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Post Info TOPIC: Tidal Dwarf Galaxies


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Dwarf-spheroidal satellites
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Title: Dwarf-spheroidal satellites: are they of tidal origin?
Authors: M. Metz, P. Kroupa

The Milky Way and Andromeda must have formed through an initial epoch of sub-structure merging. As a result of fundamental physical conservation laws tidal-dwarf galaxies (TDGs) have likely been produced. Here we show that such TDGs appear, after a Hubble-time of dynamical evolution in the host dark-matter halo, as objects that resemble known dSph satellite galaxies. We discuss the possibility that some of the Milky Way's satellites may be of tidal origin.

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RE: Tidal Dwarf Galaxies
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Title: Satellite Accretion Onto Massive Galaxies With Central Black Holes
Authors: Michael Boylan-Kolchin, Chung-Pei Ma (UC Berkeley)
(revised v2)

Minor mergers of galaxies are expected to be common in a hierarchical cosmology such as Lambda CDM and have the potential to significantly affect galactic structure. In this paper we dissect the case-by-case outcome from a set of numerical simulations of a single satellite elliptical galaxy accreting onto a massive elliptical galaxy. We take care to explore cosmologically relevant orbital parameters and to set up realistic initial galaxy models that include all three relevant dynamical components: dark matter halos, stellar bulges, and central massive black holes. The effects of several different parameters are considered, including orbital energy and angular momentum, satellite density and inner density profile, satellite-to-host mass ratio, and presence of a black hole at the centre of the host. Black holes play a crucial role in protecting the shallow stellar cores of the hosts, as satellites merging onto a host with a central black hole are more strongly disrupted than those merging onto hosts without black holes. Orbital parameters play an important role in determining the degree of disruption: satellites on less bound or more eccentric orbits are more easily destroyed than those on more bound or more circular orbits as a result of an increased number of pericentric passages and greater cumulative effects of gravitational shocking and tidal stripping. In addition, satellites with densities typical of faint elliptical galaxies are disrupted relatively easily, while denser satellites can survive much better in the tidal field of the host. Over the range of parameters explored, we find that the accretion of a single satellite elliptical galaxy can result in a broad variety of changes, in both signs, in the surface brightness profile and colour of the central part of an elliptical galaxy.

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Theory of our Galaxy's Birth
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New Findings Cast Doubt on Leading Theory of our Galaxy's Birth
Studies of nearby small galaxies by an international collaboration of astronomers including McDonald Observatory’s Matthew Shetrone are casting doubt on a widely held theory explaining the Milky Way’s formation. The theory holds that our galaxy and other large galaxies are born from the collisions and mergers of smaller galaxies similar to those we see today around the Milky Way.
The large survey of nearby dwarf galaxies, still in progress, is showing that stars in these supposed building blocks of the Milky Way don’t have the expected chemical composition. The research will be published in tomorrow’s edition of Astrophysical Journal Letters.

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RE: Tidal Dwarf Galaxies
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Title: Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. II. Early-type dwarfs with central star formation
Authors: Thorsten Lisker, Katharina Glatt, Pieter Westera, Eva K. Grebel (University of Basel)

Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colours caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centres reach a fraction of more than 15% of the dE population at brighter (B<=16) magnitudes. A spectral analysis of the centres of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with M(old)>=90% for all but one object. Therefore the majority of these galaxies will appear like ordinary dEs within ~one Gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies, but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centres.

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Baryon Content
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Title: The Baryon Content of Extremely Low Mass Dwarf Galaxies
Authors: M. Geha, M. R. Blanton, M. Masjedi, A. A. West

