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We present wide-field, deep K-band photometry of 98 luminous early-type galaxies (ETGs) from the MASSIVE survey based on observations taken with the WIRCam instrument on the Canada–France–Hawaii Telescope. Using these images, we extract accurate total K-band luminosities (LK) and half-light radii (Re) for this sample of galaxies. We use these new values to explore the size–luminosity and Faber–Jackson relations for massive ETGs. Within this volume-limited sample, we find clear evidence for curvature in both relations, indicating that the most luminous galaxies tend to have larger sizes and smaller velocity dispersions than expected from a simple power-law fit to less luminous galaxies. Our measured relations are qualitatively consistent with the most massive elliptical galaxies forming largely through dissipationless mergers. When the sample is separated into fast and slow rotators, we find the slow rotators to exhibit similar changes in slope with increasing LK, suggesting that low-mass and high-mass slow rotators have different formation histories. The curvatures in the Re–LK and σ–LK relations cancel, leading to a relation between dynamical mass and luminosity that is well described by a single power law: Reσ2 ∝ LKb with b ≈ 1.2. This is consistent with the tilt of the fundamental plane observed in lower mass elliptical galaxies.

 

The three-dimensional intrinsic shape of a galaxy and the mass of the central supermassive black hole provide key insight into the galaxy’s growth history over cosmic time. Standard assumptions of a spherical or axisymmetric shape can be simplistic and can bias the black hole mass inferred from the motions of stars within a galaxy. Here, we present spatially resolved stellar kinematics of M87 over a two-dimensional 250″ × 300″ contiguous field covering a radial range of 50 pc–12 kpc from integral-field spectroscopic observations at the Keck II Telescope. From about 5 kpc and outward, we detect a prominent 25 km s⁻¹rotational pattern, in which the kinematic axis (connecting the maximal receding and approaching velocities) is 40° misaligned with the photometric major axis of M87. The rotational amplitude and misalignment angle both decrease in the inner 5 kpc. Such misaligned and twisted velocity fields are a hallmark of triaxiality, indicating that M87 is not an axisymmetrically shaped galaxy. Triaxial Schwarzschild orbit modeling with more than 4000 observational constraints enabled us to determine simultaneously the shape and mass parameters. The models incorporate a radially declining profile for the stellar mass-to-light ratio suggested by stellar population studies. We find that M87 is strongly triaxial, with ratios ofp= 0.845 for the middle-to-long principal axes andq= 0.722 for the short-to-long principal axes, and determine the black hole mass to be$({5.37}_{-0.25}^{+0.37}\pm 0.22)\times {10}^9{M}_{\odot }$, where the second error indicates the systematic uncertainty associated with the distance to M87.

 

The stellar initial mass function (IMF) is a fundamental property in the measurement of stellar masses and galaxy star formation histories. In this work, we focus on the most massive galaxies in the nearby universe$\log (M_{\star }/M_{\odot })>11.2$. We obtain high-quality Magellan/LDSS-3 long-slit spectroscopy with a wide wavelength coverage of 0.4–1.01μm for 41 early-type galaxies (ETGs) in the MASSIVE survey and derive high signal-to-noise spectra within an aperture ofR_e/8. Using detailed stellar synthesis models, we constrain the elemental abundances and stellar IMF of each galaxy through full spectral modeling. All the ETGs in our sample have an IMF that is steeper than a Milky Way (Kroupa) IMF. The best-fit IMF mismatch parameter,α_IMF= (M/L)/(M/L)_MW, ranges from 1.1 to 3.1, with an average of 〈α_IMF〉 = 1.84, suggesting that on average, the IMF is more bottom heavy than Salpeter. Comparing the estimated stellar masses with the dynamical masses, we find that most galaxies have stellar masses that are smaller than their dynamical masses within the 1σuncertainty. We complement our sample with lower-mass galaxies from the literature and confirm that$\log ({\alpha }_{\mathrm{IMF}})$is positively correlated with$\log (\sigma )$,$\log (M_{\star })$, and$\log (M_{\mathrm{dyn}})$. From the combined sample, we show that the IMF in the centers of more massive ETGs is more bottom heavy. In addition, we find that$\log ({\alpha }_{\mathrm{IMF}})$is positively correlated with both [Mg/Fe] and the estimated total metallicity [Z/H]. We find suggestive evidence that the effective stellar surface density Σ_Kroupamight be responsible for the variation ofα_IMF. We conclude thatσ, [Mg/Fe], and [Z/H] are the primary drivers of the global stellar IMF variation.

