Halo Of NGC Research Articles

First resolved stellar halo kinematics of a Milky Way-mass galaxy outside the Local Group: The flat counter-rotating halo in NGC 4945

First resolved stellar halo kinematics of a Milky Way-mass galaxy outside the Local Group: The flat counter-rotating halo in NGC 4945

Extended Structures of Planetary Nebulae Detected in H2 Emission∗

Abstract We present narrowband near-infrared images of a sample of 11 Galactic planetary nebulae (PNe) obtained in the H2 2.122 μm and Brγ 2.166 μm emission lines and the K c 2.218 μm continuum. These images were collected with the Wide-field Infrared Camera on the 3.6 m Canada–France–Hawaii Telescope (CFHT); their unprecedented depth and wide field of view allow us to find extended nebular structures in H2 emission in several PNe, some of these being the first detection. The nebular morphologies in H2 emission are studied in analogy with the optical images, and indication of stellar wind interactions is discussed. In particular, the complete structure of the highly asymmetric halo in NGC 6772 is witnessed in H2, which strongly suggests interaction with the interstellar medium. Our sample confirms the general correlation between H2 emission and the bipolarity of PNe. The knotty or filamentary fine structures of the H2 gas are resolved in the inner regions of several ring-like PNe, also confirming the previous argument that H2 emission mostly comes from knots or clumps embedded within fully ionized material at the equatorial regions. Moreover, the H2 image of the butterfly-shaped Sh 1-89, after removal of field stars, clearly reveals a tilted ring structure at the waist. These high-quality CFHT images justify follow-up detailed morphokinematic studies that are desired in order to deduce the true physical structures of a few PNe in the sample.

Planetary nebulae with UVIT: Far ultra-violet halo around the Bow Tie nebula (NGC 40)

Context. NGC 40 is a planetary nebula with diffuse X-ray emission, suggesting an interaction of the high-speed wind from WC8 central star (CS) with the nebula. It shows strong C IV 1550 Å emission that cannot be explained by thermal processes alone. We present here the first map of this nebula in C IV emission using broad band filters on the Ultra-Violet Imaging Telescope (UVIT). Aim. We aim to map the hot C IV-emitting gas and its correspondence with soft X-ray (0.3–8 keV) emitting regions in order to study the shock interaction between the nebula and the ISM. We also aim to illustrate the potential of UVIT for nebular studies. Methods. We carry out a morphological study of images of the nebula obtained at an angular resolution of about 1.3″ in four UVIT filter bands that include C IV 1550 Å and [C II] 2326 Å lines as well as UV continuum. We also make comparisons with X-ray, optical, and IR images from the literature. Results. The [C II] 2326 Å images show the core of the nebula with two lobes on either side of CS similar to [N II]. The C IV emission in the core shows similar morphology and extent to that of diffuse X-ray emission concentrated in nebular condensations. A surprising UVIT discovery is the presence of a large faint far UV (FUV) halo in an FUV filter with λeff of 1608 Å. The UV halo is not present in any other UV filter. The FUV halo is most likely due to UV fluorescence emission from the Lyman bands of H2 molecules. Unlike the optical and IR halo, the FUV halo trails predominantly towards the south-east side of the nebular core, opposite to the CS’s proper motion direction. Conclusions. Morphological similarity of C IV 1550 Å and X-ray emission in the core suggests that it results mostly from the interaction of strong CS wind with the nebula. The FUV halo in NGC 40 highlights the extensive existence of H2 molecules in the regions even beyond the optical and IR halos. Thus UV studies are important to estimate the amount of H2, which is probably the most dominant molecule and significant for mass-loss studies.

Unusual Objects in the Spiral Galaxy NGC 6946

Abstract Several strange objects in the spiral galaxy NGC 6946 are described. One of these objects is the giant stellar complex noted long ago; we suggested that its sharp semicircular western edge is a result of the ram pressure, arising owing to motion of this complex through the HI halo of NGC 6946. We found another enigmatic object, proposing for it the name Red Ellipse; it is located within the isolated Northern arm of the galaxy. The enormous size of this Ellipse, and especially the spectroscopic data obtained recently with the 6-m reflector of the Special Astrophysical Observatory, made us to conclude that this object could not be a supernova remnant. The excellent image of NGC 6946 obtained with the Subaru 8-m telescope also shows a strange region with several regular crossed dark lanes, connected with a black spot.

