The shape of the dark matter halo in the early-type galaxy NGC 2974

We present H  i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H  i disc in this galaxy is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape.
We construct mass models of NGC 2974 by combining the H  i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H  i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse.
The mass-to-light ratio M / L _I increases in NGC 2974 from 4.3 M_⊙/L_⊙, _I at one effective radius to 8.5 M_⊙/L_{⊙, I} at 5  R _e. This increase of M / L already suggests the presence of dark matter: we find that within 5  R _e at least 55 per cent of the total mass is dark.     

Star formation and figure rotation in the early-type galaxy NGC 2974

We present Galaxy Evolution Explorer ( GALEX ) far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of the nearby early-type galaxy NGC 2974, along with complementary ground-based optical imaging. In the ultraviolet, the galaxy reveals a central spheroid-like component and a newly discovered complete outer ring of radius 6.2 kpc, with suggestions of another partial ring at an even larger radius. Blue FUV–NUV and UV-optical colours are observed in the centre of the galaxy and from the outer ring outwards, suggesting young stellar populations (≲1 Gyr) and recent star formation in both locations. This is supported by a simple stellar population model which assumes two bursts of star formation, allowing us to constrain the age, mass fraction and surface mass density of the young component pixel by pixel. Overall, the mass fraction of the young component appears to be just under 1 per cent (lower limit, uncorrected for dust extinction). The additional presence of a nuclear and an inner ring (radii 1.4 and 2.9 kpc, respectively), as traced by [O  iii ] emission, suggests ring formation through resonances. All three rings are consistent with a single pattern speed of  78 ± 6  km s⁻¹ kpc⁻¹, typical of S0 galaxies and only marginally slower than expected for a fast bar if traced by a small observed surface brightness plateau. This thus suggests that star formation and morphological evolution in NGC 2974 at the present epoch are primarily driven by a rotating asymmetry (probably a large-scale bar), despite the standard classification of NGC 2974 as an E4 elliptical.     

On the dynamics of the satellite galaxies in NGC 5044

The NGC 5044 galaxy group is dominated by a luminous elliptical galaxy that is surrounded by ∼160 dwarf satellites. The projected number density profile of this dwarf population deviates within ∼1/3 of the virial radius from a projected Navarro, Frenk and White (NFW) profile, which is assumed to approximate the underlying total matter distribution. By means of a semi-analytic model, we demonstrate that the interplay between gravitation, dynamical friction and tidal mass loss and destruction can explain the observed number density profile. We use only two parameters in our models: the total to stellar mass fraction of the satellite haloes and the disruption efficiency. The disruption efficiency is expressed by a minimum radius. If the tidal radius of a galaxy (halo) falls below this radius, it is assumed to become unobservable. The preferred parameters are an initial total to stellar mass fraction of ∼20 and a disruption radius of  4 kpc  . In that model, about 20 per cent of all the satellites are totally disrupted on their orbits within the group environment. Dynamical friction is less important in shaping the inner slope of the number density profile because the reduction in mass by tidal forces lowers the impact of the friction term. The main destruction mechanism is tide. In the preferred model, the total B -band luminosity of all disrupted galaxies is about twice the observed luminosity of the central elliptical galaxy, indicating that a significant fraction of stars are scattered into the intragroup medium. Dwarf galaxy satellites closer to the centre of the NGC 5044 group may exhibit optical evidence of partial tidal disruption. If dynamical friction forces the satellite to merge with the central elliptical, the angular momentum of the satellite tends to be removed at the apocentre passage. Afterwards, the satellite drops radially towards the centre.     

