Galactic Quadrant Research Articles

Towards a census of the Galactic anticentre star clusters – III. Tracing the spiral structure in the outer disc

In this paper we investigate the nature of 27 star cluster candidates, most of them projected towards the Galactic anticentre. We derive fundamental parameters for 20 confirmed clusters, among these 7 are new identifications. Four of the remaining are uncertain cases that require deeper photometry to establish their nature, and 4 are probably field fluctuations. In addition, we provide a partial census of the open clusters towards the Galactic anticentre. We also include in this study some interesting objects outside the anticentre region, in the second and third Galactic quadrants, mainly in the Perseus and Outer arms. These clusters confirm the extension of the Outer arm along the third quadrant. We also point out that the embedded cluster FSR 486, at a distance of 7.2 +/- 1.3 kpc from de Sun, is projected on the line of sight of the Local Group irregular dwarf galaxy IC 10. Thus, part of the unusual properties of IC 10 may be explained by a Galactic contamination. We point out the importance of embedded clusters in tracing the spiral structure.

Nearby kinematic wiggles from LEGUE

In its first two observing seasons, the LEGUE (LAMOST Experiment for Galactic Understanding and Exploration; Deng et al., Zhao et al. 2012) survey has obtained ~1.7 million science-quality spectra. We apply corrections to the PPMXL proper motions (PMs; Roeser et al. 2010) as a function of position, as determined from the measured PMs of extragalactic objects discovered in LAMOST spectra (see Fig. 1, left and center panels). LAMOST radial velocities and corrected PMs are used to derive 3D space velocities for ~480,000 F-stars (assuming MV=4 to derive distances). The right panel of Fig. 1 shows the radial component of Galactic cylindrical velocities (VR) for stars between 7.8<RGC<9.8 kpc (with R⊙,GC=7.8 kpc) as a function of height (Z) and angle (θ) from the Galactic X-axis. Each dot represents the average position of stars in a 200x200 pc box, color-coded by the mean VR of those stars. Assuming circularrotation, VR should be zero. This is true on average for θ>0° (3rd Galactic quadrant), but not for θ<0°. The velocities are also asymmetric across the Galactic plane for θ<0° (2nd quadrant), with most positions 〈 VR 〉> 0 above the disk (radially outward), and 〈 VR 〉 < 0 below the disk. Similar structure to this apparent “shearing” motion has been seen in RAVE (e.g., Williams et al. 2013; Siebert et al. 2012), and SDSS (Widrow et al. 2012).

THE DISTRIBUTION OF INFRARED DARK CLOUDS IN THE FIRST GALACTIC QUADRANT

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetimes, if IRDCs host the earliest stages of high-mass star formation then these cold, dense molecular clouds should be located in or near the spiral arms in the Galaxy. The Galactic distribution of a large sample of IRDCs should therefore provide information on Galactic structure. Moreover, determination of distances enables mass and luminosity calculations. We have observed a large sample of IRDC candidates in the first Galactic quadrant in the dense gas tracer CS (2-1) using the Mopra telescope in order to determine kinematic distances from the molecular line velocities. We find that the IRDCs are concentrated around a Galactocentric distance of ~4.5 kpc, agreeing with the results of Simon et al. This distribution is consistent with the location of the Scutum-Centaurus spiral arm. The group of IRDCs near the Sun in the first quadrant detected in 13CO (1-0) in Simon et al. is not detected in the CS data. This discrepancy arises from the differences in the critical densities between the 13CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13CO (1-0) traces a population of IRDCs with lower column densities and lower 1.1 mm flux densities in addition to more dense IRDCs detected in CS. Masses of the first quadrant IRDCs are calculated from 13CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc.

Homogeneous Distances to Several Stellar Groups in the Second Galactic Quadrant

A photometric study in the uvbyβ system of a 20° × 20° field in direction of the Cas OB6 and Per OB1 associations is presented. All currently available uvbyβ photoelectric data are used to obtain homogeneous color excesses and distances of nearly 230 stars of spectral types O-B9. The double cluster h & χ Per, NGC 663 and NGC 1502 are well represented in our sample. The sample also contains the brightest members of the young open clusters IC 1805, IC 1848, St 2, St 7 and ASCC9. We found that, within the errors, h & χ Per, NGC 663, IC 1805, IC 1848 and ASCC9, together with the Per OB1 association are located at very similar distance moduli between 11.0 to 11.3 mag. Our results indicate that the distance spread among these objects is less than the previously estimated, suggested that they could represent starforming complexes located at the same distance.