 

Abstract We present a detailed analysis of the behavior of the triaxial Schwarzschild orbit superposition method near the axisymmetric limit. Orbit superposition modeling is the primary method used to determine dynamical masses of supermassive black holes ( M BH ) in nearby galaxies; however, prior studies have reported conflicting results when comparing the outcome from axisymmetric orbit codes with that from a triaxial orbit code in the axisymmetric limit. We show that in order to achieve (oblate) axisymmetry in a triaxial code, care needs to be taken to axisymmetrize the short-axis tube orbits and to exclude both the long-axis tube and box orbits from the orbit library. Using up to 12 Gauss–Hermite moments of the line-of-sight velocity distributions as constraints, we demonstrate the effects of orbit types on the best-fit M BH in orbit modeling of the massive elliptical galaxy NGC 1453 reported in Liepold et al. In addition, we verify the efficacy of our updated code on a mock galaxy data set. We identify a subset of slowly precessing quasi-planar orbits for which the typical integration times can be insufficient to fully capture the equilibrium orbital behavior in both axisymmetric and triaxial systems with central black holes. Further investigation is needed for a more reliable treatment of these orbits. 

Recently we found compelling evidence for a gravitational-wave background with Hellings and Downs (HD) correlations in our 15 yr data set. These correlations describe gravitational waves as predicted by general relativity, which has two transverse polarization modes. However, more general metric theories of gravity can have additional polarization modes, which produce different interpulsar correlations. In this work, we search the NANOGrav 15 yr data set for evidence of a gravitational-wave background with quadrupolar HD and scalar-transverse (ST) correlations. We find that HD correlations are the best fit to the data and no significant evidence in favor of ST correlations. While Bayes factors show strong evidence for a correlated signal, the data does not strongly prefer either correlation signature, with Bayes factors ∼2 when comparing HD to ST correlations, and ∼1 for HD plus ST correlations to HD correlations alone. However, when modeled alongside HD correlations, the amplitude and spectral index posteriors for ST correlations are uninformative, with the HD process accounting for the vast majority of the total signal. Using the optimal statistic, a frequentist technique that focuses on the pulsar-pair cross-correlations, we find median signal-to-noise ratios of 5.0 for HD and 4.6 for ST correlations when fit for separately, and median signal-to-noise ratios of 3.5 for HD and 3.0 for ST correlations when fit for simultaneously. While the signal-to-noise ratios for each of the correlations are comparable, the estimated amplitude and spectral index for HD are a significantly better fit to the total signal, in agreement with our Bayesian analysis.

 

We present a stellar dynamical mass measurement of a newly detected supermassive black hole (SMBH) at the center of the fast-rotating, massive elliptical galaxy NGC 2693 as part of the MASSIVE survey. We combine high signal-to-noise ratio integral field spectroscopy (IFS) from the Gemini Multi-Object Spectrograph with wide-field data from the Mitchell Spectrograph at McDonald Observatory to extract and model stellar kinematics of NGC 2693 from the central ∼150 pc out to ∼2.5 effective radii. Observations from Hubble Space Telescope WFC3 are used to determine the stellar light distribution. We perform fully triaxial Schwarzschild orbit modeling using the latest TriOS code and a Bayesian search in 6D galaxy model parameter space to determine NGC 2693's SMBH mass (M_BH), stellar mass-to-light ratio, dark matter content, and intrinsic shape. We find$M_{\mathrm{BH}}=\left(1.7\pm 0.4\right)\times {10}^9{M}_{\odot }$and a triaxial intrinsic shape with axis ratiosp=b/a= 0.902 ± 0.009 and$q=c/a={0.721}_{-0.010}^{+0.011}$, triaxiality parameterT= 0.39 ± 0.04. In comparison, the best-fit orbit model in the axisymmetric limit and (cylindrical) Jeans anisotropic model of NGC 2693 prefer$M_{\mathrm{BH}}=\left(2.4\pm 0.6\right)\times {10}^9{M}_{\odot }$and$M_{\mathrm{BH}}=\left(2.9\pm 0.3\right)\times {10}^9{M}_{\odot }$, respectively. Neither model can account for the non-axisymmetric stellar velocity features present in the IFS data.

