31 research outputs found
A Sagittarius-Induced Origin for the Monoceros Ring
The Monoceros ring is a collection of stars in nearly-circular orbits at
roughly 18 kpc from the Galactic center. It may have originated (i) as the
response of the disc to perturbations excited by satellite companions or (ii)
from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros
stars differs from that of disc stars at comparable Galactocentric distances,
an observation that disfavours the first scenario. On the other hand, circular
orbits are difficult to accommodate in the tidal-disruption scenario, since it
requires a satellite which at the time of disruption was itself in a nearly
circular orbit. Such satellite could not have formed at the location of the
ring and, given its low mass, dynamical friction is unlikely to have played a
major role in its orbital evolution. We search cosmological simulations for
low-mass satellites in nearly-circular orbits and find that they result, almost
invariably, from orbital changes induced by collisions with more massive
satellites: the radius of the circular orbit thus traces the galactocentric
distance of the collision. Interestingly, the Sagittarius dwarf, one of the
most luminous satellites of the Milky Way, is in a polar orbit that crosses the
Galactic plane at roughly the same Galactocentric distance as Monoceros. We use
idealized simulations to demonstrate that an encounter with Sagittarius might
well have led to the circularization and subsequent tidal demise of the
progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters
(http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract
Chemical evolution during gas-rich galaxy interactions
We perform and analyse a set of galaxy interactions performed by using a self-consistent chemo-hydrodynamical model which includes star formation, supernova (SN) feedback and chemical evolution. In agreement with previous works, we find that tidally induced low-metallicity gas inflows dilute the central oxygen abundance and contribute to the flattening of the metallicity gradients. The tidally induced inflows trigger starbursts which increase the impact of Type II supernova (SN II) feedback injecting new chemical elements and driving galactic winds which modulate the metallicity distribution. Although α-enhancement in the central regions is detected as a result of the induced starbursts in agreement with previous works, our simulations suggest that this parameter can only provide a timing of the first pericentre mainly for non-retrograde encounters. In order to reproduce wet major mergers at low and high redshifts, we have run simulations with respectively 20 and 50 per cent of the disc in the form of gas. We find that the more gas-rich encounters behave similarly to the less rich ones, between the first and second pericentre, where low-metallicity gas inflows are triggered. However, the higher strength of the inflows triggered in the more gas-rich interactions produces larger metal dilution factors, which are modulated afterwards by the new chemical production by SN. We find that the more gas-rich interaction develops violent and clumpy star formation triggered by local instabilities all over the disc before the first pericentre, so that if these galaxies were observed at these early stages where no important tidally induced inflows have been able to be developed yet, they would tend to show an excess of oxygen. We find a global mean correlation of both the central abundances and the gradients with the strength of the star formation activity. However, the correlations are affected by orbital parameters, gas inflows and outflows, suggesting that it might be difficult to determine it from observations. Overall, our findings show that a consistent description of the gas dynamics and stellar evolution along the interactions is necessary to assess their effects on the chemical properties of the interstellar medium.Facultad de Ciencias Astronómicas y Geofísica
Testing SALT Approximations with Numerical Radiation Transfer Code Part 1: Validity and Applicability
Absorption line spectroscopy offers one of the best opportunities to
constrain the properties of galactic outflows and the environment of the
circumgalactic medium. Extracting physical information from line profiles is
difficult, however, for the physics governing the underlying radiation transfer
is complicated and depends on many different parameters. Idealized analytical
models are necessary to constrain the large parameter spaces efficiently, but
are typically plagued by model degeneracy and systematic errors. Comparison
tests with idealized numerical radiation transfer codes offer an excellent
opportunity to confront both of these issues. In this paper, we present a
detailed comparison between SALT, an analytical radiation transfer model for
predicting UV spectra of galactic outflows, with the numerical radiation
transfer software, RASCAS. Our analysis has lead to upgrades to both models
including an improved derivation of SALT and a customizable adaptive mesh
refinement routine for RASCAS. We explore how well SALT, when paired with a
Monte Carlo fitting procedure, can recover flow parameters from non-turbulent
and turbulent flows. When the velocity and density gradients are excluded, we
find that flow parameters are well recovered from high resolution (20
) data and moderately well from medium resolution (100
) data without turbulence at a S/N = 10, while derived quantities
(e.g., mass outflow rates, column density, etc.) are well recovered at all
resolutions. In the turbulent case, biased errors emerge in the recovery of
