Gravitational Wave Followup Overview
Welcome to the NED Gravitational Wave Follow-up (GWF) Service
The purpose of this NED service is to facilitate searches for electromagnetic (EM) counterparts to gravitational wave (GW) events. Within minutes after the Laser Interferometer Gravitational-Wave Observatory (LIGO)-Virgo-Kagra collaboration (LVK) issues an alert using the General Coordinates Network (GCN; https://gcn.nasa.gov/↗) operated by the NASA, this service responds by cross-matching in 3D the event's HEALPix map with the galaxies in the local Universe and the following results are provided: an all-sky image of the probability contours, the location of all galaxies in NED within the LVK 90% probability volume, and the top 20 galaxies sorted by the joint 3D and WISE W1 luminosity probabilities (P_3D*P_LumW1). To get started, click on "Events" in the menu above or simply click on the (blue) button below for the most recent GW event.
Note: The NED and Census of the Local Universe (CLU) projects are issuing a joint notice of candidate galaxies for each LVK GW trigger. While a large fraction of the galaxies in CLU catalog come from NED, there are galaxies in CLU that have not yet been ingested into NED, and NED is continually updating its holdings. Thus, some of the CLU top 20 galaxies listed in the GCN notices may not be in the NED list on this website. However, the overall composition of the CLU and NED galaxy lists will be similar.
Overview
We have now entered the era of gravitational wave (GW) astronomy and started to explore the Universe through this new window. Our understanding of these events is greatly enhanced by identifying and studying their EM counterparts or host galaxies, a field known as multi-messenger astronomy (MMA). The sky localizations for GW events detected by LVK are generally large (>10 square degrees; see Figure 1), making this task very challenging. However, an effective strategy to increase the efficiency of observational follow-up campaigns is to search for kilonovae or afterglows by targeting galaxies in the highest probability volume of the GW event.
This service exploits the rich and constantly growing content of the NASA/IPAC Extragalactic Database (NED) to help astronomers around the world to efficiently search for the EM counterparts to GW events. A list of galaxies in the local Universe is constructed by using available redshift and redshift-independent distances (see below for a description). Galaxy clusters, galaxy groups, and components of galaxies are omitted to keep the sample confined to candidate host galaxies for the GW event. Prior to January 2020, a distance of 200 Mpc was used to define the NED Local Universe Sample in order to match the targeted sensitivity of LIGO/Virgo to binary neutron stars (BNS). However, several BNS and neutron star-black hole (NSBH) events occurred during the LIGO/Virgo O3 run beyond 200 Mpc. Consequently, we have increased the distance limit of the NED Local Universe Sample to 1 Gpc to provide coverage of future events. We remind users that the availability of galaxy distances and redshifts are increasingly incomplete at greater distances, as further described below. Also, if an event's mean distance minus 1 standard deviation is greater than the NED-GWF distance cut of 1 Gpc the galaxy cross-match is not performed and no galaxies are provided.
An overview of the process is described here. The NED GW follow-up service listens to public alerts on the General Coordinates Network (GCN)↗, which is used by LVK to distribute GW event information. After each GW event, the HEALPix map is cross-matched in 3D to the NED galaxy list. During this process an all-sky image of the probability contours, the location of all galaxies in the 90% volume, and the top 20 galaxies sorted by the joint 3D and WISE W1 luminosity probabilities (P_3D*P_LumW1) are displayed on each event's page.
The prioritization of galaxies chosen currently is based on the only host of a BNS event (GW170817), which is a massive early-type galaxy (NGC 4993; Coulter et. al (2017)↗). Theoretical studies indicate that BNS events are likely to occur in more massive galaxies due to long merger delay time (>1 Gyr; e.g., Dominik et. al (2012)↗). We have chosen the WISE W1 luminosity as this light closely traces the older stellar population of a galaxy and hence is a proxy for stellar mass (Jarrett et. al (2013)↗). However, the expected host population of BNS events is still debated in the literature (e.g., Nugent et. al (2022)↗). Thus, we provide additional pre-computed prioritization metrics (star formation rate, P_3D*P_SFR; stellar mass, P_3D*P_Mstar; and specific star formation rate, P_3D*P_sSFR) which are available via downloading the entire galaxy list inside the event's probability volume.
Galaxies in the Local Volume Sample (NED-LVS)
The NED Local Volume Sample (NED-LVS) is a subset of ~1.9 million objects with distances out to 1000 Mpc. The distances selected for NED-LVS are based on both redshifts (98.6%) and redshift-independent distances (1.4%), where the latter are prioritized at distances less than 200~Mpc. Photometry from all-sky surveys (GALEX, 2MASS, and AllWISE) that have been joined into NED are also extracted, with 88% of NED-LVS objects having at least one measurement. These fluxes are used to derive physical properties (SFR and stellar mass) and to estimate the completeness relative to expected local luminosity densities. The completeness relative to NIR luminosities (which traces a galaxy's stellar mass) is roughly 100% at D<30 Mpc and remains moderate (70%) out to 300~Mpc. For brighter galaxies (>L★), NED-LVS is ~100% complete out to 450~Mpc. Tests of galaxy prioritization have also been performed in the volume of GW170817 with 3 physical properties, where we find that both stellar mass and inverse specific star formation rate (sSFR) place the correct host galaxy (NGC 4993) among the top ten candidates. The data in NED are updated regularly, and NED-LVS will be updated concurrently. Consequently, NED-LVS will continue to provide an increasingly complete sample of galaxies for a multitude of astrophysical research areas for years to come. For more details on properties and characterization of NED-LVS see Cook et al. 2023 (ApJS, submitted; link coming soon).
NED-LVS Download
The NED-LVS data file will be available once the paper is accepted for publication. Coming soon.
Latest Update (April 2023)
We have updated the NED-LVS galaxy list used for LVK 04 and characterized its properties and completeness in a paper submitted to ApJS (Cook et al. 2023, submitted; link coming soon). The current sample was extracted from NED on September 2021.
Acknowledgments
This service makes use of the General Coordinates Network (GCN) operated by NASA, and of the Python package ligo.skymap↗ written by Leo Singer (NASA/GSFC). We thank Leo Singer for his helpful comments and feedback.
Last update: 2023-4-17 17:58:13 PST
