Field Of NGC Research Articles

Early Results from SPARO: Instrument Characterization and Polarimetry of NGC 6334

ABSTRACTThe Submillimeter Polarimeter for Antarctic Remote Observations (SPARO) employs two nine‐element arrays of 3He‐cooled bolometers to measure linear polarization at λ = 450 μm. It is operated at the focal plane of the Viper telescope, located at the Amundsen‐Scott South Pole Station. SPARO obtains better sensitivity to degree‐scale polarized submillimeter emission than can be currently achieved with any other experiment. We describe the design of SPARO’s optics, detectors, and electronics. We also review the design of the SPARO cryostat, which has already been discussed in a previous paper. We discuss the performance of SPARO and Viper during observations at the South Pole in 2000, and we present polarimetric observations of NGC 6334 made with SPARO. Finally, we compare these observations with submillimeter polarimetric observations of the same source made at the Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope, and we discuss the implications of these three data sets for the magnetic field in NGC 6334.

ChandraObservations of Low‐Mass X‐Ray Binaries and Diffuse Gas in the Early‐Type Galaxies NGC 4365 and NGC 4382 (M85)

We used the Chandra X-Ray Observatory ACIS-S3 to image the X-ray-faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into 99 and 58 sources, respectively, most of which are low-mass X-ray binaries (LMXBs) associated with each of the galaxies. Within one effective radius of NGC 4365, about 45% of the counts are resolved into sources, 30% are attributed to unresolved LMXBs, and 25% are attributed to diffuse gas. Within two effective radii of NGC 4382, about 22% of the counts are resolved into sources, 33% are attributed to unresolved LMXBs, and 45% are attributed to diffuse gas. We identify 18 out of the 37 X-ray sources in a central field in NGC 4365 with globular clusters. The luminosity functions of the resolved sources for both galaxies are best fitted with cutoff power laws whose cutoff luminosity is ≈(0.9-3.1) × 1039 ergs s-1. These luminosities are much larger than those previously measured for similar galaxies; we do not find evidence for a break in the luminosity function at the Eddington luminosity of a 1.4 M☉ neutron star. The spatial distributions of the resolved sources for both galaxies are broader than the distribution of optical stars. In both galaxies, a hard power-law model fits the summed spectrum of all of the sources. The unresolved emission is best fitted by the sum of a soft MEKAL model, representing emission from diffuse gas, and a hard power law, presumed to be from unresolved LMXBs. There is some evidence that the temperature of the diffuse gas increases with increasing radius. A standard beta model fits the radial distribution of the diffuse gas in both galaxies. In the elliptical galaxy NGC 4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has a larger core radius. This may indicate that the gas in NGC 4382 is rotating significantly.

OH masers, molecular outflows and magnetic fields in NGC 7538

The multi-element radio-linked interferometer network (MERLIN) measurements of 1665-, 1667- and 1720-MHz OH masers associated with NGC 7538 are presented. The masers are located at the centres of three bipolar molecular outflows associated with the infrared sources IRS 1, IRS 9 and IRS 11. The distribution of OH masers in IRS 1 is more extensive than previously reported and is displaced to the south of the methanol 6.7-GHz masers. The OH masers in IRS 9 have not previously been reported. Their distribution seems to be orthogonal to the direction of the outflow and to the distribution of H2O masers. The maser distribution in IRS 11 is parallel to the dust ridge and orthogonal to the outflow. Full polarization measurements of the OH maser emission show systematic differences between the three sources. IRS 1 has moderate polarization, with linear polarization vectors partially aligned with the bipolar outflow; IRS 9 exhibits larger polarization, but little linear component; IRS 11 shows the strongest polarization and has linear polarization vectors aligned parallel to the outflow. There is also evidence for a toroidal component of the magnetic field around the IRS 11 outflow, orthogonal to the outflow direction. It is suggested that the differences in polarization trace a possible evolutionary sequence from oldest (IRS 1) to youngest (IRS 11).

Searching for Variable Stars in the Field of NGC 7789

We present the results of a time-series CCD photometric survey of variable stars in the field of the open cluster NGC 7789. In a field of about one degree centering on the cluster, a total of 28 new variable stars are discovered (14 W UMa systems, nine EA-type eclipsing binaries, one RR Lyr star, and four unclassified). In addition, we recovered 11 old variables previously discovered by other authors. Preliminary parameters are given for some of these variables.

Modeling the generation of the magnetic field in NGC 5775

A model for the generation of large-scale magnetic fields is constructed for the galaxy NGC 5775, in which the magnetic field has the form of a dipolar dynamo wave propagating along the galactic disk. The excitation of such a mode, which is unusual for galactic dynamos, can be explained by the strong variation of the galactic rotation with height above the plane of symmetry of the galactic disk.