Andromeda IV,a solitary gas‐rich dwarf galaxy

Observations are presented of the isolated dwarf irregular galaxy And IV made with the Hubble Space Telescope Advanced Camera for Surveys and the Giant Metrewave Radio Telescope in the 21 cm HI line. We determine the galaxy distance of 7.17 ± 0.31 Mpc using the Tip of Red Giant Branch method. The galaxy has a total blue absolute magnitude of –12.81 mag, linear Holmberg diameter of 1.88 kpc, and an HI ‐disk extending to 8.4 times the optical Holmberg radius. The HI massto‐blue luminosity ratio for And IV amounts 12.9 M_⊙/L_⊙. From the GMRT data we derive the rotation curve for the HI and fit it with different mass models. We find that the data are significantly better fit with an iso‐thermal dark matter halo, than by an NFW halo. We also find that MOND rotation curve provides a very poor fit to the data. The fact that the isothermal dark matter halo provides the best fit to the data supports models in which star formation feedback results in the formation of a dark matter core in dwarf galaxies. The total mass‐to‐blue luminosity ratio of 162 M_⊙/L_⊙ makes And IV among the darkest dIrr galaxies known. However, its baryonic‐to‐dark mass ratio (M_gas + M *)/M_T = 0.11 is close to the average cosmic baryon fraction of 0.15. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multi-Component Models For Disc Galaxies

We present an application of a new set of detailed, self-consistent, dynamical models for disc galaxies. We start from the hypothesis that each galaxy can be decomposed into a bulge, following the r ^1/4law, and a disc with an exponential projected density profile; and that the isodensity surfaces of each component can be represented by similar concentric spheroids. After taking into account both the asymmetric drift effects and the integration along the line of sight, we produce the rotational velocity and velocity-dispersion profile, andthe approximate shape of the line-of-sight velocity distributions for the stars as parameterized by the h ₃ and h ₄ coefficients of theGauss–Hermite expansion of the line profile. Photometric and kinematical data have been taken from the literature for the test case of the S0 galaxy NGC 5866, for which detailed stellar kinematical data are available at different positions across the galaxy. Apart from the innermost, dust-obscured regions of the galaxy, where observational effects are likely to be dominant, the model successfully reproduces the whole set of dynamical data available as well as giving a good fit to the photometry. The galaxy is shown to have an isotropic velocity-dispersion tensor, thus hinting at a dissipational formation process. This revised version was published online in July 2006 with corrections to the Cover Date.     

NGC 4650 A: A nearly edge-on ring galaxy?

The peculiar galaxy NGC 4650 A (α=12^h 42^m. 1; = δ—40° 26′; 1950·0) has been studied by means of direct and spectral observations with the ESO 3·6-m telescope. It is interpreted as a prolate, elliptical galaxy surrounded by a warped ring of H II regions, dust and stars. The distance is 47 Mpc (H ₀=55 km s⁻¹ Mpc⁻¹). The ring is seen nearly edge-on (inclination 85°) and it rotates. It has a diameter of about 21 kpc and is bluer than the elliptical galaxy for which the (M/L _v) ratio is ∼12 in solar units. The observed configuration may be the result of interaction with the nearby galaxy, NGC 4650.     

Do elliptical galaxies suffer from warp?

The present work includes the performance of detailed surface isophotometry of NGC 1700. Luminosity profiles, ellipticity curve, reduced luminosity profiles and the galaxy parameters are illustrated; the study includes the variation of position angle with the distance from the centre of the galaxy as well. An interesting feature of this object is the twisted shape of the outer isophote which does not appear as an ellipse but as an integral-sign shape which attributed to some kind of warp in the galaxy under investigation.     

Variability of emission line spectrum and physical conditions in the nucleus of the seyfert galaxy NGC 1275

The observational evidences on the variability of relative intensities of emission lines of the Seyfert galaxy NGC 1275 are discussed. The time scale of these variations is about half a year. It equals to the time scale of continuous and radio variations of the nucleus of that galaxy. The changes of the electron temperature T_e and the electron density n_e calculated from the emission line intensities were found to have a functional connection. This evidences that the physical conditions in the gaseous zone of the nucleus of NGC 1275 are recurrent.     

