941 research outputs found
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
Empirically-Driven Multiwavelength K-corrections At Low Redshift
K-corrections, conversions between flux in observed bands to flux in
rest-frame bands, are critical for comparing galaxies at various redshifts.
These corrections often rely on fits to empirical or theoretical spectral
energy distribution (SED) templates of galaxies. However, the templates limit
reliable K-corrections to regimes where SED models are robust. For instance,
the templates are not well-constrained in some bands (e.g., WISE W4), which
results in ill-determined K-corrections for these bands. We address this
shortcoming by developing an empirically-driven approach to K-corrections as a
means to mitigate dependence on SED templates. We perform a polynomial fit for
the K-correction as a function of a galaxy's rest-frame color determined in
well-constrained bands (e.g., rest-frame (g-r)) and redshift, exploiting the
fact that galaxy SEDs can be described as a one parameter family at low
redshift (0.01 < z < 0.09). For bands well-constrained by SED templates, our
empirically-driven K-corrections are comparable to the SED fitting method of
Kcorrect and SED template fitting employed in the GSWLC-M2 catalogue (the
updated medium-deep GALEX-SDSS-WISE Legacy Catalogue). However, our method
dramatically outperforms the available SED fitting K-corrections for WISE W4.
Our method also mitigates incorrect template assumptions and enforces the
K-correction to be 0 at z = 0. Our K-corrected photometry and code are publicly
available.Comment: 15 pages, 9 figures, submitted to MNRA
Curvature contraction of convex hypersurfaces by nonsmooth speeds
We consider contraction of convex hypersurfaces by convex speeds, homogeneous of degree one in the principal curvatures, that are not necessarily smooth. We show how to approximate such a speed by a sequence of smooth speeds for which behaviour is well known. By obtaining speed and curvature pinching estimates for the flows by the approximating speeds, independent of the smoothing parameter, we may pass to the limit to deduce that the flow by the nonsmooth speed converges to a point in finite time that, under a suitable rescaling, is round in the C^2 sense, with the convergence being exponential
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
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