Observations and discoveries of supernova remnants with GMRT

AbstractWe have measured HI absorption distance to the youngest Galactic supernova remnant G1.9+0.3. Absorption by known anomalous velocity features near the Galactic centre (GC) puts a lower limit on its distance from Sun as 10 kpc, 2 kpc further away from the GC. We have found a small diameter (1.6′) shell like structure G354.4+0.0, that shows polarised emission in the NVSS. Based on its morphology, angular size, HI distance and its spectrum between 1.4 GHz and 330 MHz, it is perhaps the second youngest SNR in the Galaxy that is expanding in a dense environment of an HII region surrounding it. Our pilot observation of the inner Galactic 4th quadrant within 337° < l < 354° with a fixed Galactic latitude of 0.37° has confirmed G345.1−0.2 as an SNR.

Supernova Remnants in the UWISH2 and UWIFE Surveys

AbstractWe report the preliminary results for the detection of H2 and [Fe II] line features around the Galactic supernova remnants (SNRs) from the UWISH2 and UWIFE surveys that cover the first galactic quadrant of 7°<l<65° and |b|<1.3°. By this time, we have found a total of 17 H2-emitting and 14 [Fe II]-emitting SNRs in the coverage, and more than a half of them are detected in both H2 and [Fe II] emissions, which implies that the environment of these SNRs might be complex and composed of multi-phase medium. In this paper, we present our identification strategy and some preliminary results including H2 and [Fe II] luminosity distributions.

Five old open clusters more in the outer Galactic disc

New photometric material is presented for 6 outer disk supposedly old, Galact ic star clusters: Berkeley 76, Haffner 4, Ruprecht 10, Haffner 7, Haffner 11, and Haffner 15, that are projected against the rich and complex Canis Major overde nsity at $225^o \leq l \leq 248^o $, $-7^o \leq b \leq -2^o$. This CCD data-set, in the UBVI pass-bands, is used to derive their fundamental parameters, in particular age and distance. Four of the program clusters turn out to be older than 1 Gyr. This fact makes them ideal targets for future spectroscopic campaigns aiming at deriving their metal abundances. This, in turn, contributes to increase the number of well-studied outer disk o ld open clusters. Only Haffner 15, previously considered an old cluster, is found to be a young, significantly reddened cluster, member of the Perseus arm in the third Galactic quadrant. As for Haffner~4, we suggest an age of about half a Gyr. The most interesting result we found is that Berkeley~76 is probably located at more than 17 kpc from the Galactic center, and therefore is among the most peripherical old open clusters so far detected. Besides, for Ruprecht~10 and Haffner~7, which were never studied before, we pr opose ages larger than 1 Gyr. All the old clusters of this sample are scarcely populated and show evidence o f tidal interaction with the Milky Way, and are therefore most probably in advanced st ages of dynamical dissolution.

THERE ARE NO STARLESS MASSIVE PROTO-CLUSTERS IN THE FIRST QUADRANT OF THE GALAXY

We search the lambda = 1.1 mm Bolocam Galactic Plane Survey for clumps containing sufficient mass to form ~10^4 M\odot star clusters. 18 candidate massive proto-clusters are identified in the first Galactic quadrant outside of the central kiloparsec. This sample is complete to clumps with mass M(clump) > 10^4 M_sun and radius r < 2.5 pc. The overall Galactic massive cluster formation rate is CFR(M_cluster > 10^4) ~ 5 Myr^-1, which is in agreement with the rates inferred from Galactic open clusters and M31 massive clusters. We find that all massive proto-clusters in the first quadrant are actively forming massive stars and place an upper limit of t_starless < 0.5 Myr on the lifetime of the starless phase of massive cluster formation. If massive clusters go through a starless phase with all of their mass in a single clump, the lifetime of this phase is very short.

