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 (ref. 3). It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning ∼90 kiloparsecs (kpc; refs 1, 4). 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 H I 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 galaxy's centre.
An important question in extragalactic astronomy concerns the distribution of black hole accretion rates of active galactic nuclei (AGN). Based on observations at X-ray wavelengths, the observed Eddington ratio distribution appears as a power law, while optical studies have often yielded a lognormal distribution. There is increasing evidence that these observed discrepancies may be due to contamination by star formation and other selection effects. Using a sample of galaxies from the Sloan Digital Sky Survey Data Release 7, we test if an intrinsic Eddington ratio distribution that takes the form of a Schechter function is consistent with previous work that suggests that young galaxies in optical surveys have an observed lognormal Eddington ratio distribution. We simulate the optical emission line properties of a population of galaxies and AGN using a broad instantaneous luminosity distribution described by a Schechter function near the Eddington limit. This simulated AGN population is then compared to observed galaxies via the positions on an emission line excitation diagram and Eddington ratio distributions. We present an improved method for extracting the AGN distribution using BPT diagnostics that allows us to probe over one order of magnitude lower in Eddington ratio counteracting the effects of dilution by star formation. We conclude that for optically selected AGN in young galaxies, the intrinsic Eddington ratio distribution is consistent with a possibly universal, broad power law with an exponential cutoff, as this distribution is observed in old optically selected galaxies and in X-rays.
We report on the discovery of strong tidal features around a dwarf spheroidal galaxy in the Hydra I galaxy cluster, indicating its ongoing tidal disruption. This very low surface brightness object, HCC-087, was originally classified as an early-type dwarf in the Hydra Cluster Catalogue (HCC), but our re-analysis of the ESO-VLT/FORS images of the HCC unearthed a clear indication of an S-shaped morphology and a large spatial extent. Its shape, luminosity (M V = −11.6 mag), and physical size (at a half-light radius of 3.1 kpc and a full length of ∼5.9 kpc) are comparable to the recently discovered NGC 4449B and the Sagittarius dwarf spheroidal, all of which are undergoing clear tidal disruption. Aided by N-body simulations we argue that HCC-087 is currently at its first apocenter, at 150 kpc, around the cluster center and that it is being tidally disrupted by the galaxy cluster's potential itself. An interaction with the near-by (50 kpc) S0 cluster galaxy HCC-005, at M * ∼ 3 × 10 10 M ⊙ is rather unlikely, as this constellation requires a significant amount of dynamical friction and thus low relative velocities. The S-shaped morphology and large spatial extent of the satellite would, however, also appear if HCC-087 would orbit the cluster center. These features appear to be characteristic properties of satellites that are seen in the process of being tidally disrupted, independent of the environment of the destruction. An important finding of our simulations is an orientation of the tidal tails perpendicular to the orbit.
We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000−6000 Å in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 Å feature shows a progressive central wavelength shift from ∼4600 Å to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature's red-ward shift slowing or halting at an [Fe III] blend centroid ∼4700 Å as has been proposed. The width of this feature also steadily increases with a FWHM ∼170 Å at day +100 growing to 200 Å or more by day +350. Two weaker adjacent features at around 4850 and 5000 Å exhibit similar red shifts to that of the 4700 Å feature but show no change in width until very late times. We discuss possible causes for the observed red shifts of these late-time optical features including contribution from [Co II] emission at early nebular epochs and the emergence of additional features at later times. We conclude that the ubiquitous red shift of these common late-time SN Ia spectral features is not mainly due to a decrease in line velocities of forbidden Fe emissions, but the result of decreasing line velocities and opacity of permitted Fe absorption lines.
Deep Hα images of a faint emission complex 4.0 • × 5.5 • in angular extent and located far off the Galactic plane at l = 70.0 • , b = −21.5 • reveal numerous thin filaments suggestive of a supernova remnant's shock emission. Low dispersion optical spectra covering the wavelength range 4500 -7500 A show only Balmer line emissions for one filament while three others show a Balmer dominated spectrum along with weak [N I] 5198, 5200Å, [O I] 6300, 6364Å, [N II] 6583Å, [S II] 6716, 6731Å and in one case [O III] 5007Å line emission. Many of the brighter Hα filaments are visible in near UV GALEX images presumably due to C III] 1909Å line emission. ROSAT All Sky Survey images of this region show a faint crescent shaped X-ray emission nebula coincident with the portion of the Hα nebulosity closest to the Galactic plane.The presence of long, thin Balmer dominated emission filaments with associated UV emission and coincident X-ray emission suggests this nebula is a high latitude Galactic supernova remnant despite a lack of known associated nonthermal radio emission. Relative line intensities of the optical lines in some filaments differ from commonly observed [S II]/Hα ≥ 0.4 radiative shocked filaments and typical Balmer filaments in supernova remnants. We discuss possible causes for the unusual optical SNR spectra. Subject headings: ISM: individual objects: G70.0-21.5, ISM: supernova remnant -shock waves -optical -X-rays: ISM
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