DRAFT VERSION OCTOBER 19, 2017
Typeset using LATEX twocolumn style in AASTeX61
DECAM AND DES PERSPECTIVE OF THE GW170817 HOST, NGC 4993: INDICATION FOR
DYNAMICALLY–DRIVEN FORMATION OF BINARY NEUTRON STAR IN EARLY TYPE GALAXIES
A. PALMESE,1 W. HARTLEY,1 F. TARSITANO,2 C. CONSELICE,3 O. LAHAV,1 S. ALLAM,4 J. ANNIS,4 H. LIN,4 M. SOARES-SANTOS,5, 4
D. TUCKER,4 D. BROUT,6 M. BANERJI,7, 8 K. BECHTOL,9 H. T. DIEHL,4 A. FRUCHTER,10 J. GARCÍA-BELLIDO,11 K. HERNER,4
A. J. LEVAN,12 T. S. LI,4 C. LIDMAN,13, 14 K. MISRA,15 M. SAKO,6 D. SCOLNIC,16 M. SMITH,17 T. M. C. ABBOTT,18
F. B. ABDALLA,1, 19 A. BENOIT-LÉVY,20, 1, 21 E. BERTIN,20, 21 D. BROOKS,1 E. BUCKLEY-GEER,4 A. CARNERO ROSELL,22, 23
M. CARRASCO KIND,24, 25 J. CARRETERO,26 F. J. CASTANDER,27 C. E. CUNHA,28 C. B. D’ANDREA,6 L. N. DA COSTA,22, 23
C. DAVIS,28 D. L. DEPOY,29 S. DESAI,30 J. P. DIETRICH,31, 32 A. DRLICA-WAGNER,4 T. F. EIFLER,33, 34 A. E. EVRARD,35, 36
B. FLAUGHER,4 P. FOSALBA,27 J. FRIEMAN,4, 16 E. GAZTANAGA,27 D. W. GERDES,35, 36 T. GIANNANTONIO,7, 8, 37 D. GRUEN,28, 38
R. A. GRUENDL,24, 25 J. GSCHWEND,22, 23 G. GUTIERREZ,4 K. HONSCHEID,39, 40 B. JAIN,6 D. J. JAMES,41 T. JELTEMA,42
M. W. G. JOHNSON,25 M. D. JOHNSON,25 E. KRAUSE,28 R. KRON,4, 16 K. KUEHN,14 S. KUHLMANN,43 N. KUROPATKIN,4
M. LIMA,44, 22 M. A. G. MAIA,22, 23 M. MARCH,6 J. L. MARSHALL,29 R. G. MCMAHON,7, 8 F. MENANTEAU,24, 25 C. J. MILLER,35, 36
R. MIQUEL,45, 26 E. NEILSEN,4 R. L. C. OGANDO,22, 23 A. A. PLAZAS,34 K. REIL,38 A. K. ROMER,46 E. SANCHEZ,47 R. SCHINDLER,38
R. C. SMITH,18 F. SOBREIRA,48, 22 E. SUCHYTA,49 M. E. C. SWANSON,25 G. TARLE,36 D. THOMAS,50 R. C. THOMAS,51
A. R. WALKER,18 J. WELLER,31, 52, 37 Y. ZHANG,4 AND J. ZUNTZ53
1Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
2Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
3University of Nottingham, School of Physics and Astronomy, Nottingham NG7 2RD, UK
4Fermi National Accelerator Laboratory, P. O. Box 500, Batavia, IL 60510, USA
5Department of Physics, Brandeis University, Waltham, MA 02453, USA
6Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
7Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
8Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
9LSST, 933 North Cherry Avenue, Tucson, AZ 85721, USA
10Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
11Instituto de Fisica Teorica UAM/CSIC, Universidad Autonoma de Madrid, 28049 Madrid, Spain
12Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
13ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)
14Australian Astronomical Observatory, North Ryde, NSW 2113, Australia
15Aryabhatta Research Institute of observational sciencES (ARIES), Manora Peak, Nainital 263 001 India
16Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
17School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
18Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile
19Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa
20CNRS, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France
21Sorbonne Universités, UPMC Univ Paris 06, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France
22Laboratório Interinstitucional de e-Astronomia - LIneA, Rua Gal. José Cristino 77, Rio de Janeiro, RJ - 20921-400, Brazil
23Observatório Nacional, Rua Gal. José Cristino 77, Rio de Janeiro, RJ - 20921-400, Brazil
24Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA
25National Center for Supercomputing Applications, 1205 West Clark St., Urbana, IL 61801, USA
26Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra (Barcelona) Spain
27Institute of Space Sciences, IEEC-CSIC, Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
28Kavli Institute for Particle Astrophysics & Cosmology, P. O. Box 2450, Stanford University, Stanford, CA 94305, USA
29George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and
Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA
30Department of Physics, IIT Hyderabad, Kandi, Telangana 502285, India
31Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
32Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany
33Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA
34Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
35Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
36Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
37Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians Universität München, Scheinerstr. 1, 81679 München, Germany
38SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
39Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
40Department of Physics, The Ohio State University, Columbus, OH 43210, USA
41Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA
42Santa Cruz Institute for Particle Physics, Santa Cruz, CA 95064, USA
43Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
44Departamento de Física Matemática, Instituto de Física, Universidade de São Paulo, CP 66318, São Paulo, SP, 05314-970, Brazil
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2 PALMESE ET AL.