THE GALFA-H i COMPACT CLOUD CATALOG

ABSTRACT We present a catalog of 1964 isolated, compact neutral hydrogen clouds from the Galactic Arecibo L-Band Feed Array Survey Data Release One. The clouds were identified by a custom machine-vision algorithm utilizing the difference of Gaussian kernels to search for clouds smaller than 20′. The clouds have velocities typically between |V LSR| =20 and 400 km s−1, line widths of 2.5–35 km s−1, and column densities ranging from 1 to 35 × 1018 cm−2. The distances to the clouds in this catalog may cover several orders of magnitude, so the masses may range from less than a solar mass for clouds within the Galactic disk, to greater than 104 M ☉ for high-velocity clouds (HVCs) at the tip of the Magellanic Stream. To search for trends, we separate the catalog into five populations based on position, velocity, and line width: HVCs; galaxy candidates; cold low-velocity clouds (LVCs); warm, low positive-velocity clouds in the third Galactic quadrant; and the remaining warm LVCs. The observed HVCs are found to be associated with previously identified HVC complexes. We do not observe a large population of isolated clouds at high velocities as some models predict. We see evidence for distinct histories at low velocities in detecting populations of clouds corotating with the Galactic disk and a set of clouds that is not corotating.

THE GALACTIC PLANE INFRARED POLARIZATION SURVEY (GPIPS): DATA RELEASE 1

The Galactic Plane Infrared Polarization Survey (GPIPS) covers 76 deg{sup 2} of the first Galactic quadrant midplane, 18 Degree-Sign {<=} l {<=} 56 Degree-Sign and -1 Degree-Sign {<=} b {<=} +1 Degree-Sign , in H-band (1.6 {mu}m) linear polarimetry to reveal the plane-of-the-sky orientation of the magnetic field in diffuse and denser atomic and molecular clouds. The Survey consists of 3237 overlapping 10 Multiplication-Sign 10 arcmin fields observed using the Mimir instrument on the 1.8 m Perkins telescope. Here, the first community release of GPIPS data for 559 fields (17% of the Survey) is announced and basic characteristics are described. Data products consist of H-band stellar photometry and polarimetry as well as combined images. The formats and contents of the products are described and quality cuts are explored to provide insight into opportunities and limitations of the data. The Survey probes to distances as far as the Galactic bulge, revealing magnetic field properties that correlate with spiral arms and also show significant small-scale structure. The polarizations are classified into three 'usage' samples, based on stellar brightness and polarimetric uncertainty. The brightest, lowest uncertainty polarizations are suitable for individual use and direct magnetic field mapping. The next two fainter samples aremore » useful, once averaged, for probing magnetic fields to greater distances, albeit with lower resolution. Based on this release, the full GPIPS data set will number about 5.6 million stars, with more than 1 million in the high-quality sample. This increases, by many orders of magnitude, the number of polarimetric probes of the Milky Way's magnetic field.« less

DISTANCES TO DARK CLOUDS: COMPARING EXTINCTION DISTANCES TO MASER PARALLAX DISTANCES

We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near infrared (2MASS and UKIDSS) surveys. VLBI parallax measurements of masers around massive young stars provide the most direct and bias-free measurement of the distance to these dark clouds. We compare the extinction distance estimates to these maser parallax distances. We also compare these distances to kinematic distances, including recent re-calibrations of the Galactic rotation curve. The extinction distance methods agree with the maser parallax distances (within the errors) between 66% and 100% of the time (depending on method and input survey) and between 85% and 100% of the time outside of the crowded Galactic center. Although the sample size is small, extinction distance methods reproduce maser parallax distances better than kinematic distances; furthermore, extinction distance methods do not suffer from the kinematic distance ambiguity. This validation gives us confidence that these extinction methods may be extended to additional dark clouds where maser parallaxes are not available.

EXPLORING HALO SUBSTRUCTURE WITH GIANT STARS: SUBSTRUCTURE IN THE LOCAL HALO AS SEEN IN THE GRID GIANT STAR SURVEY INCLUDING EXTENDED TIDAL DEBRIS FROM ωCENTAURI

We present the latitude-normalized radial velocity (vb) distribution of 3318 subsolar metallicity, V<13.5 stars from the Grid Giant Star Survey (GGSS) in Southern Hemisphere fields. The sample includes giants mostly within ~5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde "cloud" of stars at l~285 deg and extended, retrograde "streams" visible as relatively tight l-vb sequences. One sequence in the fourth Galactic quadrant lies within the l-vb space expected to contain tidal debris from the "star cluster" wCentauri. Not only does wCen lie precisely in this l-vb sequence, but the positions and vb of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with wCen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the wCen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in wCen. The newly discovered wCen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests wCen is a dominant contributor of retrograde giant stars in the inner Galaxy.

