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RE: NGC 4449
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Title: A multi-wavelength study of the Magellanic-type galaxy NGC 4449. I. Modelling the spectral energy distribution, the ionisation structure and the star formation history
Authors: O. . Karczewski, M. J. Barlow, M. J. Page, M. Baes, G. J. Bendo, A. Boselli, A. Cooray, D. Cormier, I. De Looze, I. Ferreras, M. Galametz, F. Galliano, N. P. M. Kuin, V. Lebouteiller, S. C. Madden, M. Pohlen, A. Rémy-Ruyer, M. W. L. Smith, L. Spinoglio

We present an integrated photometric spectral energy distribution (SED) of NGC 4449 from the far-UV to the sub-mm, including new observations acquired by Herschel. We include integrated UV photometry from Swift/UVOT using a measurement technique which is appropriate for extended sources with coincidence loss.
The available multi-wavelength data is used to infer a range of ages, metallicities and star formation rates (SFR) for the underlying stellar populations, as well as the composition and the total mass of dust in NGC 4449. Our analysis of the global optical spectrum of NGC 4449 fitted using the spectral fitting code STARLIGHT suggests that the majority of stellar mass resides in old (>~ 1 Gyr old) and metal-poor (Z/Zsolar ~ 0.2) populations, with the first onset of star formation activity deduced to have taken place at an early epoch, approximately 12 Gyr ago. A simple chemical evolution model, suitable for a galaxy continuously forming stars, suggests a ratio of carbon to silicate dust mass comparable to that of the LMC over the inferred timescales.
We present an iterative scheme, which allows us to build an in-depth and multi-component representation of NGC 4449 `bottom-up', taking advantage of the broad capabilities of the photoionisation and radiative transfer code MOCASSIN. We fit the observed SED, the global ionisation structure and the emission line intensities, and infer a recent SFR of 0.4 Msolar/yr and a total stellar mass of approximately 1e9 Msolar radiating with a bolometric luminosity of 5.7e9 Lsolar. Our fits yield a total dust mass of 2.9e6 Msolar including 2 per cent attributed to PAHs. We deduce a dust to gas mass ratio of 1/190 within the modelled region. While we do not consider possible additional contributions from even colder dust, we note that including the extended HI envelope and the molecular gas is likely to bring the ratio down to as low as ~ 1/800.

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NGC 4449B
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Title: A tidally distorted dwarf galaxy near NGC 4449
Authors: R. M. Rich, M. L. M. Collins, C. M. Black, F. M. Longstaff, A. Koch, A. Benson, D. B. Reitzel

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 galaxy's centre.

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NGC 4449
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Subaru Telescope Captures Images of the "Stealth Merger" of Dwarf Galaxies

An international team of scientists led by David Martinez-Delgado (Max Planck Institute for Astronomy, Germany) has conducted research that reveals a "stealth merger" of dwarf galaxies, where an in-falling satellite galaxy is nearly undetectable by conventional means yet has a substantial influence on its host galaxy. Aaron Romanowsky (University of California Observatories in Santa Cruz) along with graduate student Jacob Arnold (UCSC) used the Subaru Telescope to obtain high-resolution images of individual stars in a dense stream of stars in the outer regions of a nearby dwarf galaxy (NGC 4449); these outlying stars are the remains of an even smaller companion galaxy in the process of merging with its host. NGC 4449, the host galaxy, is the smallest primary galaxy in which a stellar stream from an ongoing merger has been identified and studied in detail.
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Title: Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales
Authors: David Martinez-Delgado (MPIA), Aaron J. Romanowsky (UCO), R. Jay Gabany (BBO), Francesca Annibali (Bologna), Jacob A. Arnold (UCO), Juergen Fliri (LERMA, GEPI), Stefano Zibetti (Dark Cosmology Centre), Roeland P. van der Marel (STScI), Hans-Walter Rix (MPIA), Taylor S. Chonis (Texas), Julio A. Carballo-Bello (IAC), Alessandra Aloisi (STScI), Andrea V. Maccio (MPIA), J. Gallego-Laborda (FNO), Jean P. Brodie (UCO), Michael R. Merrifield (Nottingham)

