618 research outputs found
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Measuring Dark Matter Profiles Non-Parametrically In Dwarf Spheroidals: An Application To Draco
We introduce a novel implementation of orbit-based (or Schwarzschild) modeling that allows dark matter density profiles to be calculated non-parametrically in nearby galaxies. Our models require no assumptions to be made about velocity anisotropy or the dark matter profile. The technique can be applied to any dispersion-supported stellar system, and we demonstrate its use by studying the Local Group dwarf spheroidal galaxy (dSph) Draco. We use existing kinematic data at larger radii and also present 12 new radial velocities within the central 13 pc obtained with the VIRUS-W integral field spectrograph on the 2.7 m telescope at McDonald Observatory. Our non-parametric Schwarzschild models find strong evidence that the dark matter profile in Draco is cuspy for 20 = 20 pc is well fit by a power law with slope alpha = -1.0 +/- 0.2, consistent with predictions from cold dark matter simulations. Our models confirm that, despite its low baryon content relative to other dSphs, Draco lives in a massive halo.NSF-0908639Astronom
SDSS-IV MaNGA: Identification of active galactic nuclei in optical integral field unit surveys
In this paper, we investigate 2727 galaxies observed by MaNGA as of June 2016
to develop spatially resolved techniques for identifying signatures of active
galactic nuclei (AGN). We identify 303 AGN candidates. The additional spatial
dimension imposes challenges in identifying AGN due to contamination from
diffuse ionized gas, extra-planar gas and photoionization by hot stars. We show
that the combination of spatially-resolved line diagnostic diagrams and
additional cuts on H surface brighness and H equivalent width
can distinguish between AGN-like signatures and high-metallicity galaxies with
LINER-like spectra. Low mass galaxies with high specific star formation rates
are particularly difficult to diagnose and routinely show diagnostic line
ratios outside of the standard star-formation locus. We develop a new
diagnostic -- the distance from the standard diagnostic line in the line-ratios
space -- to evaluate the significance of the deviation from the star-formation
locus. We find 173 galaxies that would not have been selected as AGN candidates
based on single-fibre spectral measurements but exhibit photoionization
signatures suggestive of AGN activity in the MaNGA resolved observations,
underscoring the power of large integral field unit (IFU) surveys. A complete
census of these new AGN candidates is necessary to understand their nature and
probe the complex co-evolution of supermassive black holes and their hosts.Comment: 18 pages, 11 figures, accepted to MNRA
Re-growth of stellar disks in mature galaxies: The two component nature of NGC 7217 revisited with VIRUS-W
Previous studies have reported the existence of two counter-rotating stellar
disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution
optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field
Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in
Texas. Our analysis confirms the existence of two components. However, we find
them to be co-rotating. The first component is the more luminous (~ 77% of the
total light), has the higher velocity dispersion (~ 170 km/s) and rotates
relatively slowly (projected = 50 km/s). The lower luminosity second
component, (~ 23% of the total light), has a low velocity dispersion (~ 20
km/s) and rotates quickly (projected = 150 km/s). The difference in
the kinematics of the two stellar components allows us to perform a kinematic
decomposition and to measure the strengths of their Mg and Fe Lick indices
separately. The rotational velocities and dispersions of the less luminous and
faster component are very similar to those of the interstellar gas as measured
from the [OIII] emission. Morphological evidence of active star formation in
this component further suggests that NGC7217 may be in the process of
(re)growing a disk inside a more massive and higher dispersion stellar halo.
The kinematically cold and regular structure of the gas disk in combination
with the central almost dust-free morphology allows us to compare the dynamical
mass inside of the central 500pc with predictions from a stellar population
analysis. We find agreement between the two if a Kroupa stellar initial mass
function is assumed.Comment: accepted for publication by MNRA
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Reconsolidation-Extinction Interactions in Fear Memory Attenuation: The Role of Inter-Trial Interval Variability
Fear extinction typically results in the formation of a new inhibitory memory that
suppresses the original conditioned response. Evidence also suggests that extinction
training during a retrieval-induced labile period results in integration of the extinction
memory into the original fear memory, rendering the fear memory less susceptible to
reinstatement. Here we investigated the parameters by which the retrieval-extinction
paradigm was most effective in memory updating. Specifically, we manipulated the intertrial intervals (ITIs) between conditional stimulus (CS) presentations during extinction,
examining how having interval lengths with different degrees of variability affected the
strength of memory updating. We showed that randomizing the ITI of CS presentations
during extinction led to less return of fear via reinstatement than extinction with a fixed
ITI. Subjects who received variable ITIs during extinction also showed higher freezing
during the ITI, indicating that the randomization of CS presentations led to a higher
general reactivity during extinction, which may be one potential mechanism for memory
updating
Are Milky-Way-like galaxies like the Milky Way? A view from SDSS-IV/MaNGA
In this paper, we place the Milky Way (MW) in the context of similar-looking
