490 research outputs found
3D ISM-Shock Spectral Emission: X-ray models for Radio Galaxy SED Modeling
Galaxies form out of small fluctuations in a smoothly expanding Universe.
However, the initial gravitational collapse phase is accompanied by the
formation of supermassive black holes and clusters of massive stars. Black
holes and star clusters generate powerful outflows in the form of jets and
superwinds that interact with still infalling gas, possibly regulating the
galaxy formation process, initiating new sites of star formation, and carrying
chemically enriched gas to the intergalactic medium. Unfortunately, beyond this
qualitative description our detailed theoretical understanding is poor.
New results from 3D simulations of a GPS/CSS galaxy, with gravitational
potentials included, shed some new light on the jet driven outflow process in
particular. New code capabilites to predict detailed X-ray spectra from
multi-dimensional time-dependent dynamics simulations of Galaxy Feedback, and
will be useful for future interpretation of X-ray and radio SEDs of forming
galaxies.Comment: 10 pages, 5 figures, Invited Conference Talk, The Spectral Energy
Distribution of Gas Rich Galaxies: Confronting Models with Data, Heidelberg,
Germany,October 4-8, 200
The Infrared Emission from the Narrow Line Region
We present models for the mid- and far- infrared emission from the Narrow
Line Region (NLR) of Active Galactic Nuclei (AGN). Using the MAPPINGS III code
we explore the effect of typical NLR parameters on the spectral characteristics
of the IR emission. These include useful IR emission line ratio diagnostic
diagrams for the determination of these parameters, as well as Star
formation--AGN mixing diagnostics. We also examine emission line to continuum
correlations which would assist in separating the IR emission arising from the
NLR from that coming from the inner torus. We find for AGN like NGC 1068 and
NGC 4151 that the NLR only contributes ~10% to the total IRAS 25 mum flux, and
that other components such as a dusty torus are necessary to explain the total
AGN IR emission.Comment: 15 pages, 12 figures, accepted for publication in A&A. Paper with
full resolution figures available at
http://www.mpa-garching.mpg.de/~brent/publications/bgrovesnlrIRpaper.pd
Ultrafaint Dwarfs - Star Formation and Chemical Evolution in the Smallest Galaxies
In earlier work we showed that a dark matter halo with a virial mass of
M can retain a large percentage of its baryons in the face of
the pre-ionization phase and supernova explosion from a 25 star. Here
we expand on the results of that work, investigating the star formation and
chemical evolution of the system beyond the first supernova. In a galaxy with a
mass M, sufficient gas is retained by the potential for
a second period of star formation to occur. The impact of a central explosion
is found to be much stronger than that of an off-centred explosion both in
blowing out the gas and in enriching it, as in the off-centered case most of
the supernova energy and metals escape into the IGM. We model the star
formation and metallicity given the assumption that stars form for 100, 200,
400 and 600 Myr and discuss the results in the context of recent observations
of very low mass galaxies. We show that we can account for most features of the
observed relationship between [/Fe] and [Fe/H] in ultra-faint dwarf
galaxies with the assumption that the systems formed at a low mass, rather than
being remnants of much larger systems
Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization
New observations indicate that ultrafaint dwarf galaxies (UFD) -- the least
luminous systems bound by dark matter halos (<10^5 Lsun) -- may have formed
before reionization. The extrapolated virial masses today are uncertain with
estimates ranging from 10^8 Msun to 10^9 Msun. We show that the progenitor halo
masses of UFDs can be as low as Mvir = 10^7 Msun. Under the right conditions,
such a halo can survive the energy input of a supernova and its radiative
progenitor. A clumpy medium is much less susceptible to both internal and
external injections of energy. It is less prone to SN sweeping because the
coupling efficiency of the explosive energy is much lower than for a diffuse
ISM. With the aid of the 3D hydro/ionization code Fyris, we show that
sufficient baryons are retained to form stars following a single supernova
event in dark matter halos down to Mvir ~ 10^7 Msun with radiative cooling. The
gas survives the SN explosion, is enriched with the abundance yields of the
discrete events, and reaches surface densities where low mass stars can form.
