108 research outputs found
Observational Signatures of Carbon-Oxygen White Dwarf Merger Remnants
Many double white dwarf (WD) mergers likely do not lead to a prompt
thermonuclear explosion. We investigate the prospects for observationally
detecting the surviving remnants of such mergers, focusing on the case of
mergers of Carbon-Oxygen WDs. For yr, the merger remnant is
observationally similar to an extreme AGB star evolving to become a massive WD.
Identifying merger remnants is thus easiest in galaxies with high stellar
masses (high WD merger rate) and low star formation rates (low birth rate of
stars). Photometrically identifying merger
remnants is challenging even in these cases because the merger remnants appear
similar to He stars and post-outburst classical novae. We propose that the most
promising technique for discovering WD merger remnants is through their unusual
surrounding photoionized nebulae. We use CLOUDY photoionization calculations to
investigate their unique spectral features. Merger remnants should produce weak
hydrogen lines and strong carbon and oxygen recombination and fine-structure
lines in the UV, optical and IR. With integral field spectrographs, we predict
that hundreds of candidates are detectable in M87 and other nearby massive
galaxies. Dust somewhat reduces the optical line emission for nebula radii
cm, but the sources spend most of their time with larger
radii when dust is less important. Our models roughly reproduce the WISE nebula
surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict
detectable [Ne VI] and [Mg VII] lines with JWST but that the mid-IR WISE
emission is dominated by dust not fine-structure lines.Comment: 12 pages, 8 figures, 2 tables, submitted to MNRA
Modeling the SED of the AGN inside NGC 4395
We study the broad-band spectral energy distribution (SED) of the
prototypical low-mass active galactic nucleus (AGN) in NGC 4395. We jointly
model the optical through mid-infrared SED with a combination of galaxy and AGN
light, and find that on arcsecond scales, the AGN dominates at most
wavelengths. However, there is still some ambiguity about emission from the
galaxy, owing partially to the strong short-term variability of the black hole.
We investigate the use of smooth and clumpy-torus models in order to
disentangle the nuclear infrared emission, as well as exploring the use of
poloidal wind emission to account for the blue spectral slope observed in the
near-IR. Even when simultaneously fitting the full optical-IR spectral range,
we find that degeneracies still remain in the best-fit models. We conclude that
high spatial resolution and wider wavelength coverage with the James Webb Space
Telescope is needed to understand the mid-infrared emission in this complex
highly-variable object, which is the best nearby example to provide a blueprint
to finding other low-mass AGN via their mid-infrared emission in the future.Comment: 17 pages, 8 figure
Ultra-Diffuse Galaxies as Extreme Star-forming Environments I: Mapping Star Formation in HI-Rich UDGs
Ultra-Diffuse Galaxies are both extreme products of galaxy evolution and
extreme environments in which to test our understanding of star formation. In
this work, we contrast the spatially resolved star formation activity of a
sample of 22 HI-selected UDGs and 35 low-mass galaxies from the NASA Sloan
Atlas (NSA) within 120 Mpc. We employ a new joint SED fitting method to compute
star formation rate and stellar mass surface density maps that leverage the
high spatial resolution optical imaging data of the Hyper Suprime-Cam Subaru
Strategic Program (HSC-SSP) and the UV coverage of GALEX, along with HI radial
profiles estimated from a subset of galaxies that have spatially resolved HI
maps. We find that the UDGs have low star formation efficiencies as a function
of their atomic gas down to scales of 500 pc. We additionally find that the
stellar mass-weighted sizes of our UDG sample are unremarkable when considered
as a function of their HI mass -- their stellar sizes are comparable to the NSA
dwarfs at fixed HI mass. This is a natural result in the picture where UDGs are
forming stars normally, but at low efficiencies. We compare our results to
predictions from contemporary models of galaxy formation, and find in
particular that our observations are difficult to reproduce in models where
UDGs undergo stellar expansion due to vigorous star formation feedback should
bursty star formation be required down to .Comment: Accepted to ApJ, 27 pages, 18 figure
Spectropolarimetric measurements of hidden broad lines in nearby megamaser galaxies: a lack of clear evidence for a correlation between black hole masses and virial products
High-accuracy black hole (BH) masses require excellent spatial resolution
that is only achievable for galaxies within ~100 Mpc using present-day
technology. At larger distances, BH masses are often estimated with
