0.35, are understood to be of in situ origin. The Gaia-Enceladus/Sausage (GE/S), Thamnos and Sequoia, with peak metallicity of about − 1.31, − 1.36, and − 1.56, are found to be about 11.66, 12.89, and 12.18 Gyr, respectively, and are understood to be remnants of dwarf galaxies merged with the Milky Way. The HD, Splash, and Thamnos have experienced chemical evolution similar to the thick disc, while GE/S, Sequoia, and Helmi stream have experienced distinct chemical enrichment of iron and α -process elements. [ABSTRACT FROM AUTHOR]

Galactic outflows are ubiquitous in galaxies containing active star formation or supermassive black hole activity. The presence of a large-scale outflow from the center of our own Galaxy was confirmed after the discovery of two large (~ 8–10 kpc) γ -ray bubbles using the Fermi-LAT telescope. These bubbles, known as the Fermi Bubbles, are highly symmetric about the Galactic disk as well as around the Galactic rotation axis and appear to emanate from the center of our Galaxy. The sharp edges of these bubbles suggest that they are related to the Galactic outflow. These bubbles are surrounded by two even bigger (~ 12–14 kpc) X-ray structures, known as the eROSITA bubbles. Together, they represent the characteristics of an outflow from the Galaxy into the circumgalactic medium. Multi-wavelength observations such as in radio, microwave, and UV toward the Fermi Bubbles have provided us with much information in the last decade. However, the origin and the nature of these bubbles remain elusive. In this review, I summarize the observations related to the Fermi/eROSITA Bubbles at different scales and wavelengths, and give a brief overview of our current understanding of them. [ABSTRACT FROM AUTHOR]

We present the calibration of the [ $\alpha$ /Fe] element in terms of ultra-violet excess for 465 dwarf stars with spectral type F0-K2. We used a single calibration, fitted to a third degree polynomial with a square of the correlation coefficient 0.74 and standard deviation 0.05 mag, for all stars due to their small colour range, $0.1<(g-r)_{0} \leq 0.6$ mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [ $\alpha$ /Fe] elements in the range $(-0.05, 0.35]$ dex. We applied the procedure to a high-latitude field, $85^{\circ }\leq b \leq 90^{\circ }$ with size 78 deg and we could estimate the [ $\alpha$ /Fe] elements of 23,414 dwarf stars which occupy a Galactic region up to a vertical distance of $z=9$ kpc. We could detect a small positive gradient, $d[\alpha \text{/Fe}]/dz=+0.032 \pm 0.002~\mbox{dex}\,\mbox{kpc}^{-1}$ , for the range $0< z<5$ kpc, while the distribution of the [ $\alpha$ /Fe] element is flat for further $z$ distances. [ABSTRACT FROM AUTHOR]

We recently found that the halo of the Milky Way contains a large reservoir of warm-hot gas that accounts for large fraction of the missing baryons from the Galaxy. The average physical properties of this circumgalactic medium (CGM) are determined by combining average absorption and emission measurements along several extragalactic sightlines. However, there is a wide distribution of both, the halo emission measure and the O vii column density, suggesting that the Galactic warm-hot gaseous halo is anisotropic. We present Suzaku observations of fields close to two sightlines along which we have precise O vii absorption measurements with Chandra. The column densities along these two sightlines are similar within errors, but we find that the emission measures are different: 0.0025±0.0006 cm pc near the Mrk 421 direction and 0.0042±0.0008 cm pc close to the PKS 2155-304 sightline. Therefore the densities and pathlengths in the two directions must be different, providing a suggestive evidence that the warm-hot gas in the CGM of the Milky Way is not distributed uniformly. However, the formal errors on derived parameters are too large to make such a claim. In the Mrk 421 direction we derive the density of $1.6^{+2.6}_{-0.8} \times 10^{-4}~\mbox{cm}^{-3}$ and pathlength of $334^{+685}_{-274}~\mbox{kpc}$. In the PKS 2155-304 direction we measure the gas density of $3.6^{+4.5}_{-1.8} \times10^{-4}~\mbox{cm}^{-3}$ and path-length of $109^{+200}_{-82}~\mbox{kpc}$. Thus the density and pathlength along these sightlines are consistent with each other within errors. The average density and pathlength of the two sightlines are similar to the global averages, so the halo mass is still huge, over 10 billion solar masses. With more such studies, we will be able to better characterize the CGM anisotropy and measure its mass more accurately. We can then compare the observational results with theoretical models and investigate if/how the CGM structure is related to the larger scale environment of the Milky Way. We also show that the Galactic disk makes insignificant contribution to the observed O vii absorption; a similar conclusion was also reached by Henley and Shelton () about the emission measure. We further argue that any density inhomogeneity in the warm-hot gas, be it from clumping, from the disk, or from a non-constant density gradient, would strengthen our result in that the Galactic halo path-length and the mass would become larger than what we estimate here. As such, our results are conservative and robust. [ABSTRACT FROM AUTHOR]