A model of the formation of elliptical galaxies

An analytical model of elliptical galaxy formation is formulated that yields predicted mass distributions. A good fit is obtained when the model is tested against the luminosity profile of NGC 3379. The present theory is identical to a parallel theory of solar system formation which suggests that the mass distribution of the solar system should be similar to that of NGC 3379. Comparison supports this conjecture.Work performed under the auspices of the USERDA.     

Optical multiband surface photometry of a sample of Seyfert galaxies: III. Global,isophotal, and bar parameters

This paper is third in a series, studying the optical properties of a sample of Seyfert galaxies. Here we present a homoge neous set of global (ellipticity, position angle, inclination, and total magnitude) and isophotal (semi‐major axis and colour indices at 24V mag arcsec^–2) parameters of the galaxy sample. We find the following median corrected isophotal colour indices: (B – I_C)⁽⁰⁾₂₄ = 1.9 mag arcsec^–2 and (V – I_C)⁽⁰⁾₂₄ = 1.1 mag arcsec^–2. A set of bar parameters (ellipticity, position angle, semi‐major axis corresponding to the ellipticity maximum in the bar region, and length) are also reported; deprojection has been applied to the bar ellipticity, length, and relative length in terms of galaxy isophotal semi‐major axis. Regarding bar length estimation, we use a method, based on the relation between the behaviour of the profiles and orbit analysis. The so estimated bar length tightly correlates with the semi‐major axis, corresponding to the ellipticity maximum with a median ratio of the former to the latter of 1.22. The median of the deprojected bar ellipticity, length, and relative length are 0.39, 5.44 kpc, and 0.44, respectively. There is a correlation between the deprojected bar length and the corrected isophotal semi‐major axis at 24V mag arcsec^–2. Three of the 17 large‐scale bars appear strong, based on the deprojected bar ellipticity as a first‐order approximation of bar strength. The deprojected relative bar length does not appear to correlate with the bar ellipticity (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionized gas characteristics in the cavities of the gas and dust disc of the spiral galaxy NGC 6946

The parameters of the ionized gas in NGC 6946 (in the [NII] λλ6548, 6583, H _α and [SII] λλ6717, 6731 lines) are investigated with the SAO RAS BTA telescope along three positions of the long slit of the SCORPIO focal reducer, passing through a number of large and small cavities of the gaseous disc of the galaxy. These cavities correspond exactly to the cavities in warm dust, visible at 5 − 8μm. We found that everywhere in the direction of NGC 6946 the lines of ionized gas are decomposed into two Gaussians, one of which shows almost constant [SII]/H _α and [NII]/H _α ratios, as well as an almost constant radial velocity within the measurement errors (about −35… − 50 km/s). This component is in fact the foreground radiation from the diffuse ionized gas of our Galaxy, which is not surprising, given the low (12°) latitude of NGC 6946; a similar component is also present in the emission of neutral hydrogen. The analysis of the component of ionized gas, occurring inNGC 6946, has revealed that it shows signs of shock excitation in the cavities of the gaseous disc of the galaxy. This shock excitation is as well typical for the extraplanar diffuse ionized gas (EDIG), observed in a number of spiral galaxies at their high Z-coordinates. This can most likely be explained by low density of the gas in the NGC 6946 disc (with the usual photoionization) inside the cavities, due to what we see the spectral features of the EDIG gas of NGC 6946, projected onto them, and located outside the plane of the galaxy. In the absence of separation of ionized gas into two components by radial velocities, there is an increasing contribution to the integral line parameters by the EDIG of our Galaxy when the gas density in NGC 6946 decreases, which explains some strange results, obtained in the previous studies. Themorphology of warmdust, visible in the infrared range and HI is almost the same (except for the peripheral parts of the galaxy, where there are no sources of dust heating).Moreover, the shock excitation of the ionized gas is detected in the smallest holes, distinguishable only in the IR images.     