45Institució Catalana de Recerca i Estudis Avançats, E-08010 Barcelona, Spain
46Department of Physics and Astronomy, Pevensey Building, University of Sussex, Brighton, BN1 9QH, UK
47Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
48Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, 13083-859, Campinas, SP, Brazil
49Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
50Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, UK
51Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
52Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748 Garching, Germany
53Institute for Astronomy, University of Edinburgh, Edinburgh EH9 3HJ, UK
ABSTRACT
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 Sérsic index n = 4.0 and low asymmetry (A = 0.04± 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 RgalNSM = 5.7
+0.57
−3.3 ×
10−6yr−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 ∼ 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 tmer . 200 Myr prior to the BNS coalescence.
Keywords: gravitational waves — galaxy: evolution — galaxy: structure
1. INTRODUCTION
The first identification of the optical counterpart (LIGO
Scientific Collaboration et al. 2017) of a gravitational wave
(GW) signal (Abbott et al. 2017) marks the beginning of a
new era for multi-messenger astronomy. The coalescence
of neutron stars is expected to have strong optical and near-
infrared signatures in the form of a kilonova, the ejecta from
which are heated by the decay of heavy nuclei produced via
r-processes. Short Gamma–Ray Bursts (sGRB) are likely
to be related to the same coalescence events (Eichler et al.
1989), but the formation of the binary and the physics in-
volved in merging are still a matter of debate (Lipunov et al.
1997; Faber & Rasio 2012).
The optical counterpart to the BNS coalescence signal
GW170817 was discovered independently by several collab-
orations using optical telescopes, including the Dark Energy
Camera (DECam; Flaugher et al. 2015) GW team (Soares-
Santos et al. 2017). In this work we use this DECam data
and supplement it with Hubble Space Telescope, AAT spec-
troscopic data and with publicly available datasets to under-
stand the source in the context of its host galaxy and the local
environment.
In particular, we relate the BNS formation to the dynam-
ics and stellar evolution of the host over time, asking whether
the binary system was born as such, or whether dynamical in-
teractions caused its formation. Dynamically–driven binary
formation has been proposed for BBH (e.g. Rodriguez et al.
2016).
Previous studies (Carter et al. 1988) classified this galaxy
as an atypical elliptical galaxy with faint concentric shells
and spectral features suggesting that the galaxy has under-
gone a merging event. Several analytical and numerical stud-
ies support the galaxy merger scenario for the formation of
shells in galaxies (e.g. Quinn 1984; Pop et al. 2017), and
show that the distribution of shells can constrain the time of
the merger event. We study the evolution of this galaxy to
discern between different BNS formation scenarios and esti-
mate the rate of BNS formation in early-type galaxies, using
Dark Energy Survey (DES) data to place NGC4993 in the
context of the galaxy population.
2. DATA
2.1. Photometric data: DECam, VHS and HST
The DECam images used in this work were taken as part
of the DECam-GW follow up program between the nights
of August 17th and September 1st 2017, using ugrizY fil-
ters. We also use public ugrizY DECam data from June 2015
to avoid contamination in the transient region. In addition,
we extract Y JK data from the VISTA Hemisphere Survey
(VHS; McMahon et al. 2013), covering the host galaxy. The
images are coadded and registered to a common pixel scale
(0.2636′′) using SWARP (Bertin et al. 2002) with 3.5 sigma
DECAM GW170817 HOST GALAXY FOLLOW UP 3
Figure 1. Left panel: DECam coadded image of NCG4993 in gri. Shell structures indicative of a recent galaxy merger are clearly visible.