We study the effect of the photoionization cross sections for the ground state of Al I on the inferred aluminium abundance in stellar atmospheres. We match the theoretical and observed line profiles of the resonance λλ 3944.01, 3961.52 Å and subordinate λλ 6696.03, 6698.68 Å doublets in high-resolution spectra of the metal-poor solar-type stars HD22879 and HD201889. We determine the parameters of these stars from their photometric and spectroscopic data. Our computations show that the profiles can be matched and a single aluminium abundance inferred simultaneously from both groups of spectral lines only with low photoionization cross sections (about 10-12 Mb). Larger cross sections (about 58-65 Mb) make such fits impossible. We therefore conclude that small photoionization cross sections should be preferred for the determination of aluminium abundances in metal-poor stars. We redetermine the aluminium abundances in the atmospheres of halo stars. The resulting abundances prove to be lower by 0.1-0.15 dex than our earlier determinations which does not affect the conclusions based on our earlier estimates. In particular, the NLTE [Al/Fe]-[Fe/H] dependence, on the whole, agrees only qualitatively with the results of theoretical predictions. Therefore further refinement of the theory of nuclear synthesis of aluminium in the process of the chemical evolution of the Galaxy remains a task of current importance. [ABSTRACT FROM AUTHOR]

Recent observational studies have discovered very small dwarf spheroidal galaxies (dSphs) which are the faintest member of the local group of galaxies. This paper examines their faintness because of the following reason: Comparing their M/ L (mass-luminosity ratio) to that of the other normal dSphs, we find very small dSphs are faint for their dark matter mass. This indicates their star formation is suppressed. There are two possibilities for the suppression: (1) ram pressure of IGM (intra-group medium), (2) wind from the Milky Way (MW). Owing to the ram pressure, interstellar medium of very small dSphs is possible to be stripped because of the shallowness of their gravitational potential. That is, star formation can be terminated during their evolution. However, the latter is difficult at the moment since their distance is far from MW. The author suggests star formation was terminated only when very small dSphs were beside MW whose wind was strong. [ABSTRACT FROM AUTHOR]

It is known that slow baryon outflow influences the evolution of dwarf spheroidals (dSphs) around the Milky Way. However, one may ask: what dSphs are too small? Actually, the depth of the gravitational potential of very small dSphs discovered recently is estimated as the energy of only one supernova explosion. This means that the loss of interstellar medium should be fast. Then, adopting a dSph formation scenario, which considers the fast baryon loss, we investigate which mass loss is important. According to our estimates, when only baryon mass loss is considered, the observational radius is larger than that expected in that scenario for all of the very small dSphs. This indicates that the expansion of the system is caused by slow dark matter outflow. [ABSTRACT FROM AUTHOR]

In this paper, the author proposes a formation scenario of dwarf spheroidal galaxies (dSphs); part of dSphs form as a result of collision of dark matter (DM) halos which retain ISM (interstellar medium). First of all, a primordial dSph forms compact when collision of DM halos occurs. After the collision, the primordial dSph leaves a mass-radius relation among old stellar systems. This intermediate evolutionary phase finishes when the galactic wind occurs. Finally, it is re-virialized. Once this scenario is adopted, we explain the reason why large M/ L (mass-to-light ratio) is a result of galactic wind. Then, we conclude it is possible for part of dSphs to be remnants of DM halo collisions. [ABSTRACT FROM AUTHOR]

Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia. [ABSTRACT FROM AUTHOR]

Absorption lines of O vii at redshift zero are observed in high quality Chandra spectra of extragalactic sightlines. The location of the absorber producing these lines, whether from the corona of the Galaxy or from the Local Group or even larger scale structure, has been a matter of debate. Here we study another poor group like our Local Group to understand the distribution of column density from galaxy to group scales. We show that we cannot yet rule out the group origin of z=0 systems. We further conclude that to argue for a Galactic or an extragalactic origin of z=0 systems is premature as they likely contain both components and predict that future higher resolution observations will resolve the z=0 systems into multiple components. [ABSTRACT FROM AUTHOR]

Two fundamental constraints on the earliest star formation conditions in the Galaxy are an apparent empirical low-metallicity threshold of-4 ≲ [Fe/H], an and upper limit to the fraction of Population III halo stars of F III < 4 × 10-4. How do these observed constraints compare with predictions of simple models? This is investigated within the framework of element dispersal from clustered core-collapse SNe. Simple arguments considering turbulent mixing within multi-phase ISM suggest that the observed low-metallicity threshold is consistent with rough expected values. However, the observed limit on F III is two orders of magnitude larger than predictions from this simple, one-zone inhomogeneous chemical evolution. [ABSTRACT FROM AUTHOR]

We present preliminary results of a long-term project to investigate the process of accretion and tidal disruption of dSph satellites in the Galactic halo and, in particular, to search for new tidal tails in a sample of nearby dSph satellites of the Milky Way. Here we present our finding of extra-tidal debris in the Ursa Minor dSph and discuss the detection by the Sloan Digitized Sky Survey team of what could be a tidal stream associated to the Sagittarius dSph. [ABSTRACT FROM AUTHOR]

Efficient spectrographs at large telescopes have made it possible to obtain high-resolution spectra of stars with high signal-to-noise ratio and advances in model atmosphere analyses have enabled estimates of high-precision differential abundances of the elements from these spectra, i.e. with errors in the range 0.01-0.03 dex for F, G, and K stars. Methods to determine such high-precision abundances together with precise values of effective temperatures and surface gravities from equivalent widths of spectral lines or by spectrum synthesis techniques are outlined, and effects on abundance determinations from using a 3D non-LTE analysis instead of a classical 1D LTE analysis are considered. The determination of high-precision stellar abundances of the elements has led to the discovery of unexpected phenomena and relations with important bearings on the astrophysics of galaxies, stars, and planets, i.e. (i) Existence of discrete stellar populations within each of the main Galactic components (disk, halo, and bulge) providing new constraints on models for the formation of the Milky Way. (ii) Differences in the relation between abundances and elemental condensation temperature for the Sun and solar twins suggesting dust-cleansing effects in proto-planetary disks and/or engulfment of planets by stars; (iii) Differences in chemical composition between binary star components and between members of open or globular clusters showing that star- and cluster-formation processes are more complicated than previously thought; (iv) Tight relations between some abundance ratios and age for solar-like stars providing new constraints on nucleosynthesis and Galactic chemical evolution models as well as the composition of terrestrial exoplanets. We conclude that if stellar abundances with precisions of 0.01-0.03 dex can be achieved in studies of more distant stars and stars on the giant and supergiant branches, many more interesting future applications, of great relevance to stellar and galaxy evolution, are probable. Hence, in planning abundance surveys, it is important to carefully balance the need for large samples of stars against the spectral resolution and signal-to-noise ratio needed to obtain high-precision abundances. Furthermore, it is an advantage to work differentially on stars with similar atmospheric parameters, because then a simple 1D LTE analysis of stellar spectra may be sufficient. However, when determining high-precision absolute abundances or differential abundance between stars having more widely different parameters, e.g. metal-poor stars compared to the Sun or giants to dwarfs, then 3D non-LTE effects must be taken into account. [ABSTRACT FROM AUTHOR]