Galactocentric Distance Research Articles

No Large Dependence of Planet Frequency on Galactocentric Distance

Abstract Gravitational microlensing is currently the only technique that helps study the Galactic distribution of planets as a function of distance from the Galactic center. The Galactic location of a lens system can be uniquely determined only when at least two of the three quantities that determine the mass–distance relations are measured. However, even if only one mass–distance relation can be obtained, a large sample of microlensing events can be used to statistically discuss the Galactic distribution of the lenses. In this study, we extract the Galactic distribution of planetary systems from the distribution of the lens-source proper motion, μ rel, for a given Einstein radius crossing time, t E, measured for the 28 planetary events in the statistical sample by Suzuki et al. Because microlensing is randomly caused by stars in our Galaxy, the observational distribution can be predicted using a Galactic model. We incorporate the planet-hosting probability, P host ∝ M L m R L r , into a Galactic model for random-selected stars, where M L is the lens mass (∼host mass), and R L is the Galactocentric distance. By comparing the observed distribution with the model-predicted μ rel distribution for a given t E at various combinations of (m, r), we obtain an estimate r = 0.2 ± 0.4 under a plausible uniform prior for m of 0 < m < 2. This indicates that the dependence of the planet frequency on the Galactocentric distance is not large, and suggests that the Galactic bulge does have planets.

The Discovery of the Largest Gas Filament in Our Galaxy, or a New Spiral Arm?

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we detect a giant HI filamentary structure in the sky region of 307$.\!\!^{\circ}$7 $<$ $\alpha$ $<$ 311$.\!\!^{\circ}$0 and 40$.\!\!^{\circ}$9 $<$ $\delta$ $<$ 43$.\!\!^{\circ}$4. The structure has a velocity range of $-$170 km s$^{-1}$ to $-$130 km s$^{-1}$, and a mean velocity of $-$150 km s$^{-1}$, putting it to a Galactocentric distance of 22 kpc. The HI structure has a length of 1.1 kpc, which appears to be so far the furthest and largest giant filament in the Galaxy and we name it Cattail. Its mass is calculated to be 6.5 $\times$ 10$^4$ M$_{\odot}$ and the linear mass density is 60 M$_{\odot}$ pc$^{-1}$. Its width is 207 pc, corresponding to an aspect ratio of 5:1. Cattail possesses a small velocity gradient (0.02 km s$^{-1}$ pc$^{-1}$) along its major axis. Together with the HI4PI data, we find that Cattail could have an even larger length, up to 5 kpc. We also identify another new elongated structure to be the extension into the Galactic first quadrant of the Outer Scutum-Centaurus (OSC) arm, and Cattail appears to be located far behind the OSC. The question about how such a huge filament is produced at the extreme Galactic location remains open. Alternatively, Cattail might be part of a new arm beyond the OSC, though it is puzzling that the structure does not fully follow the warp of the Galactic disk.

Vertical Structure of Galactic Disk Kinematics from LAMOST K Giants

Abstract We examine the vertical structure of Galactic disk kinematics over a Galactocentric radial distance range of R = 5–15 kpc and up to 3 kpc away from the Galactic plane, using the K-type giants surveyed by LAMOST. Based on robust measurements of three-dimensional velocity moments, a wobbly disk is detected in a phenomenological sense. An outflow dominates the radial motion of the inner disk, while in the outer disk there exist alternate outward and inward flows. The vertical bulk velocities is a combination of breathing and bending modes. A contraction-like breathing mode with amplitudes increasing with the distance to the plane and an upward bending mode dominate the vertical motion outside R 0, and there are reversed breathing and bending modes at R &lt; R 0, with amplitudes much smaller than those outside R 0. The mean azimuthal velocity decreases with the increasing distance to the plane, with gradients shallower for larger R. Stars in the south disk are rotating faster than stars in the north. The velocity ellipsoid orientation differs between different R: in the range of 5 &lt; R &lt; 9 kpc , the gradient of the tilt angle with respect to arctan ( Z / R ) decreases from ∼0.83 for the inner disk to ∼0.52 for the outer disk; within 9 &lt; R &lt; 15 kpc , the tilt of velocity ellipsoid deviates from vertical antisymmetry. A clear flaring signature is found for both north and south disks based on the observed vertical structures of velocity ellipsoid.

