High-velocity stars in the Gal. halo from LAMOST & Gaia
Short Name:
J/ApJS/252/3
Date:
03 Mar 2022 11:59:54
Publisher:
CDS
Description:
In this paper, we report 591 high-velocity star candidates (HiVelSCs) selected from over 10 million spectra of Data Release 7 (DR7) of the Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia data release, with three-dimensional velocities in the Galactic rest frame larger than 445km/s. We show that at least 43 HiVelSCs are unbound to the Galaxy with escape probabilities larger than 50%, and this number decreases to eight if the possible parallax zero-point error is corrected. Most of these HiVelSCs are metal-poor and slightly {alpha}-enhanced inner halo stars. Only 14% of them have [Fe/H]>-1, which may be the metal-rich "in situ" stars in the halo formed in the initial collapse of the Milky Way or metal-rich stars formed in the disk or bulge but kinematically heated. The low ratio of 14% implies that the bulk of the stellar halo was formed from the accretion and tidal disruption of satellite galaxies. In addition, HiVelSCs on retrograde orbits have slightly lower metallicities on average compared with those on prograde orbits; meanwhile, metal-poor HiVelSCs with [Fe/H]{<}-1 have an even faster mean retrograde velocity compared with metal-rich HiVelSCs. To investigate the origins of HiVelSCs, we perform orbit integrations and divide them into four types, i.e., hypervelocity stars, hyper-runaway stars, runaway stars and fast halo stars.
The High-z Supernova Search Team has discovered and observed eight new supernovae in the redshift interval z=0.3-1.2. These independent observations, analyzed by similar but distinct methods, confirm the results of Riess (1998AJ....116.1009R) and Perlmutter (1999ApJ...517..565P) and coworkers that supernova luminosity distances imply an accelerating universe.
Our aims is to create a catalog of images of HII galaxies and their individual star-formation regions in order to study the distribution of the gas emission and their underlying stellar continuum. We have used H{beta} narrow-band images of 43 selected HII galaxies obtained at ESO NTT 3.58m telescope using the SUSI2 camera. Surface photometric H{beta} fluxes and equivalent widths for all objects and star-formation regions are presented in this catalog.
From a new mosaic image in the H-{alpha} line of the complete disc of the spiral galaxy M100, a catalogue is composed listing 1948 individual HII regions. For each HII region, the catalogue gives its position relative to the centre of the galaxy, its deprojected distance to the centre, its radius, and its calibrated luminosity. An indication is included as to whether the HII region is located in the arms, between them, or in the circumnuclear star-forming region. The H-{alpha} image of M100 was obtained during two observing runs with the 4.2m WHT on La Palma, using the TAURUS camera in imaging mode. Since the field of view in this setup is limited by the filter size to around 5' diameter, four fields of the galaxy were imaged, two (eastern half of M100) during the night of 27 May 1991, and two (western half) during the night of 14 March 1992. Narrow band redshifted H-{alpha} filters with width of 15{AA} were used for the observations, centred at 6601{AA} for the H-{alpha} line observations (redshifted using the galaxy's systemic velocity of 1571km/s) and at 6577{AA} and 6565{AA} for the continuum. Exposure times were 1200 seconds for both the on-line and the continuum image on the first night, and 2x900 seconds on the second night. An EEV CCD chip was used during both observing runs, with a projected pixel size of 0.279arcsec, and a size of 1180x1280 pixels.
HII region catalogue of the barred galaxy NGC 7479. The catalogue contains the position, mean radius and the calibrated H-alpha luminosity of each HII region. The HII region catalogue has been composed using a new semi-automated technique applied to a high quality continuum-subtracted H-alpha image of the galaxy obtained from observations through the TAURUS camera in imaging mode with the 4.2m WHT on La Palma. An EEV CCD 7 detector was used with a projected pixel size of 0.279". More details about the reduction and the catalogue production procedures are given in the paper.
From an H-{alpha} image of the spiral barred galaxy NGC 3359, an HII region catalogue is composed. The catalogue comprises the position, mean radius and the calibrated H-alpha luminosity of each HII region. The observations of the image of NGC 3359 in H-{alpha} were carried out in February 1996, on the Isaac Newton telescope in La Palma in the context of the BARS international time project of the Canary Islands Observatories. A CCD TeK-7 detector was used with a projected pixel size of 0.59".
We present long-slit optical spectroscopy of 67 HII regions in 21 dwarf irregular galaxies to investigate the enrichment of oxygen, nitrogen, neon, sulfur, and argon in low-mass galaxies. Oxygen abundances are obtained via direct detection of the temperature-sensitive emission lines for 25 HII regions; for the remainder of the sample, oxygen abundances are estimated from strong-line calibrations. The direct abundance determinations are compared to the strong-line abundance calibrations of both McGaugh (1991ApJ...380..140M) and Pilyugin (2000A&A...362..325P).
Properties of HII region populations in spiral galaxies provide important information about the physics of star formation as well as regional turbulent motions in the interstellar medium. We present a set of 376 photometrically calibrated HII regions in the nearby late-type spiral galaxy NGC 628. We have studies the mean velocity and velocity dispersion for the H-alpha-emitting gas in NGC 628 and found with widely distributed star formation in the disc plane. The H-alpha emission from the HII regions dominates any emission from the diffuse component in this galaxy.
The galaxy NGC6384 has been observed with an IPCS through H{alpha} and [NII] narrow-band interference filters for direct imagery with the 2.6-m Byurakan telescope. We studied the main physical parameters of the identified 98 HII regions, their diameter and luminosity functions, as well the [NII]/H{alpha} ratio distribution.
Using the data from the NOAO Local Group Survey, we have measured the H{alpha} fluxes of 291 nebulae associated with 21 of the van den Bergh OB associations. We have combined these data together with six-color HST WFPC2 photometry, in order to identify the most UV-bright stars in the region. The simple purpose of this article is to explore the spatial relationships between these components.