Photometry of the low-luminosity spiral galaxy NGC 4136

Multicolor BVRI surface photometry of the low-luminosity (M _V ≈?18^m) spiral galaxy NGC 4136 is presented. The photometric parameters of its components and the color distribution over the galactic disk are estimated. The color indices and the corresponding effective ages are determined for the brightest star-forming regions. The disk-to-dark halo mass ratio is derived from the measured rotation curve of the galaxy. The disk mass dominates within the optical boundaries of the galaxy, so its disk can be considered as a self-gravitating system.     

A minor-merger model for NGC 7479

The overall morphology of the barred spiral galaxy NGC 7479 is modelled in numerical simulations of a minor merger. Special attention is paid to the morphology and velocity field of the asymmetric spiral structure and the strong stellar bar. The mass of the satellite galaxy is 1/10 of the mass of the primary disc, or 1/30 of the total mass of the primary. The satellite is placed initially in a circular prograde orbit at six disc scalelengths from the centre of the primary. We follow the evolution of the merger until the secondary galaxy reaches the nuclear region of the primary. A comparison between the modelled and observed morphologies of the stellar and the ionized and neutral gas distributions and velocity fields supports the hypothesis that the transient look of NGC 7479 is a result of a minor merger. We vary several of the initial parameters of the merger and discuss their effects on the resulting morphology. The merging satellite galaxy is likely to lie within the bar of NGC 7479. We identify a possible candidate in the observational data. We discuss briefly the most probable future evolution of NGC 7479 in the light of our minor-merger simulations, and conclude that NGC 7479 is likely to evolve toward an earlier Hubble type.     

The NGC 1275 enigma

The following arguments suggest that NGC 1275 does not consist of a giant elliptical (E) galaxy that is colliding with (or is superimposed on) a late-type spiral (L):

• 1 The total diameter of the region containing young associations is 33 (100/H) kpc. This size is characteristic of ScI galaxies. Neither the morphology nor the integrated luminosity of the L component of NGC 1275 supports such a classification.
• 2 The chaotic appearance of the L component of NGC 1275 is unlikely to be due to tidal damage. This is so because: (a) the E and L components are still approaching each other, (b) their relative velocity is ≈︁ 3000 km s⁻¹, (c) no stripped galaxy core (which would survive a catastrophic tidal encounter) is seen near NGC 1275.
• 3 The core of the Perseus cluster contains only one (anemic) spiral. The a priori probability that NGC 1275 represents a chance superimposition (or collision) of a spiral and an elliptical galaxy is therefore low.
• 4 The assumption that the L component of NGC 1275 is superimposed on, but not interacting with, the E component does not account for (a) the presence of an active SEYFERT nucleus, (b) the peculiar filamentary HII shell, discovered by LYNDS , (c) the presence of recently-formed stars, (d) the X-ray emission and the radio emission of NGC1275.

     

Possible dark matter in spiral galaxies of the coma cluster

The relationship between the rotation curves for the galaxies and the distribution of mass and angular momentum within the galaxies is examined. The theory of angular momentum transfer is applied to the observed properties of the galaxies. The coupling between the dynamical mass of a spiral galaxy and its luminosity is studied. Most of the spiral galaxies in subclusters surrounding NGC 4889, NGC 4874, and NGC 4839 in the Coma cluster are galaxies that have lower luminosities, with M_B fainter than −21^m.5. These galaxies are characterized by a higher mass-to-luminosity ratio than that of the galaxies with higher luminosities M_B brighter than −21^m.5, which suggests the presence of a large fraction of dark matter in the spiral galaxies of the subclusters. Translated from Astrofizika, Vol. 52, No. 1, pp. 75–84 (February 2009).     

Is C/O enhanced in NGC 253?

We have mapped the nuclear region of the starburst galaxy NGC 253 in the³ P ₁ ³ P ₀ line of neutral carbon using the JCMT. Carbon is widespread across the nuclear region with a similiar distribution to CO as expected. Previous studies of Galactic star-forming regions showed that carbon emission is enhanced in photon-dominated regions (where UV photons impinge upon molecular clouds). Previous observations of other PDR tracers such as ionized carbon and FIR continuum constrain the physical conditions in the PDR gas of NGC 253. The carbon we have observed is far brighter than predicted by theoretical models of PDRs with solar elemental values. This indicates that carbon emission is not a reliable diagnostic of the physical conditions in the nuclear region of a galaxy undergoing a burst of star formation.     