WIYN Open Cluster Study. VIII. The Geometry and Stability of the NOAO CCD Mosaic Imager

The field of direct imaging is experiencing a considerable growth in the number of available CCD mosaic imagers, especially on large telescopes. To fully exploit the astrometric potential of these imaging devices, we develop a technique, utilizing an astrometric standard, for precision transformation of pixel coordinates into a global coordinate system. We have constructed a new astrometric standard set of 1863 stars in the field of NGC 188 to derive the CCD chip constants for the NOAO CCD Mosaic Imager. The multiple-epoch data on the Mosaic's metrics indicate that this CCD mosaic device may have experienced a one-time nonelastic expansion. We also present a new determination of the pixel scale and the optical field angle distortion constants for the KPNO Mayall 4 m telescope prime focus field corrector. To establish a reliable history of the CCD mosaic imager metrics for current and potential future astrometric applications, we recommend obtaining astrometric calibrations for CCD mosaic imagers on a regular basis. Apart from mechanical positioning of the CCD mosaic camera on the telescope, noticeable changes in the thermal environment of CCD mosaic chips should also prompt new astrometric calibrations. It is shown that, following all precautions, the NOAO CCD Mosaic Imager can produce excellent astrometric results on the Mayall 4 m telescope.

Circumstellar Disk Candidates Identified in NGC 2264

We present an optical and near-infrared study of a 45' × 45' field in NGC 2264, which includes both S Mon and the Cone Nebula. We report photometry at optical (UBVRCIC) and near-infrared (JHK) wavelengths for ~5600 stars and spectroscopic classifications for ~400 of these stars. We identify circumstellar disk candidates using three techniques: excess ultraviolet (U-V) emission, excess near-IR (I-K and H-K) emission, and Hα emission-line equivalent widths for those stars with spectra. We find generally good correlation between disk indicators thought to originate from different physical processes. We find little if any evolution of disk fraction with stellar age or mass. However, when we derive mass accretion rates () from the excess emission at U, we find that decreases with age over the age range spanned by our data, ~0.1–5 Myr, and increases with mass over the range ~0.25–1 M⊙. These findings are comparable to results found previously by us in the Orion Nebula cluster flanking fields.

Photometric Standard Stars in the BVI System in a Wide Field Centered on the Spiral Galaxy NGC 30011Based on observations obtained with the 1.3 m Warsaw telescope at the Las Campanas Observatory of the Carnegie Institution of Washington and with the 2.2 m ESO/Max-Planck-Institut telescope at the European Southern Observatory.

ABSTRACTBased on 13 nights of observations of four fields in NGC 300, we have set up an extensive sequence of stars with accurate BVI photometry covering a relatively large (25′ × 25′) region centered on this galaxy. This sequence of standard stars is very useful for calibrating the photometry of variable stars and other objects in NGC 300 and other galaxies obtained from wide-field mosaic images. Our standard-star list contains B, V, and I measurements for 390 stars. The accuracy of the zero points in the V filter and B−V color is better than 0.02 mag, and about 0.03 mag for the V−I color. We found very good agreement between our measurements and those previously obtained by Walker for 26 stars near NGC 300.

A CCD Photometric Survey of Variable Stars in the Field of NGC 188

The results of a recent CCD photometry survey of variables in a 1° field covering the old open cluster NGC 188 are presented. A total of 16 short-period variables were identified during five nights, among which eight are new variables in the program field. By adopting the method of phase-dispersion minimization, the periods for seven of the eight new variables are determined. The main properties, the classifications, as well as the memberships are discussed for all of these variable stars, based on the behavior of light curves, the locations in the color-magnitude diagram, and the results of proper motions. Among the eight new objects, six are W UMa binaries, in which one definitely is and two probably are members of the cluster. A detached or semidetached eclipsing binary with an orbital period longer than 1 day is found, which is a likely cluster star. In addition, a field star with a peculiar light curve is detected, which can be classified as a RRd–type variable. Moreover, revised basic data for the eight old variables are given according to our observations. V8 can be classified as an RS CVn–type star with light variations larger than 0.2 mag.

