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Register a new userThe Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817
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The historic detection of gravitational waves from a binary neutron star merger (GW170817) and its electromagnetic counterpart led to the first accurate (sub-arcsecond) localization of a gravitational-wave event. The transient was found to be $sim$10 from the nucleus of the S0 galaxy NGC 4993. We report here the luminosity distance to this galaxy using two independent methods. (1) Based on our MUSE/VLT measurement of the heliocentric redshift ($z_{rm helio}=0.009783pm0.000023$) we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be $v_{rm CMB}=3231 pm 53$ km s$^{-1}$. Using constrained cosmological simulations we estimate the line-of-sight peculiar velocity to be $v_{rm pec}=307 pm 230$ km s$^{-1}$, resulting in a cosmic velocity of $v_{rm cosmic}=2924 pm 236$ km s$^{-1}$ ($z_{rm cosmic}=0.00980pm 0.00079$) and a distance of $D_z=40.4pm 3.4$ Mpc assuming a local Hubble constant of $H_0=73.24pm 1.74$ km s$^{-1}$ Mpc$^{-1}$. (2) Using Hubble Space Telescope measurements of the effective radius (15.5 $pm$ 1.5) and contained intensity and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10 clusters containing 226 galaxies, this yields a distance estimate of $D_{rm FP}=44.0pm 7.5$ Mpc. The combined redshift and FP distance is $D_{rm NGC 4993}= 41.0pm 3.1$ Mpc. This electromagnetic distance estimate is consistent with the independent measurement of the distance to GW170817 as obtained from the gravitational-wave signal ($D_{rm GW}= 43.8^{+2.9}_{-6.9}$ Mpc) and confirms that GW170817 occurred in NGC 4993.
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Recently, the optical counterpart of a gravitational wave source GW170817 has been identified in NGC 4993 galaxy. Together with evidence from observations in electromagnetic waves, the event has been suggested as a result of a merger of two neutron stars. We analyze the multi-wavelength data to characterize the host galaxy property and its distance to examine if the properties of NGC 4993 are consistent with this picture. Our analysis shows that NGC 4993 is a bulge-dominated galaxy with reff ~ 2-3 kpc and the Sersic index of n = 3-4 for the bulge component. The spectral energy distribution from 0.15 to 24 micron indicates that this galaxy has no significant ongoing star formation, the mean stellar mass of (0.3 - 1.2) times 10^11 Msun,the mean stellar age greater than ~3 Gyr, and the metallicity of about 20% to 100% of solar abundance. Optical images reveal dust lanes and extended features that suggest a past merging activity. Overall, NGC 4993 has characteristics of normal, but slightly disturbed elliptical galaxies. Furthermore, we derive the distance to NGC 4993 with the fundamental plane relation using 17 parameter sets of 7 different filters and the central stellar velocity dispersion from literature, finding an angular diameter distance of 37.7 +- 8.7 Mpc. NGC 4993 is similar to some host galaxies of short gamma-ray bursts but much different from those of long gamma-ray bursts, supporting the picture of GW170817 as a result of a merger of two NSs.
The current tension between the direct and the early Universe measurements of the Hubble Constant, $H_0$, requires detailed scrutiny of all the data and methods used in the studies on both sides of the debate. The Cepheids in the type Ia supernova (SNIa) host galaxy NGC 5584 played a key role in the local measurement of $H_0$. The SH0ES project used the observations of this galaxy to derive a relation between Cepheids periods and ratios of their amplitudes in different optical bands of the Hubble Space Telescope (HST), and used these relations to analyse the light curves of the Cepheids in around half of the current sample of local SNIa host galaxies. In this work, we present an independent detailed analysis of the Cepheids in NGC 5584. We employ different tools for our photometric analysis and a completely different method for our light curve analysis, and we do not find a systematic difference between our period and mean magnitude measurements compared to those reported by SH0ES. By adopting a period-luminosity relation calibrated by the Cepheids in the Milky Way, we measure a distance modulus $mu=31.810pm0.047$ (mag) which is in agreement with $mu=31.786pm0.046$ (mag) measured by SH0ES. In addition, the relations we find between periods and amplitude ratios of the Cepheids in NGC 5584 are significantly tighter than those of SH0ES and their potential impact on the direct $H_0$ measurement will be investigated in future studies.