Magnetic field structure and halo in NGC 4631

In order to clarify whether NGC 4631 has a unique magnetic field configuration in the central region along its disk, we present high-resolution Faraday-corrected polarization data. Radio continuum observations of NGC 4631 at 4.85 GHz were performed with the VLA. In addition, observations were made with the Effelsberg telescope at 4.85 GHz and at 8.35 GHz. These were analyzed together with archival VLA-data at 8.35 GHz. The vertical scale heights in NGC 4631 vary significantly in different regions within the galaxy and their mean values at 4.85 GHz are with 2.3 kpc (370 pc) for the thick (thin) disk higher than the mean values found so far in six other edge-on spiral galaxies. This may originate in the tidal interaction of NGC 4631 with its neighbouring galaxies. The total field strengths in the halo are of the order of the total magnetic field strength in the disk, whereas the ordered field strengths in the halo seem to be higher than the value in the disk. The derived distribution of rotation measures implies that NGC 4631 has a large-scale regular magnetic field configuration. Despite the strong Faraday depolarization along the galactic plane and the strong beam depolarization in the transition zone between the disk and halo, our research strongly indicates that the magnetic field orientation along the central 5-7 kpc of the disk is also plane-parallel. Therefore, we claim that NGC 4631 also has a magnetic field structure plane-parallel along its entire disk.

The globular cluster kinematics and galaxy dark matter content of NGC 3923

This paper presents further results from our spectroscopic study of the globular cluster (GC) system of the group elliptical NGC 3923. From observations made with the GMOS instrument on the Gemini South Telescope, an additional 50 GC and ultra-compact dwarf (UCD) candidates have been spectroscopically confirmed as members of the NGC 3923 system. When the recessional velocities of these GCs are combined with the 29 GC velocities reported previously, a total sample of 79 GC/UCD velocities is produced. This sample extends to over 6 arcmin (> 6 Re∼ 30 kpc) from the centre of NGC 3923 and is used to study the dynamics of the GC system and the dark matter content of NGC 3923. It is found that the GC system of NGC 3923 displays no appreciable rotation, and that the projected velocity dispersion is constant with radius within the uncertainties. The velocity dispersion profiles of the integrated light and GC system of NGC 3923 are indistinguishable over the region in which they overlap. We find some evidence that the diffuse light and GCs of NGC 3923 have radially biased orbits within ∼130 arcsec. The application of axisymmetric orbit-based models to the GC and integrated light velocity dispersion profiles demonstrates that a significant increase in the mass-to-light ratio (from M/LV= 8 to 26) at large galactocentric radii is required to explain this observation. We therefore confirm the presence of a dark matter halo in NGC 3923. We find that dark matter comprises 17.5 per cent of the mass within 1 Re, 41.2 per cent within 2 Re and 75.6 per cent within the radius of our last kinematic tracer at 6.9 Re. The total dynamical mass within this radius is found to be M⊙. In common with other studies of large ellipticals, we find that our derived dynamical mass profile is consistently higher than that derived by X-ray observations, by a factor of around 2.

The globular cluster kinematics and galaxy dark matter content of NGC 4649 (M60)

From observations with the GMOS multislit spectrograph on the Gemini North telescope, we have obtained spectra for 39 globular cluster (GC) candidates in the Virgo giant elliptical galaxy NGC 4649 (M60), of which 38 are confirmed GCs. The clusters extend out to a radius of 260 arcsec (3.5 effective radii). We find no rotation of the GC system, with an upper limit of v/σ < 0.6 at a confidence level of 95 per cent. The GC velocity dispersion is constant with radius, within the uncertainties. We fit isotropic models to the GC and stellar kinematics; these models yield an (M/L)V around 16 at 200 arcsec radius (16 kpc), an increase of a factor of 2 from the central M/L. We also use the mass profile as derived from X-rays to determine the orbital structure. Using axisymmetric orbit-based models and the X-ray mass profile, we find the orbital distribution is close to isotropic within 100 arcsec, and becomes tangentially biased beyond. Furthermore, when using the X-ray profile, we find a better fit to the kinematics compared to using a constant M/L model. Thus, both isotropic and axisymmetric orbit-based models give support for the presence of a dark matter halo in NGC 4649.