Searching for the Perseus arm in the anticenter direction

Here we propose a two step approach to provide new insights on the outer spiral arm pattern of the Milky Way: 1) a Stromgren photometric survey to map the stellar space density of moderate young type stars tracing the Perseus arm, and 2) a spectroscopic survey to determine the velocity perturbation due to the density wave through accurate Radial Velocity (RV, hereafter). First reduced preliminary data suggest an overdensity around 1.5–2 kpc, probably associated with the Perseus arm. Results from this project will present a significant step towards mapping the spiral structure between the second and the third galactic quadrant, where the determination of kinematic distances is not possible.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. IX. THE OUTER ARM IN THE FIRST QUADRANT

We report a trigonometric parallax measurement with the Very Long Baseline Array for the water maser in the distant high-mass star-forming region G75.30+1.32. This source has a heliocentric distance of 9.25+-0.45 kpc, which places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above the Galactic plane and is associated with a substantial HI enhancement at the border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc, G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped toward the North Galactic Pole. While the star-forming region has an instantaneous Galactic orbit that is nearly circular, it displays a significant motion of 18 km/s toward the Galactic plane. The present results, when combined with two previous maser studies in the Outer arm, yield a pitch angle of about 12 degrees for a large section of the arm extending from the first quadrant to the third.

The Ara OB1a association

Context: The Ara OB1a association is a nearby complex in the fourth Galactic quadrant where a number of young/embedded star clusters are projected close to more evolved, intermediate age clusters. It is also rich in interstellar matter, and contains evidence of the interplay between massive stars and their surrounding medium, such as the rim HII region NGC 6188. Aims: We provide robust estimates of the fundamental parameters (age and distance) of the two most prominent stellar clusters, NGC 6167 and NGC 6193, that may be used as a basis for studing the star formation history of the region. Methods: The study is based on a photometric optical survey (UBVIHa) of NGC 6167 and NGC 6193 and their nearby field, complemented with public data from 2MASS-VVV, UCAC3, and IRAC-Spitzer in this region. Results: We produce a uniform photometric catalogue and estimate more robustly the fundamental parameters of NGC 6167 and NGC 6193, in addition to the IRAS 16375-4854 source. As a consequence, all of them are located at approximately the same distance from the Sun in the Sagittarius-Carina Galactic arm. However, the ages we estimate differ widely: NGC 6167 is found to be an intermediate-age cluster (20-30 Myr), NGC 6193 a very young one (1-5 Myr) with PMS, H? emitters and class II objects, and the IRAS 16375-4854 source is the youngest of the three containing several YSOs. Conclusions: These results support a picture in which Ara OB1a is a region where star formation has proceeded for several tens of Myr until the present. The difference in the ages of the different stellar groups can be interpreted as a consequence of a triggered star formation process. In the specific case of NGC 6193, we find evidence of possible non-coeval star formation.

A MOLECULAR SPIRAL ARM IN THE FAR OUTER GALAXY

We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center. After tracing the arm in existing 21 cm surveys, we searched for molecular gas using the CfA 1.2 meter telescope and detected CO at 10 of 220 positions. The detections are distributed along the arm from l = 13 deg, v = -21 km/s to l = 55 deg, v = -84 km/s and coincide with most of the main H I concentrations. One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 Mo. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way. The new arm appears to be the continuation of the Scutum-Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.