Our highest resolution simulations reveal why cooling is so effective in
retaining gas compared to any other factor. In the early stages, the super-hot
metal-enriched SN ejecta exhibit strong cooling, leading to much of the
explosive energy being lost. Consistent with earlier work, the baryons do *not*
survive in smooth or adiabatic models in the event of a supernova. The smallest
galaxies carry signatures of the earliest epochs of star formation, which may
distinguish a small primordial galaxy from one that was stripped down to its
present size through tidal interaction. We discuss these results in the context
of local UFDs and damped Ly-alpha systems (z~2) at very low metallicity ([Fe/H]
~ -3). We show that both classes of objects are consistent with primordial
low-mass systems that have experienced only a few enrichment events.Comment: 32 pages, 25 figures, accepted by Ap
The Magellanic Stream: break up and accretion onto the hot Galactic corona
The Magellanic HI Stream (~2x10^9 Msun [d/55 kpc]^2) encircling the Galaxy at
a distance 'd' is arguably the most important tracer of what happens to gas
accreting onto a disk galaxy. Recent observations reveal that the Stream's mass
is in fact dominated (3:1) by its ionised component. Here we revisit the origin
of the mysterious H-alpha recombination emission observed along much of its
length that is overly bright (~150-200 milli-Rayleigh) for the known Galactic
ultraviolet background (~20-40 mR / [d/55 kpc]^2). In an earlier model, we
proposed that a slow shock cascade was operating along the Stream due to its
interaction with the extended Galactic hot corona. We find that, for a smooth
coronal density profile, this model can explain the bright H-alpha emission if
the coronal density satisfies 2 < (n / 10^{-4} cm^{-3}) < 4 at d = 55 kpc. But
in view of updated parameters for the Galactic halo and mounting evidence that
most of the Stream must lie far beyond the Magellanic Clouds (d>55 kpc), we
revisit the shock cascade model in detail. At lower densities, the HI gas is
broken down by the shock cascade but mostly mixes with the hot corona without
significant recombination. At higher densities, the hot coronal mass (including
the other baryonic components) exceeds the baryon budget of the Galaxy. If the
H-alpha emission arises from the shock cascade, the upper limit on the smooth
coronal density constrains the Stream's mean distance to < 75 kpc. If, as some
models indicate, the Stream is even further out, either the shock cascade is
operating in a regime where the corona is substantially mass-loaded with recent
gas debris, or an entirely different ionization mechanism is responsible.Comment: Significant expansion of the parameter space explored in response to
referee's comments. ApJ accepte
The Centaurus A Northern Middle Lobe as a Buoyant Bubble
We model the northern middle radio lobe of Centaurus A (NGC 5128) as a
buoyant bubble of plasma deposited by an intermittently active jet. The extent
of the rise of the bubble and its morphology imply that the ratio of its
density to that of the surrounding ISM is less than 10^{-2}, consistent with
our knowledge of extragalactic jets and minimal entrainment into the precursor
radio lobe. Using the morphology of the lobe to date the beginning of its rise
through the atmosphere of Centaurus A, we conclude that the bubble has been
rising for approximately 140Myr. This time scale is consistent with that
proposed by Quillen et al. (1993) for the settling of post-merger gas into the
presently observed large scale disk in NGC 5128, suggesting a strong connection
between the delayed re-establishment of radio emission and the merger of NGC
5128 with a small gas-rich galaxy. This suggests a connection, for radio
galaxies in general, between mergers and the delayed onset of radio emission.
In our model, the elongated X-ray emission region discovered by Feigelson et
al. (1981), part of which coincides with the northern middle lobe, is thermal
gas that originates from the ISM below the bubble and that has been uplifted
and compressed. The "large-scale jet" appearing in the radio images of Morganti
et al. (1999) may be the result of the same pressure gradients that cause the
uplift of the thermal gas, acting on much lighter plasma, or may represent a
jet that did not turn off completely when the northern middle lobe started to
buoyantly rise. We propose that the adjacent emission line knots (the "outer
filaments") and star-forming regions result from the disturbance, in particular
the thermal trunk, caused by the bubble moving through the extended atmosphere
of NGC 5128.Comment: 38 pages, 13 figures, submitted to ApJ; a version with higher
resolution figures is available at
http://www.mso.anu.edu.au/~saxton/papers/cena.pd
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