The joint detection of gravitational waves and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the $r$-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the standard siren technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC4993. In this paper we measure the surface brightness fluctuation (SBF) distance to NGC4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $m{-}M=33.05pm0.08pm0.10$ mag, or a linear distance $d=40.7pm1.4pm1.9$ Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC4993 implies a Hubble constant $H_0=71.9pm 7.1~km~s^{-1}~Mpc^{-1}$. A comparison of our result to the GW-inferred value of $H_0$ indicates a binary orbital inclination of $i,{gtrsim},137~deg$. The SBF technique can be applied to early-type host galaxies of BNS mergers to ${sim,}100$ Mpc with HST and possibly as far as ${sim,}300$ Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW signals at distances where many future BNS mergers are likely to be detected.
We present a study of NGC 4993, the host galaxy of the GW170817 gravitational wave event, the GRB170817A short gamma-ray burst (sGRB) and the AT2017gfo kilonova. We use Dark Energy Camera imaging, AAT spectra and publicly available data, relating our findings to binary neutron star (BNS) formation scenarios and merger delay timescales. NGC4993 is a nearby (40 Mpc) early-type galaxy, with $i$-band Sersic index $n=4.0$ and low asymmetry ($A=0.04pm 0.01$). These properties are unusual for sGRB hosts. However, NGC4993 presents shell-like structures and dust lanes indicative of a recent galaxy merger, with the optical transient located close to a shell. We constrain the star formation history (SFH) of the galaxy assuming that the galaxy merger produced a star formation burst, but find little to no on-going star formation in either spatially-resolved broadband SED or spectral fitting. We use the best-fit SFH to estimate the BNS merger rate in this type of galaxy, as $R_{NSM}^{gal}= 5.7^{+0.57}_{-3.3} times 10^{-6} {rm yr}^{-1}$. If star formation is the only considered BNS formation scenario, the expected number of BNS mergers from early-type galaxies detectable with LIGO during its first two observing seasons is $0.038^{+0.004}_{-0.022}$, as opposed to $sim 0.5$ from all galaxy types. Hypothesizing that the binary system formed due to dynamical interactions during the galaxy merger, the subsequent time elapsed can constrain the delay time of the BNS coalescence. By using velocity dispersion estimates and the position of the shells, we find that the galaxy merger occurred $t_{rm mer}lesssim 200~{rm Myr}$ prior to the BNS coalescence.
Gravitational waves produced from the merger of binary neutron stars (BNSs) are accompanied by electromagnetic counterparts, making it possible to identify the associated host galaxy. We explore how properties of the host galaxies relate to the astrophysical processes leading to the mergers. It is thought that the BNS merger rate within a galaxy at a given epoch depends primarily on the galaxys star-formation history as well as the underlying merger time-delay distribution of the binary systems. The stellar history of a galaxy, meanwhile, depends on the cosmological evolution of the galaxy through time, and is tied to the growth of structure in the Universe. We study the hosts of BNS mergers in the context of structure formation by populating the Universe Machine simulations with gravitational-wave events~ according to a simple time-delay model. We find that different time-delay distributions predict different properties of the associated host galaxies, including the distributions of stellar mass, star-formation rate, halo mass, and local and large-scale clustering of hosts. BNSs that merge today with short delay times prefer to be in hosts that have high star-formation rates, while those with long delay times live in dense regions within massive halos that have low star formation. We show that with ${mathcal O}(10)$ events from current gravitational-wave detector networks, it is possible to make preliminary distinctions between formation channels which trace stellar mass, halo mass, or star-formation rate. We also find that strategies to follow up gravitational-wave events with electromagnetic telescopes can be significantly optimized using the clustering properties of their hosts.
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