We investigate the gas content and baryonic Tully-Fisher relationship for extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 > Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists of 101 galaxies for which we have obtained follow-up HI observations using the Arecibo Observatory and Green Bank Telescope. This represents the largest homogeneous sample of dwarfs at low luminosities with well-measured HI and optical properties. The sample spans a range of environments, from dense groups to truly isolated galaxies. The average neutral gas fraction is fgas=0.6, significantly exceeding that of typical gas-rich galaxies at higher luminosities. Dwarf galaxies are therefore less efficient at turning gas into stars over their lifetimes. The strong environmental dependence of the gas fraction distribution demonstrates that while internal processes can reduce the gas fractions to roughly fgas=0.4, external processes are required to fully remove gas from a dwarf galaxy. The average rotational velocity of our sample is vrot=50 km/s. Including more massive galaxies from the literature, we fit a baryonic Tully-Fisher slope of Mbaryon \propto vrot^(3.70 ± 0.15). This slope compares well with CDM models that assume an equal baryon to dark matter ratio at all masses. While gas stripping or other processes may modify the baryon to dark matter ratio for dwarfs in the densest environments, the majority of dwarf galaxies in our sample have not preferentially lost significant baryonic mass relative to more massive galaxies.

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NGC5291
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When galaxies collide (as our galaxy, the Milky Way, eventually will with the nearby Andromeda galaxy), what happens to matter that gets spun off in the collision's wake?

With help from the Spitzer Space Telescope's infrared spectrograph (IRS) and infrared array camera (IRAC), Cornell astronomers are beginning to piece together an answer to that question. Specifically, they are gaining new insight into how some ubiquitous dwarf galaxies form, interact, and arrange themselves into new systems.

Dwarf galaxies, with stellar masses around 0.1 percent that of the Milky Way, are far more common than their more massive spiral or starburst counterparts. Some may be primordial remnants of the Big Bang; but others -- called tidal dwarfs -- formed later as a result of gravitational interactions after galactic collisions.
To understand which dwarf galaxies are tidal in origin and how those galaxies differ from primordial dwarf galaxies, Cornell researcher Sarah Higdon and her colleagues studied a galactic merger called NGC 5291, which is 200 million light-years from Earth and roughly four times the size of the Milky Way. At the system's centre are two colliding galaxies; behind them trail a string of much smaller dwarfs.


This false-colour infrared image from NASA's Spitzer Space Telescope shows little "dwarf galaxies" forming in the "tails" of two larger galaxies that are colliding together. The big galaxies are at the centre of the picture, while the dwarfs can be seen as red dots in the red streamers, or tidal tails. The two blue dots above the big galaxies are stars in the foreground.
Position (2000): RA: 13h47m24.5s Dec: -30d24m25s
Size: 8.5 x 7.1 arcmin

The researchers focused on the system because they knew from earlier analyses that the trailing dwarfs were formed tidally as a result of the central collision. Until recently, though, they hadn't been able to look closely enough at the tidal dwarfs to catalogue their properties for comparison with those of similar galaxies.
Spitzer's sharp eye has changed that. Using it to look for compounds that indicate star-forming activity, Higdon's team found that when it comes to fostering new star formation, the colliding galaxies at the system's centre are fairly dull. The exciting place to be, they found, is in the tidal dwarfs at the system's edges.
Specifically, the team found that the tidal dwarfs show strong emission from organic compounds, found in crude petroleum, burnt toast, and (more relevantly) stellar nurseries, known as PAHs -- for polycyclic aromatic hydrocarbons. And for the first time, the researchers detected warm molecular hydrogen -- another indicator of star formation, and one that has never before been directly measured in tidal dwarf galaxies.

"We know molecular hydrogen is out there. Now we have the sensitivity to measure it" - Sarah Higdon.

Higdon and Cornell colleagues James Higdon and Jason Marshall describe the features of the NGC 5291 system in a forthcoming issue of the Astrophysical Journal.

"Nearly everything at some stage interacts. This is a part of the puzzle. But we've only just started looking. We don't know how long lived (the tidal dwarf galaxies) will be, or how many formed like this" - Sarah Higdon.

Next, the team plans to search for new tidal dwarf galaxies using the Spitzer surveys and compare their properties to the newly catalogued galaxies in NGC 5291.

Source

-- Edited by Blobrana at 15:01, 2005-12-01

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Posts: 131433
Date:
Tidal Dwarf Galaxies
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A new method to detect small, faint galaxies that spring up in the wake of violent galactic collisions has been devised by astronomers using the Spitzer Space Telescope. The method could shed light on how most of the galaxies near our own formed.