single-epoch scaling relations for active galactic nuclei. This method requires
only luminosity and the velocity dispersion of the broad line region (BLR) to
calculate a virial product, and an additional virial factor, , to determine
BH mass. The accuracy of these single-epoch masses, however, is unknown, and
there are few empirical constraints on the variance of between objects. We
attempt to calibrate single-epoch BH masses using spectropolarimetric
measurements of nine megamaser galaxies from which we measure the velocity
distribution of the BLR. We do not find strong evidence for a correlation
between the virial products used for single-epoch masses and dynamical mass,
both for the megamaser sample alone and when combined with dynamical masses
from reverberation mapping modeling. Furthermore, we find evidence that the
virial parameter varies between objects, but we do not find strong evidence
for a correlation with other observable parameters such as luminosity or broad
line width. Although we cannot definitively rule out the existence of any
correlation between dynamical mass and virial product, we find tension between
allowed values for masers and those widely used in the literature. We
conclude that the single-epoch method requires further investigation if it is
to be used successfully to infer BH masses.Comment: 27 pages, 16 figures, resubmitted to ApJ after incorporating
reviewer's comments. Corrected Figure 8, main results do not chang
The MASSIVE Survey - X. Misalignment between Kinematic and Photometric Axes and Intrinsic Shapes of Massive Early-Type Galaxies
We use spatially resolved two-dimensional stellar velocity maps over a
field of view to investigate the kinematic features of 90
early-type galaxies above stellar mass in the MASSIVE
survey. We measure the misalignment angle between the kinematic and
photometric axes and identify local features such as velocity twists and
kinematically distinct components. We find 46% of the sample to be well aligned
(), 33% misaligned, and 21% without detectable rotation
(non-rotators). Only 24% of the sample are fast rotators, the majority of which
(91%) are aligned, whereas 57% of the slow rotators are misaligned with a
nearly flat distribution of from to . 11
galaxies have and thus exhibit minor-axis ("prolate")
rotation in which the rotation is preferentially around the photometric major
axis. Kinematic misalignments occur more frequently for lower galaxy spin or
denser galaxy environments. Using the observed misalignment and ellipticity
distributions, we infer the intrinsic shape distribution of our sample and find
that MASSIVE slow rotators are consistent with being mildly triaxial, with mean
axis ratios of and . In terms of local kinematic features,
51% of the sample exhibit kinematic twists of larger than , and 2
galaxies have kinematically distinct components. The frequency of misalignment
and the broad distribution of reported here suggest that the most
massive early-type galaxies are mildly triaxial, and that formation processes
resulting in kinematically misaligned slow rotators such as gas-poor mergers
occur frequently in this mass range.Comment: Accepted to MNRA
ELVES IV: The Satellite Stellar-to-Halo Mass Relation Beyond the Milky-Way
Quantifying the connection between galaxies and their host dark matter halos
has been key for testing cosmological models on various scales. Below , such studies have primarily relied on the satellite galaxy
population orbiting the Milky Way. Here we present new constraints on the
connection between satellite galaxies and their host dark matter subhalos using
the largest sample of satellite galaxies in the Local Volume () to date. We use confirmed and candidate dwarf
satellites around 27 Milky Way (MW)-like hosts from the Exploration of Local
VolumE Satellites (ELVES) Survey and use the semi-analytical SatGen model for
predicting the population of dark matter subhalos expected in the same volume.
Through a Bayesian model comparison of the observed and the forward-modeled
satellite stellar mass functions (SSMF), we infer the satellite stellar-to-halo
mass relation. We find that the observed SSMF is best reproduced when subhalos
at the low mass end are populated by a relation of the form , with a moderate slope of and a low scatter, constant as a function of the peak halo mass, of
. A model with a steeper slope
() and a scatter that grows with decreasing
is also consistent with the observed SSMF but is not
required. Our new model for the satellite-subhalo connection, based on hundreds
of Local Volume satellite galaxies, is in line with what was previously derived
using only the Milky Way satellites.Comment: Accepted for publication in ApJ. Figure 8 shows the key result -- the
Satellite Stellar to Halo Mass relation obtained in this work, in comparison
to previous studie
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