The KBSS–KCWI survey: the connection between extended Ly α haloes and galaxy azimuthal angle at z ∼ 2–3

ABSTRACT We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $\rm Ly\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $\rm Ly\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $\rm Ly\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling.

Comparing hypervelocity star populations from the Large Magellanic Cloud and the Milky Way

ABSTRACT We predict and compare the distributions and properties of hypervelocity stars (HVSs) ejected from the centres of the Milky Way (MW) and the Large Magellanic Cloud (LMC). In our model, HVSs are ejected at a constant rate – equal in both galaxies – via the Hills mechanism and are propagated in a combined potential, where the LMC orbits the MW on its first infall. By selecting $m \gt 2\, \mathrm{M_\odot }$ HVSs well separated from the Magellanic Clouds and Galactic mid-plane, we identify mock HVSs which would stand out from ordinary stars in the stellar halo in future data releases from the Gaia satellite and the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). We find that in these deep surveys, LMC HVSs will outnumber MW ones by a factor of ∼2.5, as HVSs can more easily escape from the shallower potential of the LMC. At an assumed HVS ejection rate of $10^{-4} \, \mathrm{yr^{-1}}$, HVSs detectable in the final Gaia data release and LSST from the LMC (MW) will number $125_{-12}^{+11}$ ($50_{-8}^{+7}$) and $140_{-11}^{+10}$ ($42_{-7}^{+6}$), respectively. The MW and LMC HVS populations show different kinematics and spatial distributions. While LMC HVSs have more modest total velocities and larger Galactocentric distances clustered around those of the LMC itself, HVSs from the MW show broader distributions, including a prominent high-velocity tail above $500 \, \mathrm{km \ s^{-1}}$ that contains at least half of the stars. These predictions are robust against reasonable variation of the Galactic potential and of the LMC central black hole mass.

An intriguing globular cluster in the Galactic bulge from the VVV survey

Context. Globular clusters (GCs) are the oldest objects known in the Milky Way, so each discovery of a new GC is astrophysically important. In the inner Galactic bulge regions these objects are difficult to find due to extreme crowding and extinction. However, recent near-IR surveys have discovered a number of new bulge GC candidates that need to be further investigated. Aims. Our main objective is to use public data from the Gaia mission, the VISTA Variables in the Via Lactea (VVV) survey, the Two Micron All Sky Survey, and the Wide-field Infrared Survey Explorer to measure the physical parameters of Minni 48, a new candidate globular star cluster located in the inner bulge of the Milky Way at l = 359.35 deg, b = 2.79 deg. The specific goals are to measure its main astrophysical parameters, such as size, proper motions, metallicity, reddening and extinction, distance, total luminosity, and age. Methods. Even though there is a bright foreground star contaminating the field, this cluster appears quite bright in near- and mid-IR images. The size of Minni 48 is derived from the cluster radial density profile, while its reddening and extinction are estimated from optical and near-IR reddening maps. We obtain statistically decontaminated optical and near-IR colour-magnitude diagrams (CMDs) for this cluster. Mean cluster proper motions are measured from Gaia data. The heliocentric cluster distance is determined from both the red clump (RC) and the red giant branch (RGB) tip magnitudes in the near-IR CMD, while the cluster metallicity is estimated from the RGB slope and the fit to theoretical stellar isochrones. Results. The size of this GC is found to be r = 6′±1′, and the reddening and extinction values are E(J − Ks) = 0.60 ± 0.05 mag, AG = 3.23 ± 0.10 mag, and AKs = 0.45 ± 0.05 mag. The resulting mean cluster proper motions are μα = −3.5 ± 0.5 mas yr−1 and μδ = −6.0 ± 0.5 mas yr−1. We also study the RR Lyrae stars recognized in the field, and we argue that they are not members of this GC. The magnitude of the RC in the near-IR CMD yields an accurate distance modulus estimate of (m − M)0 = 14.61 mag, equivalent to a distance D = 8.4 ± 1.0 kpc. Such a distance is consistent with the optical distance estimate, (m − M)0 = 14.67 mag, D = 8.6 ± 1.0 kpc, as well as with the distance estimated using the tip of the RGB, (m − M)0 = 14.45 mag, D = 7.8 ± 1.0 kpc. We also derive a cluster metallicity of [Fe/H] = − 0.20 ± 0.30 dex. Adopting these values of metallicity and distance, a good fit to the PARSEC stellar isochrones is obtained in all CMDs using Age = 10 ± 2 Gyr. The total absolute magnitude of this GC is estimated to be MKs = −9.04 ± 0.66 mag. Conclusions. Based on its position, kinematics, metallicity, and age, we conclude that Minni 48 is a genuine GC, similar to other well-known metal-rich bulge GCs. It is located at a projected galactocentric angular distance of 2.9 deg, equivalent to 0.4 kpc, situating this cluster as one of the closest GCs to the Galactic centre currently known.