individual parameters, but derived quantities are still well recovered
Chemical evolution during gas-rich galaxy interactions
We perform and analyse a set of galaxy interactions performed by using a self-consistent chemo-hydrodynamical model which includes star formation, supernova (SN) feedback and chemical evolution. In agreement with previous works, we find that tidally induced low-metallicity gas inflows dilute the central oxygen abundance and contribute to the flattening of the metallicity gradients. The tidally induced inflows trigger starbursts which increase the impact of Type II supernova (SN II) feedback injecting new chemical elements and driving galactic winds which modulate the metallicity distribution. Although α-enhancement in the central regions is detected as a result of the induced starbursts in agreement with previous works, our simulations suggest that this parameter can only provide a timing of the first pericentre mainly for non-retrograde encounters. In order to reproduce wet major mergers at low and high redshifts, we have run simulations with respectively 20 and 50 per cent of the disc in the form of gas. We find that the more gas-rich encounters behave similarly to the less rich ones, between the first and second pericentre, where low-metallicity gas inflows are triggered. However, the higher strength of the inflows triggered in the more gas-rich interactions produces larger metal dilution factors, which are modulated afterwards by the new chemical production by SN. We find that the more gas-rich interaction develops violent and clumpy star formation triggered by local instabilities all over the disc before the first pericentre, so that if these galaxies were observed at these early stages where no important tidally induced inflows have been able to be developed yet, they would tend to show an excess of oxygen. We find a global mean correlation of both the central abundances and the gradients with the strength of the star formation activity. However, the correlations are affected by orbital parameters, gas inflows and outflows, suggesting that it might be difficult to determine it from observations. Overall, our findings show that a consistent description of the gas dynamics and stellar evolution along the interactions is necessary to assess their effects on the chemical properties of the interstellar medium.Facultad de Ciencias Astronómicas y Geofísica
Clumpy disc and bulge formation
We present a set of hydrodynamical/N-body controlled simulations of isolated gas-rich galaxies that self-consistently include supernova (SN) feedback and a detailed chemical evolution model, both tested in cosmological simulations. The initial conditions are motivated by the observed star-forming galaxies at z̃2-3.We find that the presence of a multiphase interstellar media in our models promotes the growth of disc instability favouring the formation of clumps which, in general, are not easily disrupted on time-scales compared to the migration time. We show that stellar clumps migrate towards the central region and contribute to form a classicallike bulge with a Séersic index, n>2. Our physically motivated SNfeedback has a mild influence on clump survival and evolution, partially limiting the mass growth of clumps as the energy released per SN event is increased, with the consequent flattening of the bulge profile. This regulation does not prevent the building of a classical-like bulge even for the most energetic feedback tested. Our SN feedback model is able to establish self-regulated star formation, producing mass-loaded outflows and stellar age spreads comparable to observations. We find that the bulge formation by clumps may coexist with other channels of bulge assembly such as bars and mergers. Our results suggest that galactic bulges could be interpreted as composite systems with structural components and stellar populations storing archaeological information of the dynamical history of their galaxy.Facultad de Ciencias Astronómicas y Geofísica
The Physics of Indirect Estimators of Lyman Continuum Escape and their Application to High-Redshift JWST Galaxies
Reliable indirect diagnostics of LyC photon escape from galaxies are required
to understand which sources were the dominant contributors to reionization.
While multiple escape fraction () indicators have been proposed to
trace favourable conditions for LyC leakage from the interstellar medium of
low-redshift ``analog'' galaxies, it remains unclear whether these are
applicable at high redshifts where LyC emission cannot be directly observed.
Using a library of 14,120 mock spectra of star-forming galaxies with redshifts
from the SPHINX cosmological radiation
hydrodynamics simulation, we develop a framework for the physics that leads to
high . We investigate LyC leakage from our galaxies based on the
criteria that successful LyC escape diagnostics must \textit{i)} track a high
specific star formation rate, \textit{ii)} be sensitive to stellar population
age in the range ~Myr representing the times when supernova first
explode to when LyC production significantly drops, and \textit{iii)} include a
proxy for neutral gas content and gas density in the interstellar medium. , , M, and H equivalent width
select for one or fewer of our criteria, rendering them either necessary but
insufficient or generally poor diagnostics. In contrast, UV slope (),
and match two or more of our criteria, rendering them good
diagnostics (albeit with significant scatter). Using our library,
we build a quantitative model for predicting based on ,
, H, M, , and .
