TITLE:   GCN CIRCULAR
NUMBER:  21669
SUBJECT: LIGO/Virgo G298048: Properties of NGC 4993
DATE:    17/08/24 01:39:20 GMT
FROM:    Mansi M. Kasliwal at Caltech  <mansi@astro.caltech.edu>

Po-Chieh Yu (NCU), C.-C. Ngeow (NCU), W.-H. Ip (NCU) on behalf of the
GROWTH (Global Relay of Observatories Watching Transients Happen
collaboration)


Ogando et al. (2008) measured velocity dispersion and Lick indices for the
candidate host galaxy NGC 4993 using long-slit spectra obtained with the
1.52m ESO telescope. From their measurements of the central velocity
dispersion (163 km/s), we obtained the central black holes mass of
log(Mbh/Msun) ~ 7.7 using the relation given by Ferrarese & Merritt (2000).
We decomposed Pan-STARRS stacked r-band image of the galaxy using Galfit
(Peng et al. 2010). Our initial analysis obtained that the Sersic index n
=1.27. Using the Mbh-n relation (Savorgnan et al. 2013) gives a similar
estimation of the black hole mass log(Mbh/Msun) = 7.25. We also obtained the
effective radius Re = 2.8 kpc; the OT is located at ~2.2 kpc (0.78 Re) from
the nucleus.


We estimated the bolometric luminosity L_bol = 8.96e40 erg/s by adopting
Lx=5.6e39 erg/s (Evans et al. LVC GCN 21612) and using bolometric
correction Lbol/Lx = 16 for low-luminosity AGNs (LLAGNs; Ho 2008). We
obtained the Eddington luminosity L_Edd = 1.26e45.7 erg/s by using the Mbh
= 10e7.7 Msun given in this circular. This gives the Eddington ratio
L_bol/L_Edd = 7e(-5.7), which is similar to ratios of other LLAGNs (Ho
2008).


The galaxy has 18 companions, and it is considered to be located in medium
density regions (4 < N < 22) (Ogando et al. 2008). The galaxy mass
estimated by the effective radius and the central velocity dispersion
(Burstein et al.1997) is log(M*/Msun)=10.64, which is similar to the mean
value of the S0 sample of Ogando et al. (2008).


Our residual image of subtracting an IRAF elliptical model from the HST
archival ACS image  (F606W) shows clearly complicated dust lanes that are
extended from the nucleus to outer kpc regions, as mentioned in Foley et
al. (LVC GCN 21536). NGC 4993 could be a radio AGN (LVC GCN 21537, 21548,
21645). HST images show that some radio galaxies have optical jets (M87,
3C15, 3C78, 3C264). But no optical jets can be seen in our residual map of
NGC 4993.


In summary, the properties of the candidate host galaxy NGC 4993 seem to be
similar to normal early type galaxies, as indicated by Sadler et al. (LVC
GCN 21645).