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GCN Circular 21645

Subject
LIGO/Virgo G298048: Global properties and star-formation rate of NGC 4993
Date
2017-08-22T16:55:17Z (7 years ago)
From
David Kaplan at UW-Milwaukee <kaplan@uwm.edu>
E. M. Sadler (University of Sydney), J. R. Allison (University of Sydney), D. L. Kaplan (UWM), T. Murphy (University of Sydney) on behalf of the VAST collaboration.

The global properties of the candidate host galaxy NGC 4993 appear entirely consistent with those of a normal early-type galaxy, with a negligible level of ongoing star formation. 

An archival optical spectrum from the 6dF Galaxy Survey (6dFGS; Jones et al. 2009) is dominated by the light of an old stellar population along with weak H-alpha and [NII] emission lines. The relatively high [NII]/H-alpha emission-line ratio is suggestive of a low-ionisation AGN rather than star formation. The galaxy is devoid of any large neutral gas reservoir, with an upper limit to the neutral hydrogen mass of M_HI < 2e9 Msun (HIPASS; Meyer et al. 2004).

The 6dFGS fibre (6 arcsecond diameter) only covers the central region of the galaxy, and excludes the location of the putative optical counterpart, so it is still possible that star formation could be occurring outside the region covered by the 6dFGS spectrum. To check this, we looked at several photometric indicators of the current rate of massive star formation across the galaxy as a whole. 

We caution that all the values derived here should be considered as upper limits because of the possibility of contamination from a central AGN. From GALEX photometry (S_NUV ~ 1.08e-4 Jy) we obtain a near UV luminosity of L_NUV ~ 1.9e19 W/Hz. Using the relations from A. Hopkins (PhD thesis) and Cowie et al. (1997) gives an estimated NUV-derived star formation rate of SFR(NUV) ~ 0.003 Msun/yr for massive stars with M > 5 Msun. This low star formation rate is consistent with the global upper limit from far infrared photometry.  NGC 4993 is undetected by IRAS, so we adopt a conservative upper limit of 0.25 Jy for the FIR flux density at 60 microns. This gives a 60 micron luminosity L_FIR < 4.3e22 W/Hz. From Condon et al. (1992), this translates to a star formation rate (for stars more massive than 5 Msun) of SFR(FIR) < 0.08 Msun/yr. 

The existence of a low-luminosity radio-emitting active galactic nucleus in this galaxy is not surprising. The bivariate radio luminosity function of Mauch & Sadler (2007) implies that approximately 10% of galaxies of similar stellar mass to NGC 4993 host a radio AGN at least as strong as the central radio source reported for NGC 4993 (LVC GCNs 21537, 21548, and others), so the presence of this source is not unusual. The nuclear dust lanes evident in the HST ACS images (Foley et al. LVC GCN 21536) are similar to those seen in many other early-type galaxies, and may be the product of a minor merger than occurred as long as several Gyr ago.  

In summary, there is persuasive evidence that the current rate of massive-star formation in NGC 4993 is very low. We therefore speculate that the massive-star progenitors of the binary neutron-star system are likely to have formed and exploded several Gyr ago, implying a long in-spiral time for the neutron-star pair that produced the GW event.
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