The Central Velocity Field in NGC 253: Possible Indication of a Bar

We have investigated whether motion of gas in a barlike potential can account for the peculiar but systematic velocity —eld observed in the nuclear region of the starburst galaxy NGC 253. This unusual velocity —eld with gradients along both major and minor axes was revealed in a high-resolution (1A.8 H92a recombination line observation by Anantharamaiah & Goss. A simple logarithmic poten] 1A.0) tial is used to model the bar. Assuming that the bulk of the gas —ows along closed and nonintersecting bar and antibar orbits of the bar potential, we have computed the expected velocity —eld and x 1 x 2 position-velocity diagrams and compared them with the observations. A comparison of the integrated CO intensity maps with the spatial distribution of the and orbits in the model indicates that the x 1 x 2 nuclear molecular gas in NGC 253 lies mainly on the orbits. We also —nd that the velocity —eld x 2 observed in the central 100 pc region in the H92a recombination line is well accounted for by the bar model if most of the ionized gas resides in the inner orbits. However, the model is unable to explain x 2 the velocity —eld on a larger scale of D500 pc observed using the Owens Valley Radio Observatory interferometer with a resolution of 5@@ ] 3@@. The direction of the observed CO velocity —eld appears twisted compared to the model. We suggest that this perturbation in the velocity —eld may be due to an accretion event that could have occurred 107 years ago. Subject headings: galaxies: individual (NGC 253) ¨ galaxies: kinematics and dynamics ¨ galaxies: structure

The White Dwarf Cooling Age of the Open Cluster NGC 2420

We have used deep HST WFPC2 observations of two fields in NGC 2420 to produce a cluster CMD down to V = 27. After imposing morphological selection criteria we find eight candidate white dwarfs in NGC 2420. Our completeness estimates indicate that we have found the terminus of the WD cooling sequence. We argue that the cluster distance modulus is likely to be close to 12.10 with E(B-V) = 0.04. With these parameters we find a white dwarf cooling age for NGC 2420 of 2.0 +- 0.20 (1 sigma) Gyrs. The 0.20 Gyr uncertainty includes errors in the photometry, sequence fitting, precursor time scales, and theoretical white dwarf cooling time scales. Comparing the cluster white dwarf cooling age to ages derived from stellar isochrone fitting we find a preference for ages derived from models incorporating convective overshoot.

The Ionizing Star Clusters of Giant H [CSC]ii[/CSC] Regions in NGC 2403

We present the results of a study on the massive star population down to about MV ~ -3.1, or 12–15 M⊙, of the most luminous giant H II regions in the nearby spiral galaxy NGC 2403, based on Hubble Space Telescope images and ground-based spectrograms. Particular emphasis is placed on the distribution of the Wolf-Rayet and red supergiant stars and the information they provide about the recent star-forming history of these large complexes. We find direct evidence for the presence of Wolf-Rayet (WR) stars in five of the six giant H II regions investigated; 25–40 WR stars are inferred for the sole NGC 2403-I giant H II region. Red supergiant (RSG) stars are mainly distributed over a more extended halo, while the young blue stars and most WR stars are in or close to a compact core. One appears to be seeing young cores of O and WR stars surrounded by older halos containing red supergiants. We propose a scenario in which RSG stars belonging to an early phase of star formation were followed by a more recent burst corresponding to a very blue mean sequence. Delayed trigger with preheating over several 100 pc by the first generation of massive stars allowed the build-up of the required confinement for the production of parsec-scale cluster cores with luminosity up to a few times 106 L⊙. Finally, we present some interesting objects found in the field of NGC 2403 outside the giant H II regions, such as field WR stars, globular clusters and background galaxies.

TheHubble Space TelescopeKey Project on the Extragalactic Distance Scale. XVII. The Cepheid Distance to NGC 4725

The distance to NGC 4725 has been derived from Cepheid variables as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered, with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 ± 0.10 mag and E(V-I)=0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.50 ± 0.16 (random) ± 0.17(systematic) mag was determined for NGC 4725. The corresponding of distance of 12.6 ± 1.0 (random) ± 1.0 (systematic) Mpc is in excellent agreement with that found with an independent analysis based upon the DoPHOT photometry package. With a foreground reddening of only E(V-I)=0.02, the inferred intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the most highly reddened fields encountered by the HST Key Project to date.

Modeling Polarization Maps of External Galaxies.

