2,089 research outputs found
Non parametric reconstruction of distribution functions from observed galactic disks
A general inversion technique for the recovery of the underlying distribution
function for observed galactic disks is presented and illustrated. Under the
assumption that these disks are axi-symmetric and thin, the proposed method
yields the unique distribution compatible with all the observables available.
The derivation may be carried out from the measurement of the azimuthal
velocity distribution arising from positioning the slit of a spectrograph along
the major axis of the galaxy. More generally, it may account for the
simultaneous measurements of velocity distributions corresponding to slits
presenting arbitrary orientations with respect to the major axis. The approach
is non-parametric, i.e. it does not rely on a particular algebraic model for
the distribution function. Special care is taken to account for the fraction of
counter-rotating stars which strongly affects the stability of the disk. An
optimisation algorithm is devised -- generalising the work of Skilling & Bryan
(1984) -- to carry this truly two-dimensional ill-conditioned inversion
efficiently. The performances of the overall inversion technique with respect
to the noise level and truncation in the data set is investigated with
simulated data. Reliable results are obtained up to a mean signal to noise
ratio of~5 and when measurements are available up to . A discussion of
the residual biases involved in non parametric inversions is presented.
Prospects of application to observed galaxies and other inversion problems are
discussed.Comment: 11 pages, 13 figures; accepted for publication by MNRA
Non-parametric reconstruction of distribution functions from observed galactic discs
A general inversion technique for the recovery of the underlying distribution function for observed galactic discs is presented and illustrated. Under the assumption that these discs are axisymmetric and thin, the proposed method yields a unique distribution compatible with all the observables available. The derivation may be carried out from the measurement of the azimuthal velocity distribution arising from positioning the slit of a spectrograph along the major axis of the galaxy. More generally, it may account for the simultaneous measurements of velocity distributions corresponding to slits presenting arbitrary orientations with respect to the major axis. The approach is non-parametric, i.e. it does not rely on a particular algebraic model for the distribution function. Special care is taken to account for the fraction of counter-rotating stars, which strongly affects the stability of the disc. An optimization algorithm is devised — generalizing the work of Skilling &38; Bryan — to carry this truly two-dimensional ill-conditioned inversion efficiently. The performance of the overall inversion technique with respect to the noise level and truncation in the data set is investigated with simulated data. Reliable results are obtained up to a mean signal-to-noise ratio of 5, and when measurements are available up to 4 Re. A discussion of the residual biases involved in non-parametric inversions is presented. The prospects of application of the algorithm to observed galaxies and other inversion problems are discusse
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘ex situ’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M ∗ > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.Peer reviewedFinal Accepted Versio
Stellar Content from high resolution galactic spectra via Maximum A Posteriori
This paper describes STECMAP (STEllar Content via Maximum A Posteriori), a
flexible, non-parametric inversion method for the interpretation of the
integrated light spectra of galaxies, based on synthetic spectra of single
stellar populations (SSPs). We focus on the recovery of a galaxy's star
formation history and stellar age-metallicity relation. We use the high
resolution SSPs produced by PEGASE-HR to quantify the informational content of
the wavelength range 4000 - 6800 Angstroms.
A detailed investigation of the properties of the corresponding simplified
linear problem is performed using singular value decomposition. It turns out to
be a powerful tool for explaining and predicting the behaviour of the
inversion. We provide means of quantifying the fundamental limitations of the
problem considering the intrinsic properties of the SSPs in the spectral range
of interest, as well as the noise in these models and in the data.
We performed a systematic simulation campaign and found that, when the time
elapsed between two bursts of star formation is larger than 0.8 dex, the
properties of each episode can be constrained with a precision of 0.04 dex in
age and 0.02 dex in metallicity from high quality data (R=10 000,
signal-to-noise ratio SNR=100 per pixel), not taking model errors into account.
The described methods and error estimates will be useful in the design and in
the analysis of extragalactic spectroscopic surveys.Comment: 31 pages, 23 figures, accepted for publication in MNRA
Propagators in Lagrangian space
It has been found recently that propagators, e.g. the cross-correlation
spectra of the cosmic fields with the initial density field, decay
exponentially at large-k in an Eulerian description of the dynamics. We explore
here similar quantities defined for a Lagrangian space description. We find
that propagators in Lagrangian space do not exhibit the same properties: they
are found not to be monotonic functions of time, and to track back the linear
growth rate at late time (but with a renormalized amplitude). These results
have been obtained with a novel method which we describe alongside. It allows
the formal resummation of the same set of diagrams as those that led to the
known results in Eulerian space. We provide a tentative explanation for the
marked differences seen between the Eulerian and the Lagrangian cases, and we
point out the role played by the vorticity degrees of freedom that are specific
to the Lagrangian formalism. This provides us with new insights into the
late-time behavior of the propagators.Comment: 14 pages, 5 figure
Probing magnetic fields with multi-frequency polarized synchrotron emission
We investigate the problem of probing the local spatial structure of the
magnetic field of the interstellar medium using multi-frequency polarized maps
of the synchrotron emission at radio wavelengths. We focus in this paper on the
three-dimensional reconstruction of the largest scales of the magnetic field,
relying on the internal depolarization (due to differential Faraday rotation)
of the emitting medium as a function of electromagnetic frequency. We argue
that multi-band spectroscopy in the radio wavelengths, developed in the context
of high-redshift extragalactic HI lines, can be a very useful probe of the 3D
magnetic field structure of our Galaxy when combined with a Maximum A
Posteriori reconstruction technique. When starting from a fair approximation of
the magnetic field, we are able to recover the true one by using a linearized
version of the corresponding inverse problem. The spectral analysis of this
problem allows us to specify the best sampling strategy in electromagnetic
frequency and predicts a spatially anisotropic distribution of posterior
errors. The reconstruction method is illustrated for reference fields extracted
from realistic magneto-hydrodynamical simulations
The distribution of nearby stars in phase space mapped by Hipparcos: I. The potential well and local dynamical mass
Hipparcos data provide the first, volume limited and absolute magnitude
limited homogeneous tracer of stellar density and velocity distributions in the
solar neighbourhood. The density of A-type stars more luminous than
can be accurately mapped within a sphere of 125 pc radius, while proper motions
in galactic latitude provide the vertical velocity distribution near the
galactic plane. The potential well across the galactic plane is traced
practically hypothesis-free and model-free. The local dynamical density comes
out as \rho_{0}=0.076 \pm0.015~M_{\sun}~{pc}^{-3} a value well below all
previous determinations leaving no room for any disk shaped component of dark
matter.Comment: 24 pages, 13 figures, latex. To appear in A&A (main journal
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