galaxies in terms of their star-formation and chemical evolution histories. We
select a sample of 138 Milky-Way analogues (MWAs) from the SDSS-IV/MaNGA survey
based on their masses, Hubble types, and bulge-to-total ratios. To compare
their chemical properties to the detailed spatially-resolved information
available for the MW, we use a semi-analytic spectral fitting approach, which
fits a self-consistent chemical-evolution and star-formation model directly to
the MaNGA spectra. We model the galaxies' inner and outer regions assuming that
some of the material lost in stellar winds falls inwards. We also incorporate
chemical enrichment from type II and Ia supernovae to follow the alpha-element
abundance at different metallicities and locations. We find some MWAs where the
stellar properties closely reproduce the distribution of age, metallicity, and
alpha enhancement at both small and large radii in the MW. In these systems,
the match is driven by the longer timescale for star formation in the outer
parts, and the inflow of enriched material to the central parts. However, other
MWAs have very different histories. These divide into two categories:
self-similar galaxies where the inner and outer parts evolve identically; and
centrally-quenched galaxies where there is very little evidence of late-time
central star formation driven by material accreted from the outer regions. We
find that, although selected to be comparable, there are subtle morphological
differences between galaxies in these different classes, and that the
centrally-quenched galaxies formed their stars systematically earlier.Comment: 16 pages, 9 figures, MNRAS accepted versio
Spectroscopic decomposition of NGC 3521: unveiling the properties of the bulge and disc
We study the kinematics and the stellar populations of the bulge and disc of
the spiral galaxy NGC 3521. At each position in the field of view, we separate
the contributions of the bulge and the disc from the total observed spectrum
and study their kinematics, age, and metallicities independently. Their
properties are clearly distinct: the bulge rotates more slowly, has a higher
velocity dispersion, and is less luminous than the disc. We identify three main
populations of stars in NGC 3521: old ( Gyr), intermediate ( 3
Gyr), and young (1 Gyr). The mass and light of NGC 3521 are dominated by
the intermediate stellar population. The youngest population contributes mostly
to the disc component and its contribution increases with radius. We also study
the luminosity-weighed properties of the stars in NGC 3521. Along the
photometric major axis, we find: i) no age gradient for the stars in the bulge,
and a negative age gradient for the stars in the disc; ii) negative metallicity
gradients and sub-solar -enhancement for both the bulge and the disc.
We propose the following picture for the formation of NGC 3521: initial
formation a long time ago ( Gyr), followed by a second burst of star
formation or a merger ( 3 Gyrs ago), which contributed predominantly
to the mass-build up of the bulge. Recently ( Gyr), the disc of NGC
3521 experienced an additional episode of star formation that started in the
innermost regions.Comment: 13 pages, 11 figures, accepted for publication in MNRA
SDSS-IV MaNGA: Variations in the N/O -- O/H relation bias metallicity gradient measurements
In this paper we use strong line calibrations of N/O and O/H in MaNGA spaxel
data to explore the systematics introduced by variations in N/O on various
strong-line metallicity diagnostics. We find radial variations in N/O at fixed
O/H which correlate with total galaxy stellar-mass; and which can induce systematic uncertainties in oxygen abundance gradients when
nitrogen-dependent abundance calibrations are used. Empirically, we find that
these differences are associated with variation in the local star formation
efficiency, as predicted by recent chemical evolution models for galaxies, but
we cannot rule out other processes such as radial migration and the accretion
of passive dwarf galaxies also playing a role.Comment: 8 pages, 4 figures. Submitted to ApJL. All comments are welcom
SDSS-IV MaNGA: Radial Gradients in Stellar Population Properties of Early-Type and Late-Type Galaxies
We derive ages, metallicities, and individual element abundances of early-
and late-type galaxies (ETGs and LTGs) out to 1.5 R. We study a large
sample of 1900 galaxies spanning in stellar mass,
through key absorption features in stacked spectra from the SDSS-IV/MaNGA
survey. We use mock galaxy spectra with extended star formation histories to
validate our method for LTGs and use corrections to convert the derived ages
into luminosity- and mass-weighted quantities. We find flat age and negative
metallicity gradients for ETGs and negative age and negative metallicity
gradients for LTGs. Age gradients in LTGs steepen with increasing galaxy mass,
from Gyr/R for the lowest mass galaxies to
Gyr/R for the highest mass ones. This strong
gradient-mass relation has a slope of . Comparing local age and
metallicity gradients with the velocity dispersion within galaxies
against the global relation with shows that internal processes
regulate metallicity in ETGs but not age, and vice versa for LTGs. We further
find that metallicity gradients with respect to local show a much
stronger dependence on galaxy mass than radial metallicity gradients. Both
galaxy types display flat [C/Fe] and [Mg/Fe], and negative [Na/Fe] gradients,
whereas only LTGs display gradients in [Ca/Fe] and [Ti/Fe]. ETGs have
increasingly steep [Na/Fe] gradients with local reaching
dex/ km/s for the highest masses. [Na/Fe] ratios are correlated with
metallicity for both galaxy types across the entire mass range in our sample,
providing support for metallicity dependent supernova yields.Comment: 21 pages, 21 figures, 4 tables + Appendi
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