Orbits of 152 globular clusters of the MilkyWay galaxy constructed from Gaia DR2

We present orbits and their properties for 152 globular clusters of the Milky Way galaxy obtained using average Gaia DR2 proper motions and other astrometric data from the list of Vasiliev. For orbital integration we have applied the axisymmetric model of the Galactic potential based on the Navarro-Frenk- White dark halo, and modified by Bajkova & Bobylev utilizing circular velocities of Galactic objects in a wide region of Galactocentric distances (up to 200 kpc) from the Bhattacharjee et al. catalog. Based on the analysis of the obtained orbits, we have modified the composition of the subsystems of globular clusters presented in Massari et al.

Influence of the Galactic bar on the kinematics of the disc stars with Gaia EDR3 data

ABSTRACT A model of the Galaxy with the outer ring R1R2 can explain the observed distribution of the radial, VR, and azimuthal, VT, velocity components along the Galactocentric distance, R, derived from the Gaia EDR3 data. We selected stars from the Gaia EDR3 catalogue with reliable parallaxes, proper motions, and line-of-sight velocities lying near the Galactic plane, |z| &amp;lt; 200 pc, and in the sector of the Galactocentric angles |θ| &amp;lt; 15° and calculated the median velocities VR and VT in small bins along the distance R. The distribution of observed velocities appears to have some specific features: the radial velocity VR demonstrates a smooth fall from +5 km s−1 at the distance of R ≈ R0 − 1.5 kpc to −3 km s−1 at R ≈ R0 + 1.0 kpc while the azimuthal velocity VT shows a sharp drop by 7 km s−1 in the distance interval R0 &amp;lt; R &amp;lt; R0 + 1.0 kpc, where R0 is the solar Galactocentric distance. We build a model of the Galaxy including bulge, bar, disc, and halo components, which reproduces the observed specific features of the velocity distribution in the Galactocentric distance interval |R − R0| &amp;lt; 1.5 kpc. The best agreement corresponds to the time 1.8 ± 0.5 Gyr after the start of the simulation. A model of the Galaxy with the bar rotating at the angular velocity of Ωb = 55 ± 3 km s−1 kpc−1, which sets the OLR of the bar at the distance of R0 − 0.5 ± 0.4 kpc, provides the best agreement between the model and observed velocities. The position angle of the bar, θb, corresponding to the best agreement between the model and observed velocities is θb = 45 ± 15°.

Giant star-forming complexes in high-z main-sequence galaxy analogues: the internal structure of clumps in DYNAMO galaxies

ABSTRACT To indirectly study the internal structure of giant clumps in main sequence galaxies at z ∼ 1–3, we target very turbulent and gas-rich local analogues from the DYNAMO sample with the Hubble Space Telescope, over a wavelength range of ∼200–480 nm. We present a catalogue of 58 clumps identified in six DYNAMO galaxies, including the WFC3/UVIS F225W, F336W, and F467M photometry where the (225–336) and (336–467) colours are sensitive to extinction and stellar population age, respectively. We measure the internal colour gradients of clumps themselves to study their age and extinction properties. We find a marked colour trend within individual clumps, where the resolved colour distributions show that clumps generally have bluer (336–467) colours (denoting very young ages) in their centres than at their edges, with little variation in the (225–336) colour associated with extinction. Furthermore, we find that clumps whose colours suggest they are older, are preferentially located closer towards the centres of their galaxies, and we find no young clumps at small galactocentric distances. Both results are consistent with simulations of high-redshift star-forming systems that show clumps form via violent disc instability, and through dynamic processes migrate to the centres of their galaxies to contribute to bulge growth on time-scales of a few 100 Myr, while continually forming stars in their centres. When we compare the DYNAMO clumps to those in these simulations, we find the best agreement with the long-lived clumps.