Polarimetry in the Outskirts of NGC 6611

Fil: Orsatti, Ana Maria. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Astrofisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas. Instituto de Astrofisica la Plata; Argentina

The Spatial Distribution of Faint Fuzzy Star Clusters in NGC 5195

We present a study of a faint fuzzy star cluster system in the nearby SB0 galaxy NGC 5195 interacting with the famous spiral galaxy NGC 5194 (M51), based on HST ACS BVI mosaic images taken by the Hubble Heritage Team. We have found about 50 faint fuzzy star clusters around NGC 5195 that are larger than typical globular clusters with effective radii reff > 7 pc and that are red with (V - I) > 1.0. They are mostly fainter than MV ≈ -8.3 mag. From the comparison of BVI photometry of these clusters with the simple stellar population models, we find that they are as massive as ≈105 M☉ and older than 1 Gyr. Strikingly, most of these clusters are found to be scattered in an elongated region almost perpendicular to the northern spiral arm of NGC 5194, and the center of the region is slightly north of the NGC 5195 center, while normal compact red clusters of NGC 5195 are located around the bright optical body of the host galaxy. This is in contrast to the cases of NGC 1023 and NGC 3384 where the spatial distribution of faint fuzzy clusters shows a ring structure around the host galaxy. We suggest that at least some faint fuzzy clusters are experiencing tidal interactions with the companion galaxy NGC 5194 and must be associated with the tidal debris in the western halo of NGC 5195.

The Dark Halo of NGC 5963 as a Constraint on Dark Matter Self‐Interaction at the Low‐Velocity Regime

Self-interacting dark matter has been proposed as a hypothesis to explain the shallow central slopes of the density profiles of dark matter halos in galaxies. In order to be consistent with observational studies at scales of galaxy clusters, the cross section should scale inversely with the velocity of collision. In this paper we consider the mass density profile of the halo of the low surface brightness (LSB) galaxy NGC 5963 to place an upper limit on the dark matter cross section for collisions with velocities ~150 km s-1, i.e., at the low-velocity regime. After calibrating against cosmological simulations, we found that the large inferred dark matter concentration and central dark matter density in NGC 5963 are inconsistent with an effective collisional cross section per unit of mass >0.2 cm2 g-1. Corrections were applied in order to account for reduction of the core by the adiabatic contraction caused by cooling baryons. Our limits, which involve a number of simplifying, but always conservative, assumptions, exclude the last permitted interval for velocity-dependent cross sections to explain the flat density core in LSB galaxies. Implications for the nature of dark matter are also discussed.

Giant halos in dwarf irregular galaxies vs. dwarf elliptical galaxies

We present a discovery of a giant stellar halo in NGC 6822, a dwarf irregular galaxy in the Local Group. This halo is mostly made of old red giants, showing striking features: 1) it is several times larger than the main body of the galaxy seen in the optical images, and 2) it is elongated in the direction almost perpendicular to the HI disk of NGC 6822. The structure of this stellar halo looks similar to the shape of dwarf elliptical galaxies, indicating that the halos of dwarf irregular galaxies share the same origin with those of the dwarf elliptical galaxies.

Globular Clusters in Early-Type Galaxies with GMOS

AbstractWe present recent results from our long-term Gemini/GMOS study of globular clusters (GCs) in early-type galaxies. To date, we have obtained photometry and spectroscopy for GCs in NGCs 3379, 4649, 524, 7332, and IC 1459. We find a clear bimodality in the NGC 4649 GC color distribution, with the fraction of blue/red clusters increasing with galacto-centric radius. We derive ages and metallicities for 22 GCs in NGC 3379, finding that most of the clusters appear old (10–15 Gyr); however, there is a group of 4 metal-rich, younger clusters with ages of 2–6 Gyr. The NGC 3379 GC velocity dispersion decreases with radius, as does the inferred (local) mass-to-light ratio: there is no evidence for a dark matter halo in NGC 3379 based on our GC data.