 

We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1μas resolution and ∼1μJy sensitivity could access >10⁶SMBH shadows. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2μas resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1μas resolution and ≲5μJy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.

 

We have used Hubble Space Telescope (HST) images, SAURON Integral Field Spectroscopy (IFS), and adaptative optics assisted Gemini NIFS near-infrared K-band IFS to map the stellar and gas distribution, excitation and kinematics of the inner few kpc of the nearby edge-on S0 galaxy NGC 4111. The HST images map its ≈450 pc diameter dusty polar ring, with an estimated gas mass ≥107 M⊙. The NIFS data cube maps the inner 110 pc radius at ≈7 pc spatial resolution, revealing a ≈220 pc diameter polar ring in hot (2267 ± 166 K) molecular H2 1–0 S(1) gas embedded in the polar ring. The stellar velocity field shows disc-dominated kinematics along the galaxy plane both in the SAURON large scale and in the NIFS nuclear-scale data. The large-scale [O iii] λ5007 Å velocity field shows a superposition of two disc kinematics: one similar to that of the stars and another along the polar ring, showing non-circular motions that seem to connect with the velocity field of the nuclear H2 ring, whose kinematics indicate accelerated inflow to the nucleus. The estimated mass inflow rate is enough not only to feed an active galactic nucleus (AGN) but also to trigger circumnuclear star formation in the near future. We propose a scenario in which gas from the polar ring, which probably originated from the capture of a dwarf galaxy, is moving inwards and triggering an AGN, as supported by the local X-ray emission, which seems to be the source of the H2 1–0 S(1) excitation. The fact that we see neither near-UV nor Br γ emission suggests that the nascent AGN is still deeply buried under the optically thick dust of the polar ring.

 

We present a measurement of the Hubble constantH₀from surface brightness fluctuation (SBF) distances for 63 bright, mainly early-type galaxies out to 100 Mpc observed with the WFC3/IR on the Hubble Space Telescope (HST). The sample is drawn from several independent HST imaging programs using the F110W bandpass, with the majority of the galaxies being selected from the MASSIVE survey. The distances reach the Hubble flow with a median statistical uncertainty per measurement of 4%. We construct the Hubble diagram with these IR SBF distances and constrainH₀using four different treatments of the galaxy velocities. For the SBF zero-point calibration, we use both the existing tie to Cepheid variables, updated for consistency with the latest determination of the distance to the Large Magellanic Cloud from detached eclipsing binaries, and a new tie to the tip of the red giant branch (TRGB) calibrated from the maser distance to NGC 4258. These two SBF calibrations are consistent with each other and with theoretical predictions from stellar population models. From a weighted average of the Cepheid and TRGB calibrations, we deriveH₀= 73.3 ± 0.7 ± 2.4 km s⁻¹Mpc⁻¹, where the error bars reflect the statistical and systematic uncertainties. This result accords well with recent measurements ofH₀from Type Ia supernovae, time delays in multiply lensed quasars, and water masers. The systematic uncertainty could be reduced to below 2% by calibrating the SBF method with precision TRGB distances for a statistical sample of massive early-type galaxies out to the Virgo cluster measured with the James Webb Space Telescope.

 

Analyses of pulsar timing data have provided evidence for a stochastic gravitational wave background in the nanohertz frequency band. The most plausible source of this background is the superposition of signals from millions of supermassive black hole binaries. The standard statistical techniques used to search for this background and assess its significance make several simplifying assumptions, namely (i) Gaussianity, (ii) isotropy, and most often, (iii) a power-law spectrum. However, a stochastic background from a finite collection of binaries does not exactly satisfy any of these assumptions. To understand the effect of these assumptions, we test standard analysis techniques on a large collection of realistic simulated data sets. The data-set length, observing schedule, and noise levels were chosen to emulate the NANOGrav 15 yr data set. Simulated signals from millions of binaries drawn from models based on the Illustris cosmological hydrodynamical simulation were added to the data. We find that the standard statistical methods perform remarkably well on these simulated data sets, even though their fundamental assumptions are not strictly met. They are able to achieve a confident detection of the background. However, even for a fixed set of astrophysical parameters, different realizations of the universe result in a large variance in the significance and recovered parameters of the background. We also find that the presence of loud individual binaries can bias the spectral recovery of the background if we do not account for them.