The vast majority of local galaxies are "dwarfs" – our galaxy, the Milky Way, has 1000 times more mass in stars. But it is not clear how these dwarfs form.
Some may have condensed directly from primordial gas soon after the big bang. But astronomers are not sure these lightweight galaxies would have been able to survive unscathed the relatively common galactic smash-ups that occurred in the early universe.
Now, a team of astronomers from Cornell University in Ithaca, New York, US, is trying to answer that question by studying a population of dwarf galaxies that were definitely born when two larger galaxies slammed into each other.


Position(2000): RA 13h47m24.5s Dec -30d24m25s


"It's important to compare the properties of the ones we know with those that may have lasted since the big bang" - Jim Higdon, team member.

The objects studied by the team are called tidal dwarf galaxies (TDGs), because they are created when two galaxies interact gravitationally. They form in long, tail-like structures but are so small and faint that only about a dozen had previously been observed.



But now the Cornell team has used Spitzer's exquisite sensitivity at infrared wavelengths to identify 15 TDGs around a pair of merging galaxies, together called NGC 5291, that lie 200 million light years from Earth. The galaxies are strung along two arcs of stars and gas stretching about 240,000 light years behind each of the two larger galaxies.

Previous ultraviolet measurements had suggested these arcs contained "knots" of star formation. Spitzer confirmed this, revealing these modest TDGs are actually powerhouses of star formation – boasting about 200 million sun-like stars that are just a few million years old.

The researchers also discovered the merger-induced dwarf galaxies have a telltale infrared spectrum. They are now searching for this signature in large infrared sky surveys to understand what fraction of other dwarf galaxies formed this way.

"We want to understand how the universe was built up" - Sarah Higdon, team leader.

The research will be published in a future issue of the Astrophysical Journal.




Title: NGC 5291: Implications for the Formation of Dwarf Galaxies
Authors: Malphrus, Benjamin K.; Simpson, Caroline E.; Gottesman, S. T.; Hawarden, Timothy G.

The possible formation and evolution of dwarf irregular galaxies from material derived from perturbed evolved galaxies is addressed via an H I study of a likely example, the peculiar system NGC 5291.
This system, located in the western outskirts of the cluster Abell 3574, contains the lenticular galaxy NGC 5291 which is in close proximity to a disturbed companion and is flanked by an extensive complex of numerous knots extending roughly 4 min north and 4 min south of the galaxy.
In an initial optical and radio study, Longmore et al. (1979, MNRAS, 188, 285) showed that these knots have the spectra of vigorous star-forming regions, and suggested that some may in fact be young dwarf irregular galaxies.
High resolution 21-cm line observations taken with the VLA are presented here and reveal that the H I distribution associated with this system encompasses not only the entire N-S complex of optical knots, but also forms an incomplete ring or tail that extends approximately 3 min to the west.
The H I associated with NGC 5291 itself shows a high velocity range; the Seashell is not detected. The formation mechanism for this unusual system is unclear and two models - a large, low-luminosity ram-swept disk, and a ram-swept interaction-are discussed.
The H I in the system contains numerous concentrations, mostly along the N-S arc of the star-forming complexes, which generally coincide with one or more optical knots; the larger H I features contain several x 10^9 solar mass of gas. Each of the knots is compared to a set of criteria designed to determine if these objects are bound against their own internal kinetic energy and are tidally stable relative to the host galaxy.
An analysis of the properties of the H I concentrations surrounding the optical star-forming complexes indicates that at least the largest of these is a bound system; it also possesses a stellar component. It is suggested that this object is a genuinely young dwarf irregular galaxy that has evolved from the material associated with the system and that this entire complex contains several proto- or young dwarf irregular galaxies in various stages of development. We are therefore witnessing the early evolution of a number of genuinely young galaxies.
Given the evident importance of the NGC 5291 system as a 'nursery' for young galaxies, careful modelling is required if we are to understand this remarkable galaxy.

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