Study of the Influence of an Evolving Galactic Potential on the Orbital Properties of 152 Globular Clusters with Data from the Gaia EDR3 Catalogue

We have studied the influence of an evolving gravitational potential of the Milky Way Galaxy on the orbital motion of 152 globular clusters with proper motions from the Gaia EDR3 catalogue and mean distances from Baumgardt and Vasiliev (2021). To construct a semicosmological evolving model potential with changing masses and sizes of the Galactic components, we have used the algorithm described in Haghi et al. (2015). The adopted axisymmetric three-component model potential of the Galaxy includes a spherical bulge, a flat Miyamoto--Nagai disk, and a spherical Navarro--Frenk--White dark matter halo.The orbits are integrated backward in time. We compare the orbital parameters of globular clusters derived in static and evolving potentials when integrating the orbits for 5 and 12 Gyr backward. For the first time we have studied the influence of separately a change in the masses and a change in the sizes of the Galactic components. The changes in the masses and sizes of the components are shown to act on the orbital parameters in the opposite way. At small Galactocentric distances this influence is maximally compensated for. The orbits of distant globular clusters and those with a large apocenter distance undergo the biggest changes. We show that on time scales up to $-5$~Gyr the orbits of globular clusters in the case of a potential with both changing masses and changing sizes of the components undergo, on average, minor changes compared to the case of a static potential. These changes fit into the limits of the statistical uncertainties caused by the errors in the data. So, on these time scales the Galactic potential may be deemed static. We provide tables with the orbital parameters of globular clusters derived in both static and evolving potentials.

Internal rotation of Milky Way dwarf spheroidal satellites with Gaia Early Data Release 3

ABSTRACT We present an analysis of the kinematics of 14 satellites of the Milky Way (MW). We use proper motions (PMs) from the Gaia Early Data Release 3 (EDR3) and line-of-sight velocities (vlos) available in the literature to derive the systemic 3D motion of these systems. For six of them, namely the Carina, Draco, Fornax, Sculptor, Sextans, and Ursa Minor dwarf spheroidal galaxies (dSph), we study the internal kinematics projecting the stellar PMs into radial, VR (expansion/contraction), and tangential, VT (rotation), velocity components with respect to the centre of mass. We find significant rotation in the Carina (|VT| = 9.6 ± 4.5 km s−1 ), Fornax (|VT| = 2.8 ± 1.3 km s−1 ), and Sculptor (|VT| = 3.0 ± 1.0 km s−1 ) dSphs. Besides the Sagittarius dSph, these are the first measurements of internal rotation in the plane of the sky in the MW’s classical dSphs. All galaxies except Carina show |VT|/σv &amp;lt; 1. We find that slower rotators tend to show, on average, larger sky-projected ellipticity (as expected for a sample with random viewing angles) and are located at smaller Galactocentric distances (as expected for tidal stirring scenarios in which rotation is transformed into random motions as satellites sink into the parent halo). However, these trends are small and not statistically significant, indicating that rotation has not played a dominant role in shaping the 3D structure of these galaxies. Either tidal stirring had a weak impact on the evolution of these systems or it perturbed them with similar efficiency regardless of their current Galactocentric distance.

A New Estimate of the Best Value for the Solar Galactocentric Distance

Using data from the literature, we made a list of individual estimates of the solar Galactocentric distance, which were performed after 2017 by different methods. These values have not yet been used to calculate the best value of mean $R_0$. For the sample containing 21 estimates, based on the standard approach, we found the weighted mean ${\overline R_0}=8.14$ kpc with the dispersion $\sigma=0.16$ kpc, and using the median statistics, we obtained the estimate $R_0=8.15\pm0.11$ kpc. For practical use, the value $R_0=8.1\pm0.1$ kpc can be recommended.