ChandraObservation of the Edge‐on Galaxy NGC 3556 (M108): Violent Galactic Disk‐Halo Interaction Revealed

We present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral galaxy NGC 3556, together with a multiwavelength analysis of various discrete X-ray sources and diffuse X-ray features. Among 33 discrete X-ray sources detected within the IB = 25 mag arcsec-2 isophote ellipse of the galaxy, we identify a candidate for the galactic nucleus, an ultraluminous X-ray source that might be an accreting intermediate-mass black hole, a possible X-ray binary with a radio counterpart, and two radio-bright giant H II regions. We detect large amounts of extraplanar diffuse X-ray emission, which extend about 10 kpc radially in the disk and ≳4 kpc away from the galactic plane. The diffuse X-ray emission exhibits significant substructures, possibly representing various blown-out superbubbles or chimneys of hot gas heated in massive star-forming regions. This X-ray-emitting gas has temperatures in the range of ~(2-7) × 106 K and has a total cooling rate of ~2 × 1040 ergs s-1. The energy can be easily supplied by supernova blast waves in the galaxy. These results show NGC 3556 to be a galaxy undergoing vigorous disk-halo interaction. The halo in NGC 3556 is considerably less extended, however, than that of NGC 4631, in spite of many similarities between the two galaxies. This may be due to the fact that NGC 3556 is isolated, whereas NGC 4631 is interacting. Thus, NGC 3556 presents a more pristine environment for studying the disk-halo interaction.

Do the galaxies NGC 936 and NGC 3198 possess massive spheroidal subsystems?

The condition for gravitational stability of the stellar disks of the galaxies NGC 936 and NGC 3198 makes maximum disk models unacceptable. We present mass estimates for these objects' spheroidal components. The mass of the dark halo of NGC 3198, within four disk radial scale lengths, exceeds its disk mass by a factor of 1.6 to 2. The masses of the disk and spheroidal subsystem (halo + bulge), within four radial scale lengths, are approximately the same for NGC 936.

A Search for Near‐Infrared Emission from the Halo of NGC 5907 at Radii of 10–30 Kiloparsecs

We present a search for near-infrared (3.5-5 μm) emission from baryonic dark matter in the form of low-mass stars and/or brown dwarfs in the halo of the nearby edge-on spiral galaxy NGC 5907. The observations were made using a 256 × 256 InSb array with a pixel scale of 17'' at the focus of a liquid-helium-cooled telescope carried above the Earth's atmosphere by a sounding rocket. In contrast to previous experiments that have detected a halo around NGC 5907 in the V, R, I, J, and K bands at galactic radii 6 kpc < r < 10 kpc, our search finds no evidence for emission from a halo at 10 kpc < r < 30 kpc. Assuming a halo mass density scaling as r-2, which is consistent with the flat rotation curves that are observed out to radii of 32 kpc, the lower limit of the mass-to-light ratio at 3.5-5 μm for the halo of NGC 5907 is 250 (2 σ) in solar units. This is comparable to the lower limit we have found previously for NGC 4565. Based on recent models, our nondetection implies that hydrogen-burning stars contribute less than 15% of the mass of the dark halo of NGC 5907. Our results are consistent with the previous detection of extended emission at r ≤ 10 kpc if the latter is caused by a stellar population that has been ejected from the disk because of tidal interactions. We conclude that the dark halo of NGC 5907, which is evident from rotation curves that extend far beyond 10 kpc, is not made up of hydrogen-burning stars.

A Search for a Near-Infrared Halo around NGC 4565

We present a near-infrared (3.5-5 micron) search for the integrated emission from low-mass stars and/or brown dwarfs in the halo of the nearby edge-on spiral galaxy NGC 4565. The observation was made with a liquid-helium-cooled rocket-borne telescope using a 256 x 256 InSb array with a pixel scale of 17. Images of NGC 4565 were successfully obtained with sensitivity near the natural background limit. Our search reveals no evidence of a faint halo around the galaxy, in contrast with the previous reports of a halo around NGC 5907. The lower limit of the mass-to-light ratio for the halo of NGC 4565 is 260 (2 delta) in solar units at 3.5-5 microns. This implies that hydrogen-burning stars do not contribute significantly to the mass of the dark halo in NGC 4565.

