During the second observing run of the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo Interferometer, a gravitational-wave signal consistent with a binary neutron star ...coalescence was detected on 2017 August 17th (GW170817), quickly followed by a coincident short gamma-ray burst trigger detected by the Fermi satellite. The Distance Less Than 40 (DLT40) Mpc supernova search performed pointed follow-up observations of a sample of galaxies regularly monitored by the survey that fell within the combined LIGO+Virgo localization region and the larger Fermi gamma-ray burst error box. Here we report the discovery of a new optical transient (DLT17ck, also known as SSS17a; it has also been registered as AT 2017gfo) spatially and temporally coincident with GW170817. The photometric and spectroscopic evolution of DLT17ck is unique, with an absolute peak magnitude of Mr = −15.8 0.1 and an r-band decline rate of 1.1 mag day−1. This fast evolution is generically consistent with kilonova models, which have been predicted as the optical counterpart to binary neutron star coalescences. Analysis of archival DLT40 data does not show any sign of transient activity at the location of DLT17ck down to r ∼ 19 mag in the time period between 8 months and 21 days prior to GW170817. This discovery represents the beginning of a new era for multi-messenger astronomy, opening a new path by which to study and understand binary neutron star coalescences, short gamma-ray bursts, and their optical counterparts.
Abstract
The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This ...very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at −13.6 days relative to the
B
-band maximum light, or ∼5 days after the estimated explosion—the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si
ii
λ
6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.
We present stellar and dark matter (DM) density profiles for a sample of seven massive, relaxed galaxy clusters derived from strong and weak gravitational lensing and resolved stellar kinematic ...observations within the centrally located brightest cluster galaxies (BCGs). In Paper I of the series, we demonstrated that the total density profile derived from these data, which span three decades in radius, is consistent with numerical DM-only simulations at radii > ~5-10 kpc, despite the significant contribution of stellar material in the core. Here, we decompose the inner mass profiles of these clusters into stellar and dark components. Parameterizing the DM density profile as a power law rho sub(DM) is proportional to r super(- beta ) on small scales, we find a mean slope left angle bracket beta right angle bracket = 0.50 + or - 0.10(random)(ProQuest: Formulae and/or non-USASCII text omitted)(systematic). Alternatively, cored Navarro-Frenk-White (NFW) profiles with (ProQuest: Formulae and/or non-USASCII text omitted) provide an equally good description. These density profiles are significantly shallower than canonical NFW models at radii <, ~30 kpc, comparable to the effective radii of the BCGs. The inner DM profile is correlated with the distribution of stars in the BCG, suggesting a connection between the inner halo and the assembly of stars in the central galaxy. The stellar mass-to-light ratio inferred from lensing and stellar dynamics is consistent with that inferred using stellar population synthesis models if a Salpeter initial mass function is adopted. We compare these results to theories describing the interaction between baryons and DM in cluster cores, including adiabatic contraction models and the possible effects of galaxy mergers and active galactic nucleus feedback, and evaluate possible signatures of alternative DM candidates.
Abstract
We present deep
Magellan
/Megacam stellar photometry of four recently discovered faint Milky Way satellites: Sagittarius II (Sgr II), Reticulum II (Ret II), Phoenix II (Phe II), and ...Tucana III (Tuc III). Our photometry reaches ∼2–3 magnitudes deeper than the discovery data, allowing us to revisit the properties of these new objects (e.g., distance, structural properties, luminosity measurements, and signs of tidal disturbance). The satellite color-magnitude diagrams show that they are all old (∼13.5 Gyr) and metal poor (Fe/H ≲ −2.2). Sgr II is particularly interesting, as it sits in an intermediate position between the loci of dwarf galaxies and globular clusters in the size–luminosity plane. The ensemble of its structural parameters is more consistent with a globular cluster classification, indicating that Sgr II is the most extended globular cluster in its luminosity range. The other three satellites land directly on the locus defined by Milky Way ultra-faint dwarf galaxies of similar luminosity. Ret II is the most elongated nearby dwarf galaxy currently known for its luminosity range. Our structural parameters for Phe II and Tuc III suggest that they are both dwarf galaxies. Tuc III is known to be associated with a stellar stream, which is clearly visible in our matched-filter stellar density map. The other satellites do not show any clear evidence of tidal stripping in the form of extensions or distortions. Finally, we also use archival H
i
data to place limits on the gas content of each object.
Abstract
We present a series of high-resolution echelle spectra of SN 2023ixf in M101, obtained nightly during the first week or so after discovery using PEPSI on the Large Binocular Telescope. Na
i
...D absorption in these spectra indicates a host reddening of
E
(
B
−
V
) = 0.031 mag and a systemic velocity of +7 km s
−1
relative to the average redshift of M101. Dramatic changes are seen in the strength and shape of strong emission lines emitted by circumstellar material (CSM), including He
ii
λ
4686, C
iv
λλ
5801,5811, H
α
, and N
iv
λλ
7109,7123. In general, these narrow lines broaden to become intermediate-width lines before disappearing from the spectrum within a few days, indicating a limited extent to the dense CSM of around 20–30 au (or ≲10
14.7
cm). H
α
persists in the spectrum for about a week as an intermediate-width emission line with P Cyg absorption at 700–1300 km s
−1
arising in the post-shock shell of swept-up CSM. Early narrow emission lines are blueshifted and indicate an expansion speed in the pre-shock CSM of about 115 km s
−1
, but with even broader emission in higher-ionization lines. This is faster than the normal winds of red supergiants, suggesting some mode of eruptive mass loss from the progenitor or radiative acceleration of the CSM. A lack of narrow blueshifted absorption suggests that most of the CSM is not along our line of sight. This and several other clues indicate that the CSM of SN 2023ixf is significantly aspherical. We find that CSM lines disappear after a few days because the asymmetric CSM is engulfed by the supernova photosphere.