The Challenge to MOND from Ultra-faint Dwarf Galaxies

Abstract Modified Newtonian dynamics (MOND) at low acceleration has been astonishingly powerful at explaining the rotation curve of galaxies and the relation between the baryonic content of the galaxies and their observed circular velocity, known as the baryonic Tully–Fisher relationship. It is known that MOND fails at explaining the observed velocity dispersion of the ultra-faint dwarf galaxies (UFDs) with the justification that UFDs are more prone to tidal disruption in MOND compared to cold dark matter model. We show that (i) the ratio of tidal to internal acceleration in UFDs is extremely low, (ii) there is no correlation between the deviation of UFDs from MOND’s prediction as a function of tidal susceptibility, and (iii) recent constraints from Gaia proper motion analysis on the orbital parameters of the UFDs exacerbate the challenge to MOND. In particular, Gaia data indicate that Ursa Major I is experiencing a recent infall into the Milky Way’s halo, and its inconsistency with MOND at the 7σ level cannot be attributed to being an early infall satellite. Moreover, the new data from Gaia DR2 show Willman I to have the least eccentric orbit of all UFDs, and its deviation from MOND at the 4σ level cannot be attributed to a highly eccentric orbit as previously suggested. Finally, given that Tuc III is the only UFD observed to show tidal features, Reticulum II and Segue I are two other UFDs that potentially challenge MOND as they have comparable Galactocentric distances to Tuc III while showing no tidal features.

Discovery of a new nearby globular cluster with extreme kinematics located in the extension of a halo stream

Context. We report the discovery of VVV-CL160, a new nearby globular cluster (GC) with extreme kinematics, located in the Galactic plane at l = 10.1477 deg, b = 0.2999 deg. Aims. We aim to characterize the physical properties of this new GC and place it in the context of the Milky Way, exploring its possible connection with the known GC NGC 6544 and with the Hrid halo stream. Methods. VVV-CL160 was originally detected in the VISTA Variables in the Vía Láctea (VVV) survey. We use the proper motions (PMs) from the updated VVV Infrared Astrometric Catalog (VIRAC2) to select GC members and make deep near-infrared color-magnitude diagrams to study the cluster properties. We also fit King models to the decontaminated sample to determine the GC structural parameters. Results. VVV-CL160 has an unusually large PM for a Galactic GC as measured with VIRAC2 and Gaia EDR3: μα cos (δ) = −2.3 ± 0.1 mas yr−1 and μδ = −16.8 ± 0.1 mas yr−1. The kinematics are similar to those of the known GC NGC 6544 and the Hrid halo stream. We estimate a reddening of E(J − K) = 1.95 mag and an extinction of AK = 1.40 mag for VVV-CL160. We also measure a distance modulus of (m − M) = 13.01 mag and a distance of D⊙ = 4.0 ± 0.5 kpc. This places the GC at z = 29 pc above the Galactic plane and at a galactocentric distance of RG = 4.2 kpc. We also measure a metallicity of [Fe/H] = −1.4 ± 0.2 dex for an adopted age of t = 12 Gyr; King model fits of the PM-decontaminated sample reveal a concentrated GC, with core radius rc = 22.8″ and tidal radius rt = 50′. We also estimate the absolute magnitude in the near-infrared of MK = −7.6 mag, equivalent to an optical absolute magnitude of MV = −5.1 mag. We also explore the possible association of this new GC with other GCs and halo streams. Conclusions. Based on the locations and kinematics, we suggest that VVV-CL160, along with NGC 6544, may be associated with the extension of the Hrid halo stream.

Mild radial variations of the stellar IMF in the bulge of M31

ABSTRACT Using new, homogeneous, long-slit spectroscopy in the wavelength range from ∼0.35 to $\sim 1 \, \mu$m, we study radial gradients of optical and near-infrared (NIR) initial mass function (IMF)-sensitive features along the major axis of the bulge of M31, out to a galactocentric distance of ∼200 arcsec (∼800 pc). Based on state-of-the-art stellar population synthesis models with varying Na abundance ratio, we fit a number of spectral indices, from different chemical species (including TiO’s, Ca, and Na indices), to constrain the low-mass (≲0.5 M⊙) end slope (i.e. the fraction of low-mass stars) of the stellar IMF, as a function of galactocentric distance. Outside a radial distance of ∼10 arcsec, we infer an IMF similar to a Milky Way-like distribution, while at small galactocentric distances, an IMF radial gradient is detected, with a mildly bottom-heavy IMF in the few inner arcsec. We are able to fit Na features (both NaD and $\rm Na\,{\small I}8190$), without requiring extremely high Na abundance ratios. $\rm [Na/Fe]$ is ∼0.4 dex for most of the bulge, rising up to ∼0.6 dex in the innermost radial bins. Our results imply an overall, luminosity-weighted, IMF and mass-to-light ratio for the M31 bulge, consistent with those for a Milky Way-like distribution, in contrast to results obtained, in general, for most massive early-type galaxies.