The interaction of the planetary nebula NGC 6894 with the ISM magnetic field

We have obtained images of the planetary nebula NGC 6894 and the 'stripes' near it and have analysed their morphologies. Our analysis suggests that the material of the stripes has been ionized by the central star of NGC 6894, and could have originated from the stripping of the halo of NGC 6894 by the interstellar medium (ISM). Based on the fact that the orientation of the stripes is parallel to the Galactic plane, we speculate that the morphology of the stripes is determined by the Galactic magnetic field. The above interpretation requires the motion of NGC 6894 to be in a direction which is consistent with observations. This work further stresses the significant role of the ISM magnetic field in shaping haloes of planetary nebulae, and the potential for using planetary nebulae to probe the ISM.

The Hot Gaseous Halo of the Spiral Galaxy NGC 3628 in the Leo Triplet

In a 14 ks {ital ROSAT} Position Sensitive Proportional Counter pointing we detected an extended (28{times}40 kpc) halo of hot tenuous gas around NGC 3628, with a flat emission distribution and a temperature of {ital T}{approx_equal}2{times}10{sup 6} K. The soft X-ray luminosity of the halo gas in the 0.1{endash}2.0 keV band is {approximately}8{times}10{sup 39} ergss{sup {minus}1}, and the total soft X-ray luminosity of NGC 3628 is approximately 2.1{times}10{sup 40} ergss{sup {minus}1}. The most likely heating source for the halo gas are the shocks resulting from a supernova-driven outflow from the circumnuclear starburst propagating through the halo interstellar medium. Both the morphology and the line ratios of associated optical emission-line gas are consistent with this interpretation. In many respects the soft X-ray halo of NGC 3628 resembles those of other nearby, edge-on starburst galaxies like, e.g., M82 and NGC 253. Its size and radiative cooling time indicate that halos like that of NGC 3628 have to be considered as potentially important contributors to the population of quasi-stellar object absorption-line systems. With two exceptions, most of the compact X-ray sources seen in projection within the halo of NGC 3628 are probably background active galactic nuclei. The remaining two sources might be condensationsmore » in the shock-heated halo gas. The soft X-ray properties of a few sources in the disk of NGC 3628 are consistent with those of high-mass X-ray binaries or powerful young supernova remnants. The PSPC spectrum of NGC 3627, which was within the field of view during our observations, also exhibits the typical characteristics of a starburst galaxy. It can be approximated by a power-law + Raymond-Smith model with a total soft X-ray luminosity of {ital L}{sub {ital x}}=1.1{times}10{sup 40} ergss{sup {minus}1}. {copyright} {ital 1996 The American Astronomical Society.}« less

A Search for Far-Ultraviolet Emission Lines from Diffuse Hot Gas in the Halo of NGC 4631

During the Astro-2 mission in 1995 March, the Hopkins Ultraviolet Telescope (HUT) was used to conduct a sensitive search for far-UV emission lines from hot gas in the halo of NGC 4631. This galaxy has strong radio continuum emission, extended Hα emission, and soft X-ray emission from an extended halo. Recent ROSAT observations indicate that much of the X-ray emission comes from a very soft component, with a temperature less than 6 × 105 K. Gas cooling through this temperature range should show emission in the prominent O VI λλ1032, 1038 and C IV λλ1548, 1551 lines. Hot gas interacting with cold clouds at the interface of the disk and the halo may form turbulent mixing layers, which should produce strong emission in lower ionization species such as C II λ1335 and O III] λλ1661, 1666. The aperture of the HUT spectrograph subtends 10'' × 56'' and was positioned parallel to the disk 39'' (1.4 kpc) south of the nucleus. No emission lines were detected. The derived constraints are tempered somewhat by the unknown extinction within the NGC 4631 halo. If it is low, the upper limits suggest that the mean mixing-layer temperatures are less than 2 × 105 K or that mixing layers are not a significant contributor to the observed Hα emission at the position of the HUT slit. For smooth flows, the upper limit to the O VI flux suggests that less than approximately 15 M☉ yr-1 are processed through a galactic fountain in NGC 4631.

Polarized emission-line profiles in the core and halo of NGC 7027

Polarized emission-line profiles in the core and halo of NGC 7027