We extend previous radiation transfer models of disk galaxies to include bulge plus disk systems. The Monte Carlo radiation transfer models include the polarizing effects of dust scattering and transmission through aligned grains. We assume that the dust grains are aligned by large scale toroidal magnetic fields and we model the interstellar polarization using empirical formulae derived for the Milky Way. Although our galaxy models do not include dust or star inhomogeneities arising from clumping or spiral arm structures, they do successfully reproduce the observed dust lanes and polarization patterns and null points. We apply our techniques to NGC 4565 and NGC 891 which are examples of nearly edge-on spiral galaxies with prominent dust lanes. We can successfully model the polarimetric structure of NGC 4565 with uniformly aligned grains and a smooth dust lane. However, our models fail to reproduce the structure within NGC 891's dust lane. This failure is attributed to the striking disk activity present in NGC 891 and the polarimetric evidence for vertical magnetic field structures. The dust lane activity and non-toroidal magnetic fields in NGC 891 appear to break the large scale grain alignment to such an extent that our smooth model is unable to reproduce the flux and polarization maps. Although NGC 891 is often assumed to be very similar to our own Galaxy, our modeling suggests that, polarimetrically at least, NGC 4565 behaves more like the Milky Way than does NGC 891.

Spiral Density Wave Theory, Corotation Resonance, and the Velocity Field of NGC 4321

We extend the exposition of a new kinematic method (exploiting geometric phase) for determining the corotation resonance radius of a grand-design spiral. We present an explicit formulation, in terms of first-order linear spiral density wave theory, for fitting to the velocity field of a grand-design spiral galaxy. We test the new formulation on the velocity field of the ionized gas in the grand-design spiral NGC 4321 in the Virgo Cluster using Fabry-Perot Hα data. The kinematic method as currently implemented is useful specifically for determining an upper bound for the corotation resonance radius. We illustrate the kinematic evidence for the corotation resonance in the disk of NGC 4321.

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

NGC 5982: a smooth triaxial elliptical

NGC 5982 is known to be one of those elliptical galaxies that have peculiar kinematics in their centres. In this paper, optical spectroscopic observations are presented, which show that a specific feature is present in the velocity field of NGC 5982. This suggests that the kinematical behaviour is the result of projection effects in a dynamically smooth triaxial elliptical galaxy, and that it is not due to a dynamically distinct, central mass component. Nevertheless, we suggest that it may be related to a merger event.

New Evidence for an Anisotropic Radiation Field in NGC 5252

view Abstract Citations (24) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Evidence for an Anisotropic Radiation Field in NGC 5252 Prieto, M. Almudena ; Freudling, Wolfram Abstract Observational evidence accumulated over the last decade strongly supports the idea that the ionizing radiation field in active galactic nuclei (AGNs) is anisotropic. Here we report on new evidence for this anisotropy in the particular case of the Seyfert 2 galaxy NGC 5252. H I observations of this galaxy reveal neutral gas surrounding the nucleus but located outside the bicone of ionized gas that notably characterizes this object. The neutral gas is at a distance from the nucleus where it could be reached by the ionizing radiation if this were isotropically distributed. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173426 Bibcode: 1993ApJ...418..668P Keywords: GALAXIES: ACTIVE; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 5252; GALAXIES: SEYFERT; RADIO CONTINUUM: GALAXIES full text sources ADS | data products SIMBAD (6) NED (1)

NGC 2287 - an important intermediate-age open cluster

We have obtained photoelectric UBV photometry for 100 stars, uvbyβ photometry for 39 stars and MK spectral types for 80 stars in the field of NGC 2287. After combination with data from other sources, several interesting cluster properties are apparent. Both the UBV and uvbyβ photometry point to a small but nonzero reddening, while our spectral types confirm previous results indicating a high binary frequency for the cluster. Based on our spectral and photometric data for the cluster members with −1≤M V ≤3 we find a minimum binary frequency of 40% and discuss the possibility that the results may imply a binary frequency closer to 80%. The cluster age is found to be ∼2×10 8 yr based on both the main-sequence turnoff and the red giant distribution; the width of the turnup region can probably be explained by a combination of duplicity and a range in stellar rotation

The distance to nearby galaxy NGC 3109 based on the tip of the red giant branch

Deep VI CCD photometry of ∼4200 stars in a 3'.9×3'.9 central field of NGC 3109, a bright Magellanic-type spiral galaxy (Sm IV), has been obtained. The tip of the red giant branch (TRGB) of low-mass stars is found to be at I TRGB =21.55±0.1 mag and (V−I) TRGB =1.7±0.2 mag. The mean metallicity of the red giant branch (RGB) stars has been measured using the color of the RGB stars at M I =−3.5 mag, (V−I) −3.5 =1.41±0.07 mag: [Fe/H]=−1.6±0.2 dex. The distance modulus to NGC 3109 has been estimated using the I magnitude of the TRGB to be (m−M) 0 =25.45±0.15 mag for an adopted foreground extinction of A I =0.08 mag, showing a very good agreement with the Cepheid distance modulus estimate based on very recent CCD photometry, (m−M) 0 =25.5±0.2 mag (Capaccioli, Piotto, & Bresolin)