Sharpening quasar absorption lines with ESPRESSO. Temperature of warm gas at z ~ 2, constraints on the Mg isotopic ratio, and structure of cold gas at z ~ 0.5

Aims. We aim to study several key physical properties of quasar absorption-line systems that are subtly encoded in their absorption profiles and have not yet been thoroughly investigated or constrained. Methods. We analysed a high-resolution (R = 140 000) spectrum of the bright quasar HE 0001−2340 (zem = 2.26) obtained with ESPRESSO, which was recently installed at the Very Large Telescope. We analysed three systems at z = 0.45, z = 1.65, and z = 2.19 using multiple-component Voigt-profile fitting. We also compared our spectrum with those obtained with VLT/UVES, covering a total period of 17 years. Results. We disentangle turbulent and thermal broadening in many components spread over about 400 km s−1 in the z ≈ 2.19 sub-damped Lyman-α system. We derive an average temperature of 16 000 ± 1300 K, which is about twice the canonical value of the warm neutral medium in the Galactic interstellar medium (ISM). A comparison with other high-z, low-metallicity absorbers reveals an anti-correlation between gas temperature and total H I column density. Although requiring confirmation, this could be the first observational evidence of a thermal decrease with galactocentric distance; in other words, we may be witnessing a thermal transition between the circumgalactic medium and the cooler ISM. We revisit the Mg isotopic ratios at z = 0.45 and z = 1.65 and constrain them to be ξ = (26Mg + 25Mg)/24Mg &lt; 0.6 and &lt; 1.4 in these two systems, respectively. These values are consistent with the standard solar ratio; that is, we do not confirm strong enhancement of heavy isotopes previously inferred from UVES data. Finally, we confirm the partial coverage of the quasar emission-line region by a Fe I-bearing cloud in the z = 0.45 system and present evidence for velocity substructure of the gas that has Doppler parameters of the order of only ∼0.3 km s−1. This agrees well with the low kinetic temperature of T ∼ 100 K inferred from modelling of the gas physical conditions. Conslusions. This work demonstrates the unique insight provided by high-fidelity, high-resolution optical spectrographs on large telescopes when used to investigate the thermal state of the gas in and around galaxies as well as its spatial and velocity structure on small scales, and to constrain the associated stellar nucleosynthetic history.

Peculiarities of the chemical abundance distribution in galaxies NGC 3963 and NGC 7292

ABSTRACT Spectroscopic observations of 32 H ii regions in the spiral galaxy NGC 3963 and the barred irregular galaxy NGC 7292 were carried out with the 2.5-m telescope of the Caucasus Mountain Observatory of the Sternberg Astronomical Institute using the Transient Double-beam Spectrograph with a dispersion of ≈1 Å pixel−1 and a spectral resolution of ≈3 Å. These observations were used to estimate the oxygen and nitrogen abundances and the electron temperatures in H ii regions through modern strong-line methods. In general, the galaxies have oxygen and nitrogen abundances typical of stellar systems with similar luminosities, sizes, and morphology. However, we have found some peculiarities in chemical abundance distributions in both galaxies. The distorted outer segment of the southern arm of NGC 3963 shows an excess oxygen and nitrogen abundances. Chemical elements abundances in NGC 7292 are constant and do not depend on the galactocentric distance. These peculiarities can be explained in terms of external gas accretion in the case of NGC 3963 and major merging for NGC 7292.

Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz

Context. The Andromeda galaxy is the best-known large galaxy besides our own Milky Way. Several images and studies exist at all wavelengths from radio to hard X-ray. Nevertheless, only a few observations are available in the microwave range where its average radio emission reaches the minimum. Aims. In this paper, we want to study the radio morphology of the galaxy, decouple thermal from nonthermal emission, and extract the star formation rate. We also aim to derive a complete catalog of radio sources for the mapped patch of sky. Methods. We observed the Andromeda galaxy with the Sardinia Radio Telescope at 6.6 GHz with very high sensitivity and angular resolution, and an unprecedented sky coverage. Results. Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, we confirm that, globally, the spectral index is ∼0.7−0.8, while in the star forming regions it decreases to ∼0.5. By disentangling (gas) thermal and nonthermal emission, we find that at 6.6 GHz, thermal emission follows the distribution of HII regions around the ring. Nonthermal emission within the ring appears smoother and more uniform than thermal emission because of diffusion of the cosmic ray electrons away from their birthplaces. This causes the magnetic fields to appear almost constant in intensity. Furthermore, we calculated a map of the star formation rate based on the map of thermal emission. Integrating within a radius of Rmax = 15 kpc, we obtained a total star formation rate of 0.19 ± 0.01 M⊙ yr−1 in agreement with previous results in the literature. Finally, we correlated our radio data with infrared images of the Andromeda galaxy. We find an unexpectedly high correlation between nonthermal and mid-infrared data in the central region, with a correlation parameter r = 0.93. Finally, by computing the logarithmic 24μm/21cm ratio q24 μm, we find a decreasing trend with increasing galactocentric distance and an increasing dispersion. The logarithmic far-infrared-to-radio ratio is found to be 2.41 ± 0.04.

Investigating Clumpy Galaxies in the Sloan Digital Sky Survey Stripe 82 Using the Galaxy Zoo

Abstract Giant, star-forming clumps are a common feature prevalent among high-redshift star-forming galaxies and play a critical role in shaping their chaotic morphologies and yet, their nature and role in galaxy evolution remains to be fully understood. A majority of the effort to study clumps has been focused at high redshifts, and local clump studies have often suffered from small sample sizes. In this work, we present an analysis of clump properties in the local universe, and for the first time, performed with a statistically significant sample. With the help of the citizen science-powered Galaxy Zoo: Hubble project, we select a sample of 92 z &lt; 0.06 clumpy galaxies in Sloan Digital Sky Survey Stripe 82 galaxies. Within this sample, we identify 543 clumps using a contrast-based image analysis algorithm and perform photometry as well as estimate their stellar population properties. The overall properties of our z &lt; 0.06 clump sample are comparable to the high-redshift clumps. However, contrary to the high-redshift studies, we find no evidence of a gradient in clump ages or masses as a function of their galactocentric distances. Our results challenge the inward migration scenario for clump evolution for the local universe, potentially suggesting a larger contribution of ex situ clumps and/or longer clump migration timescales.

Determination of the escape velocity of the Milky Way using a halo sample selected based on proper motion

Context. The Gaia mission has provided the largest catalogue ever of sources with tangential velocity information. However, it is difficult to use this catalogue for dynamical studies because most of the stars lack line-of-sight velocity measurements. Recently, we presented a selection of ∼107 halo stars with accurate distances that were selected based on their photometry and proper motions. Aims. Using this sample, we model the tail of the velocity distribution in the stellar halo locally and as a function of distance. Our goal is to measure the escape velocity, and based on this, to constrain the mass of our Galaxy. Methods. We fitted the tail of the velocity distribution with a power-law distribution, a commonly used approach that has long been established. For the first time, we used tangential velocities that were accurately measured for an unprecedented number of halo stars to estimate the escape velocity. Results. In the solar neighbourhood, we obtain a very precise estimate of the escape velocity, which is 497−8+8 km s−1. This estimate is most likely biased low, our best guess is by 10%. As a result, the true escape velocity is most likely closer to 550 km s−1. The escape velocity directly constrains the total mass of the Milky Way. To find the best-fitting halo mass and concentration parameter, we adjusted the dark (spherical Navarro-Frenk-White) halo of a realistic Milky Way potential while keeping the circular velocity at the solar radius fixed at vc(R⊙) = 232.8 km s−1. The resulting halo parameters are M200+10% = 1.11−0.07+0.08 · 1012 M⊙, and the concentration parameter is c+10% = 11.8−0.3+0.3, where we use the explicit notation to indicate that they are corrected for the 10% bias. The slope of the escape velocity with galactocentric distance is as expected in the inner Galaxy based on Milky Way models. Curiously, we find a disagreement beyond the solar radius where the estimated escape velocity is higher than at the solar radius. This result is likely an effect of a change in the shape of the velocity distribution and could be related to the presence of velocity clumps. A tentative analysis of the escape velocity as a function of (R, z) shows that the slope is shallower than expected for a spherical halo when standard values are used for the characteristic parameters describing the galactic disc.