NGC 1705: The missing link between blue compact dwarf and dwarf elliptical galaxies?

We report results of a detailed study of the nearby blue compact dwarf galaxy NGC 1705. There are two stellar populations in NGC 1705: an underlying population around a Gyr old or older, and a young population of fairly continuous star formation for the last 250 Myr. An off-centre nucleus, which is probably a young (10 Myr old), massive (10⁶ M ) globular cluster, is part of this young population. The H morphology and emission line kinematics are consistent with an expulsive flow triggered by the formation of the nucleus. We show that NGC 1705 is losing ionized material in this galactic wind with neutral material probably entrained in the flow. This flow may lead to total removal of the ISM. The evolutionary fate of NGC 1705 is uncertain but the most likely outcome is rapid evolution into a nucleated dwarf elliptical galaxy.     

The early-type galaxies NGC 1407 and NGC 1400 – I. Spatially resolved radial kinematics and surface photometry

This is the first paper of a series focused on investigating the star formation and evolutionary history of the two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study.
Here, we present new high signal-to-noise ratio long-slit spectroscopic data obtained at the ESO 3.6-m telescope and high-resolution multiband imaging data from the Hubble Space Telescope /Advanced Camera for Surveys and wide-field imaging from Subaru Suprime-Cam. We spatially resolved integrated spectra out to ∼0.6 (NGC 1407) and ∼1.3 (NGC 1400) effective radii. The radial profiles of the kinematic parameters v _rot, σ, h ₃ and h ₄ are measured. The surface brightness profiles are fitted to different galaxy light models and the colour distributions analysed. The multiband images are modelled to derive isophotal shape parameters and residual galaxy images. The parameters from the surface brightness profile fitting are used to estimate the mass of the possible central supermassive black hole in NGC 1407. The galaxies are found to be rotationally supported and to have a flat core in the surface brightness profiles. Elliptical isophotes are observed at all radii and no fine structures are detected in the residual galaxy images. From our results, we can also discard a possible interaction between NGC 1400, NGC 1407 and the group intergalactic medium. We estimate a mass of  ∼1.03 × 10⁹ M_⊙  for the supermassive black hole in NGC 1407 galaxy.     

Survey of galaxies in the local volume in Hα: Faint companions of M31

Observations on the 6-m telescope in the Hα line and in the continuum are reported for 10 dwarf companions of the galaxy M31: And I, And II, And III, And V, And IX, And X, Cass dSph, Peg DSph, NGC147, NGC221, and one irregular dwarf galaxy in the background, And IV. All the observed companions of M31 have current star formation rates (SFR) on the order of or less than 10⁻⁶ M_⊙ /yr. On a “star formation rate-neutral hydrogen mass” diagram for galaxies in the local volume, the dwarf spheroidal companions of Andromeda lie in the region of extremely low values for these parameters. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 337–350 (August 2006).     

Theoretical estimates of intrinsic galaxy alignment

It has recently been argued that the observed ellipticities of galaxies may be determined at least in part by the primordial tidal gravitational field in which the galaxy formed. Long-range correlations in the tidal field could thus lead to an ellipticity–ellipticity correlation for widely separated galaxies. We present a new model relating ellipticity to angular momentum, which can be calculated in linear theory. We use this model to calculate the angular power spectrum of intrinsic galaxy shape correlations. We show that, for low-redshift galaxy surveys, our model predicts that intrinsic correlations will dominate correlations induced by weak lensing, in good agreement with previous theoretical work and observations. We find that our model produces ' E -mode' correlations enhanced by a factor of 3.5 over B -modes on small scales, making it harder to disentangle intrinsic correlations from those induced by weak gravitational lensing.