We map and analyse a stellar stream in the halo of the nearby dwarf starburst galaxy NGC 4449, detecting it in deep integrated-light images using the Black Bird Observatory 0.5-meter telescope, and resolving it into red giant branch stars using Subaru/Suprime-Cam. The properties of the stream imply a massive dwarf spheroidal progenitor, which will continue to disrupt and deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ~1:50, while the dynamical mass-ratio when including dark matter may be ~1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

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Title: Cluster 77 in NGC 4449: The nucleus of a satellite galaxy being transformed into a globular cluster?
Authors: Francesca Annibali (INAF-OABo), Monica Tosi (INAF-OABo), Alessandra Aloisi (STScI), Roeland P. van der Marel (STScI), David Martinez-Delgado (MPIA)

We report the discovery in our HST ACS B, V, and I images of NGC 4449 of a globular cluster (GC) which appears associated with two tails of blue stars. The cluster is massive (M~1.7x10^6 M_sun) and highly flattened (\epsilon~0.24). From the color-magnitude diagrams of the resolved stars we infer active star formation in the tails over the past ~200 Myr. In a diagram of mean projected mass density inside r_e versus total mass the cluster lies at the upper end of the GC distribution, where galaxy nuclei are. The north-west tail is associated with a concentration of HI and infrared (dust/PAHs) emission which appears as part of a much longer stream wrapping around the galaxy. These properties suggest that the cluster may be the nucleus of a former gas-rich satellite galaxy undergoing tidal disruption by NGC 4449. If so, the cluster is seen in an earlier phase compared to other suggested nuclei of disrupted galaxies such as \omega Cen (Milky Way) and G1 (M31).

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Zoom in the Galaxy NGC 4449

[youtube=http://youtube.com/watch?v=h-4WLVyZtdY]

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A small galaxy ablaze with a burst of star formation appears in a Hubble Space Telescope image released on Tuesday. A merger with another galaxy may have triggered the colourful display.
The galaxy, called NGC 4449, lies 12.5 million light-years away in the constellation Canes Venatici and is much smaller than our own Milky Way.
Bluish-white clusters of massive, young stars and reddish patches where new stars are forming behind veils of dust suggest the dwarf galaxy is giving birth to stars at a tremendous rate.

Watch an animation zooming in on the NGC 4449. (2.74mb, mpeg)

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Nearly 12.5 million light-years away in the dwarf galaxy NGC 4449 a veritable stellar "fireworks" is on display here shown in exquisite detail through the eyes of the Hubble Space Telescope.

Hundreds of thousands of vibrant blue and red stars are visible in this new image of galaxy NGC 4449 taken by the NASA/ESA Hubble Space Telescope. Hot bluish white clusters of massive stars are scattered throughout the galaxy, interspersed with numerous dustier reddish regions of current star formation. Massive dark clouds of gas and dust are silhouetted against the flaming starlight.

july_NGC4449_e1
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NGC 4449, part of a group of galaxies in the constellation Canes Venatici.
Credit: NASA, ESA, A. Aloisi (ESA/STScI)


NGC 4449 has been forming stars since several billion years ago, but currently it is experiencing a star formation event at a much higher rate than in the past. This unusual explosive and intense star formation activity qualifies as a starburst. At the current rate, the gas supply that feeds the stellar production would only last for another billion years or so.
Starbursts usually occur in the central regions of galaxies, but NGC 4449 has a more widespread star formation activity, since the very youngest stars are observed both in the nucleus and in streams surrounding the galaxy.
A "global" starburst like NGC 4449 resembles primordial star forming galaxies which grew by merging with and accreting smaller stellar systems. Since NGC 4449 is close enough to be observed in great detail, it is the ideal laboratory for the investigation of what may have occurred during galactic formation and evolution in the early Universe.
Its likely that the current widespread starburst was triggered by interaction or merging with a smaller companion. NGC 4449 belongs to a group of galaxies in the constellation Canes Venatici, the Hunting Dogs. Astronomers think that NGC 4449s star formation has been influenced by interactions with several of its neighbours.

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