FIRST CHEMICAL ANALYSIS OF STARS IN THE TRIANGULUM--ANDROMEDA STAR CLOUD

We undertake the first high resolution spectroscopic study of the Triangulum-Andromeda (TriAnd) star cloud --- an extended, mid-latitude Milky Way halo substructure about 20 kpc away in the second Galactic quadrant --- through six M giant star candidates selected to be both spatially and dynamically associated with this system. The abundance patterns of [Ti/Fe], [Y/Fe] and [La/Fe] as a function of [Fe/H] for these stars support TriAnd as having an origin in a dwarf galaxy with a chemical enrichment history somewhat similar to that of the Sagittarius dwarf spheroidal (dSph) galaxy. We also investigate the previously proposed hypothesis that TriAnd is an outlying, dynamically older piece of the Monoceros Stream (also known as the Galactic Anticenter Stellar Structure, "GASS") under the assumption that both features come from the tidal disruption of the same accreted Milky Way satellite and find that net differences in the above abundance patterns between the TriAnd and GASS stars studied suggest that these two systems are independent and unrelated.

UWISH2 - the UKIRT Widefield Infrared Survey for H2

We present the goals and preliminary results of an unbiased, near-infrared, narrow-band imaging survey of the First Galactic Quadrant (10deg<l<65deg ; -1.3deg<b<+1.3deg). This area includes most of the Giant Molecular Clouds and massive star forming regions in the northern hemisphere. The survey is centred on the 1-0S(1) ro-vibrational line of H2, a proven tracer of hot, dense molecular gas in star-forming regions, around evolved stars, and in supernova remnants. The observations complement existing and upcoming photometric surveys (Spitzer-GLIMPSE, UKIDSS-GPS, JCMT-JPS, AKARI, Herschel Hi-GAL, etc.), though we probe a dynamically active component of star formation not covered by these broad-band surveys. Our narrow-band survey is currently more than 60% complete. The median seeing in our images is 0.73arcsec. The images have a 5sigma detection limit of point sources of K=18mag and the surface brightness limit is 10^-19Wm^-2arcsec^-2 when averaged over our typical seeing. Jets and outflows from both low and high mass Young Stellar Objects are revealed, as are new Planetary Nebulae and - via a comparison with earlier K-band observations acquired as part of the UKIDSS GPS - numerous variable stars. With their superior spatial resolution, the UWISH2 data also have the potential to reveal the true nature of many of the Extended Green Objects found in the GLIMPSE survey.

CONSTRAINTS ON THE COSMIC-RAY DENSITY GRADIENT BEYOND THE SOLAR CIRCLE FROMFERMIγ-RAY OBSERVATIONS OF THE THIRD GALACTIC QUADRANT

We report an analysis of the interstellar $\gamma$-ray emission in the third Galactic quadrant measured by the {Fermi} Large Area Telescope. The window encompassing the Galactic plane from longitude $210\arcdeg$ to $250\arcdeg$ has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the $\gamma$-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the \HI\ column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a cosmic-ray propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, $X_{\rm CO} = N({\rm H_{2}})/W_{\rm CO}$, is found to be $(2.08 \pm 0.11) \times 10^{20} {\rm cm^{-2} (K km s^{-1})^{-1}}$ in the Local-arm clouds and is not significantly sensitive to the choice of \HI\ spin temperature. No significant variations are found for clouds in the interarm region.

MILKY WAY DISK-HALO TRANSITION IN H I: PROPERTIES OF THE CLOUD POPULATION

Using 21cm HI observations from the Parkes Radio Telescope's Galactic All-Sky Survey, we measure 255 HI clouds in the lower Galactic halo that are located near the tangent points at 16.9 < l < 35.3 degrees and |b| < 20 degrees. The clouds have a median mass of 700 Msun and a median distance from the Galactic plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a region of the fourth quadrant (QIV) studied previously using the same data set and measurement criteria. The properties of the individual clouds in the two quadrants are quite similar suggesting that they belong to the same population, and both populations have a line of sight cloud-cloud velocity dispersion of sigma_cc ~ 16 km/s. However, there are three times as many disk-halo clouds at the QI tangent points and their scale height, at h=800 pc, is twice as large as in QIV. Thus the observed line of sight random cloud motions are not connected to the cloud scale height or its variation around the Galaxy. The surface density of clouds is nearly constant over the QI tangent point region but is peaked near R~4 kpc in QIV. We ascribe all of these differences to the coincidental location of the QI region at the tip of the Milky Way's bar, where it merges with a major spiral arm. The QIV tangent point region, in contrast, covers only a segment of a minor spiral arm. The disk-halo HI cloud population is thus likely tied to and driven by large-scale star formation processes, possibly through the mechanism of supershells and feedback.