ABSTRACT We report evidence for excess blue light from the Type Ia supernova (Sn Ia) SN 2012cg at 15 and 16 days before maximum B-band brightness. The emission is consistent with predictions for the ...impact of the supernova on a non-degenerate binary companion. This is the first evidence for emission from a companion to a normal SN Ia. Sixteen days before maximum light, the color of SN 2012cg is 0.2 mag bluer than for other normal SN Ia. At later times, this supernova has a typical SN Ia light curve, with extinction-corrected mag and . Our data set is extensive, with photometry in seven filters from five independent sources. Early spectra also show the effects of blue light, and high-velocity features are observed at early times. Near maximum, the spectra are normal with a silicon velocity vSi = −10,500 km s−1. Comparing the early data with models by Kasen favors a main-sequence companion of about six solar masses. It is possible that many other SN Ia have main-sequence companions that have eluded detection because the emission from the impact is fleeting and faint.
We present very early, high-cadence photometric observations of the nearby Type Ia SN 2017cbv. The light curve is unique in that it has a blue bump during the first five days of observations in the ...U, B, and g bands, which is clearly resolved given our photometric cadence of 5.7 hr during that time span. We model the light curve as the combination of early shocking of the supernova ejecta against a nondegenerate companion star plus a standard SN Ia component. Our best-fit model suggests the presence of a subgiant star 56 R☉ from the exploding white dwarf, although this number is highly model-dependent. While this model matches the optical light curve well, it overpredicts the observed flux in the ultraviolet bands. This may indicate that the shock is not a blackbody, perhaps because of line blanketing in the UV. Alternatively, it could point to another physical explanation for the optical blue bump, such as interaction with circumstellar material or an unusual nickel distribution. Early optical spectra of SN 2017cbv show strong carbon (C ii λ6580) absorption up through day −13 with respect to maximum light, suggesting that the progenitor system contains a significant amount of unburned material. These early results on SN 2017cbv illustrate the power of early discovery and intense follow-up of nearby supernovae to resolve standing questions about the progenitor systems and explosion mechanisms of SNe Ia.
Clusters of galaxies are excellent locations to probe the distribution of baryons and dark matter (DM) over a wide range of scales. We study a sample of seven massive, relaxed galaxy clusters with ...centrally located brightest cluster galaxies (BCGs) at z = 0.2-0.3. Using the observational tools of strong band weak gravitational lensing, combined with resolved stellar kinematics within the BCG, we measure the total radial density profile, comprising both dark and baryonic matter, over scales of ~or =3-3000 kpc. We present Keck spectroscopy yielding seven new spectroscopic redshifts of multiply imaged sources and extended stellar velocity dispersion profiles of the BCGs. We discuss the significance of our findings for understanding the physical processes governing the assembly of BCGs and cluster cores, particularly the influence of baryons on the inner DM halo.
Abstract
To better understand the formation of large, low-surface-brightness galaxies, we measure the correlation function between ultradiffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). ...We find that: (1) the projected radial distribution of UDG satellites (projected surface density ∝
r
−0.84±0.06
) is consistent with that of normal satellite galaxies; (2) the number of UDG satellites per MWA (
S
UDG
) is ∼0.5 ± 0.1 over projected radii from 20 to 250 kpc and −17 <
M
r
< −13.5; (3)
S
UDG
is consistent with a linear extrapolation of the relationship between the number of UDGs per halo versus halo mass obtained over galaxy group and cluster scales; (4) red UDG satellites dominate the population of UDG satellites (∼80%); (5) over the range of satellite magnitudes studied, UDG satellites comprise ∼10% of the satellite galaxy population of MWAs; and (6) a significant fraction of these (∼13%) have estimated total masses >10
10.9
M
⊙
or, equivalently, at least half the halo mass of the LMC, and populate a large fraction (∼18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low-mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low-mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs.
We present the first results from an optical reverberation mapping campaign executed in 2014 targeting the active galactic nuclei (AGNs) MCG+08-11-011, NGC 2617, NGC 4051, 3C 382, and Mrk 374. Our ...targets have diverse and interesting observational properties, including a "changing look" AGN and a broad-line radio galaxy. Based on continuum-Hβ lags, we measure black hole masses for all five targets. We also obtain Hγ and He ii λ4686 lags for all objects except 3C 382. The He ii λ4686 lags indicate radial stratification of the BLR, and the masses derived from different emission lines are in general agreement. The relative responsivities of these lines are also in qualitative agreement with photoionization models. These spectra have extremely high signal-to-noise ratios (100-300 per pixel) and there are excellent prospects for obtaining velocity-resolved reverberation signatures.
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