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Register a new userA Galaxy Cluster Near NGC 720
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H. Arp
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The galaxy cluster RXJ 0152.7-1357 is emitting X-rays at the high rate of 148 counts $ks^{-1}$. It would be one of the most luminous X-ray clusters known if it is at its redshift distance of z = .8325. It is conspicuously elongated, however, toward the bright, X-ray active galaxy NGC 720 about 14 arcmin away. At the same distance on the other side of NGC 720, and almost perfectly aligned, is an X-ray BSO of 5.8 cts/ks. It is reported here that the redshift of this quasar is z = .8312.
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(Abridged) We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720 to verify the existence of a dark matter (DM) halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources. For semi-major axes a<~150 (18.2h_{70}^{-1} kpc) the ellipticity of the diffuse emission is ex ~0.15, which is less than the values 0.2-0.3 obtained from ROSAT because the point sources contaminated the ROSAT values. The Chandra data confirm the ~20 deg position angle (PA) twist discovered by ROSAT, but the Chandra twist is more gradual also because of the point sources contaminating the ROSAT values. Overall the ex and PA values for a<~150 can be explained by the triaxial model of NGC 720 published by Romanowsky & Kochanek. Since the optical image displays no substantial isophote twisting, the X-ray PA twist requires a massive DM halo if the hot gas is in hydrostatic equilibrium. The mass-follows-light hypothesis is also inconsistent with the Chandra ellipticities at the 96% (98%) level for oblate (prolate) symmetry. Thus, both the PA twist and the ellipticities of the Chandra image imply a DM halo independent of the gas T profile -- evidence that cannot be obviated by alternative gravity theories such as MOND. The DM density model, rho ~a^{-2}, provides the best fit and gives ellipticities of 0.37 +/- 0.03 (0.36 +/- 0.02) for oblate (prolate) models. These moderate ellipticities for the DM halo are inconsistent with both the nearly spherical halos predicted if the DM is self-interacting and with the highly flattened halos predicted if the DM is cold molecular gas. These ellipticities may also be too large to be explained by warm DM, but are consistent with galaxy-sized halos formed in the Lambda-CDM paradigm.
NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs. It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning 90 kiloparsecs. NGC 4449 is relatively isolated, although an interaction with its nearest known companion-the galaxy DDO 125, some 40 kpc to the south-has been proposed as being responsible for the complexity of its HI structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxys centre.
Deep Effelsberg HI spectra of the one-armed, bright Virgo cluster spiral galaxy NGC 4254 are presented.Five different positions were observed in the 21 cm HI line with the Effelsberg 100-m telescope: one in the center and 4 located one HPBW to the NE, NW, SW, and SE, respectively, from the galaxy center. The spectra are compared to existing deep VLA observations, and the single dish and interferometric HI data are used to constrain a dynamical model which includes the effects of ram pressure. The peculiar, one-armed spiral pattern of NGC 4254 and its distorted and kinematically perturbed atomic gas distribution can be explained by a close and rapid encounter ~280 Myr ago with another massive Virgo galaxy, followed by ram pressure stripping that is still ongoing. The stripping occurs almost face-on, since the angle between the disk and the orbital plane is 70 degrees. The galaxy with which NGC 4254 had its encounter is tentatively identified as the lenticular NGC 4262.
We present a catalogue of 261 new infrared selected members of the 3C129 galaxy cluster. The cluster, located at $z approx$ 0.02, forms part of the Perseus-Pisces filament and is obscured at optical wavelengths due to its location in the zone of avoidance. We identified these galaxies using the $J-$ and $K-$band imaging data provided by the UKIDSS Galactic Plane Survey within an area with a radius of $1.1^{circ}$ centred on the X-ray emission of the cluster at $ell, b approx 160.52^{circ}, 0.27^{circ}$. A total of 26 of the identified galaxy members have known redshifts 24 of which are from our 2016 Westerbork HI survey and two are from optical spectroscopy. An analysis of the galaxy density at the core of the 3C129 cluster shows it to be less dense than the Coma and Norma clusters, but comparable to the galaxy density in the core of the Perseus cluster. From an assessment of the spatial and velocity distributions of the 3C129 cluster galaxies that have redshifts, we derived a velocity of $cz = 5227 pm 171$ km/s and $sigma = 1097 pm 252$ km/s for the main cluster, with a substructure in the cluster outskirts at $cz = 6923 pm 71$ km/s with $sigma = 422 pm 100$ km/s. The presence of this substructure is consistent with previous claims based on the X-ray analysis that the cluster is not yet virialised and may have undergone a recent merger.
We report the discovery of a new dwarf galaxy (NGC6503-d1) during the Subaru extended ultraviolet (XUV) disk survey. It is a likely companion of the spiral galaxy NGC6503. The resolved images, in B, V, R, i, and Halpha, show an irregular appearance due to bright stars with underlying, smooth and unresolved stellar emission. It is classified as the transition type (dIrr/dSph). Its structural properties are similar to those of the dwarfs in the Local Group, with a V absolute magnitude ~ -10.5, half-light radius ~400 pc, and central surface brightness ~25.2. Despite the low stellar surface brightness environment, one HII region was detected, though its Halpha luminosity is low, indicating an absence of any appreciable O-stars at the current epoch. The presence of multiple stellar populations is indicated by the color-magnitude diagram of ~300 bright resolved stars and the total colors of the dwarf, with the majority of its total stellar mass ~4x10^6 Msun in an old stellar population.
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