When applied to bright Ly emitters observed with JWST, we find
that the majority of them have .Comment: 16 pages, 15 figures, 1 table, submitted to MNRA
How to Quench a Dwarf Galaxy: The Impact of Inhomogeneous Reionization on Dwarf Galaxies and Cosmic Filaments
We use the SPHINX suite of high-resolution cosmological radiation
hydrodynamics simulations to study how spatially and temporally inhomogeneous
reionization impacts the baryonic content of dwarf galaxies and cosmic
filaments. The SPHINX simulations simultaneously capture the large-scale
process of reionization, model the escape of ionising radiation from thousands
of galaxies, and resolve haloes well below the atomic cooling threshold. This
makes them an ideal tool for examining how reionization impacts star formation
and the gas content of dwarf galaxies. We compare simulations with and without
stellar radiation to isolate the effects of radiation feedback from that of
supernova, cosmic expansion, and numerical resolution. We find that the gas
content of cosmic filaments can be reduced by more than 80% following
reionization. The gas inflow rates into haloes with
are strongly affected and are reduced by more
than an order of magnitude compared to the simulation without reionization. A
significant increase in gas outflow rates is found for halo masses
. Our simulations show that inflow
suppression (i.e. starvation), rather than photoevaporation, is the dominant
mechanism by which the baryonic content of high-redshift dwarf galaxies is
regulated. At fixed redshift and halo mass, there is a large scatter in the
halo baryon fractions that is entirely dictated by the timing of reionization
in the local region surrounding a halo. Finally, although the gas content of
high-redshift dwarf galaxies is significantly impacted by reionization, we find
that most haloes with can remain self-shielded
and form stars long after reionization, until their local gas reservoir is
depleted, suggesting that local group dwarf galaxies do not necessarily exhibit
star formation histories that peak prior to
Chemical evolution during gas-rich galaxy interactions
We analyse a set of galaxy interactions performed by using a self-consistent
chemo-hydrodynamical model which includes star formation, Supernova feedback
and chemical evolution. In agreement with previous works, we find that
tidally-induced low-metallicity gas inflows dilute the central oxygen abundance
and contribute to the flattening of the metallicity gradients. The
tidally-induced inflows trigger starbursts which increase the impact of SN II
feedback injecting new chemical elements and driving galactic winds which
modulate the metallicity distribution. Although -enhancement in the
central regions is detected as a result of the induced starbursts in agreement
with previous works, our simulations suggest that this parameter can only
provide a timing of the first pericentre mainly for non-retrograde encounters.
In order to reproduce wet major mergers at low and high redshifts, we have run
simulations with respectively 20 and 50 percent of the disc in form of gas. We
find that the more gas-rich encounters behave similarly to the less rich ones,
between the first and second pericentre where low-metallicity gas inflows are
triggered. However, the higher strength of the inflows triggered in the more
gas-rich interactions produces larger metal dilutions factors which are
afterward modulated by the new chemical production by Supernova. We find that
the more gas-rich interaction develops violent and clumpy star formation
triggered by local instabilities all over the disc before the first pericentre,
so that if these galaxies were observed at these early stages where no
important tidally-induced inflows have been able to develop yet, they would
tend to show an excess of oxygen. We find a global mean correlation of both the
central abundances and the gradients with the strength of the star formation
activity. [abridged]Comment: 13 pages, 13 figures, accepted for publication in MNRA
The ATLAS3D project - XXIX : The new look of early-type galaxies and surrounding fields disclosed by extremely deep optical images
Date of Acceptance: 25/09/2014Galactic archaeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data reduction, coupled with the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the ATLAS3D project, we have obtained with the MegaCam camera at the Canada-France-Hawaii Telescope extremely deep, multiband images of nearby early-type galaxies (ETGs). We present here a catalogue of 92 galaxies from the ATLAS3D sample, which are located in low- to medium-density environments. The observing strategy and data reduction pipeline, which achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey are compared to other recent deep imaging projects. The paper highlights the capability of LSB-optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar haloes of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs are one of the prime goals of the project. We provide specific examples of each type of observed structures - tidal tails, stellar streams and shells - and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.Peer reviewedFinal Accepted Versio