Middle panel: r-band residuals from GALFIT after subtraction of the best-fit single Sérsic light profile. Right panel: F606W-band HST ACS
image with a 3 component galaxy model subtracted. Dust lanes crossing the centre of the galaxy are evident. The green lines show the position
of the transient. The BNS counterpart is only present in the middle panel.
clipping to remove cosmic ray artifacts. An RGB coadded
image of the galaxy is presented in Figure 1. We build a
χ2 detection image from the r, i and z-band data and run
SEXTRACTOR (Bertin & Arnouts 1996) in dual mode on the
coadded images.
The photometry is corrected for galactic extinction. In or-
der to compare the galaxy properties to a broader sample, we
also use DES data from the first year of observations (Y1;
Drlica-Wagner et al. 2017). We use MAG_AUTO magnitudes
unless otherwise stated.
NGC4993 was also observed during HST Cycle 24
(PropID 14840, PI: Bellini) using ACS in F606W. The data
were publicly released in April 2017 and were accessed via
the Hubble Source Catalog (HSC; Whitmore et al. 2016).
2.2. Spectroscopic data: 6dF and AAT
The 6dF Galaxy Survey (Jones et al. 2004) final release
(Jones et al. 2009) includes an optical spectrum of the cen-
ter of NGC4993 with an estimated redshift (z = 0.009680±
0.000150).
Spectra of 14 galaxies with vhelio ∼3000km/s and within
one degree radius of NGC4993 were obtained in one target
of opportunity exposure of the AAOmega spectrograph at the
Anglo–Australian Telescope (AAT) on August 27th 2017. Of
those, 10 spectral fits passed quality cuts. All the spectra used
here are centered on their galaxy nucleus with a 2′′ aperture.
3. HOST MORPHOLOGY
3.1. CAS and Galfit
We begin our study of NGC4993 with an analysis of
its morphological properties, employing the CAS non-
parameteric light quantification (Conselice 2003) and para-
metric Sérsic light profile fitting with GALFIT (Peng et al.
2010). Both methods utilise a mask to exclude other sources
in the image and the location of the kilonova event. The
CAS system is able to pick out the salient features of galaxy
morphology, allowing galaxy types to be assigned and identi-
fying objects that are likely to have undergone a recent major
merger (see Conselice 2003 for details). Meanwhile, fitting
the light profile additionally provides us with an alternative
estimate of the total magnitude and can reveal more subtle
aspects of galaxy morphology within the residuals of the
model-subtracted image.
GALFIT is run on the DES and VHS images in two ways:
band-by-band and simultaneously across all bands using a
modified version, GALFIT-M (Vika et al. 2013). In the sec-
ond case the Sérsic fitting parameters are allowed to vary
with wavelength as a second-order polynomial. We extract
the PSF model required by GALFIT from the coadd images
with PSFEX (Bertin 2011) and initialize the fitting param-
eters based on measurements of the galaxy from SEXTRAC-
TOR. All parameters are left free without constraints, except
for the central position in the single-band fits. This is al-
lowed to vary by only ±1 pixel as it is well-constrained by
SEXTRACTOR already.
In order to assess the stability of GALFIT and obtain
an estimate of the uncertainties on the measurements, each
single–band run is performed 10,560 times, varying the in-
puts around their nominal values. We take the median as our
final measurement and the standard deviation as the uncer-
tainty.
3.2. Results
Following the definitions given in Conselice (2003), we
find: concentration C = 3.348±0.035, asymmetry A = 0.04±
0.01, and clumpiness S = 0.05±0.05. These values are typ-
4 PALMESE ET AL.
Filter MAG_AUTO Mag re n  θ
u 14.24 14.15 61.8 3.2 0.15 -13.9
g 12.95 12.80 62.5 3.4 0.15 -12.8
r 12.08 11.90 63.5 3.7 0.16 -11.2
i 11.65 11.45 64.4 4.0 0.16 -9.9
z 11.34 11.13 65.3 4.3 0.16 -8.4
Y 11.13 10.96 65.7 4.4 0.16 -7.7
YVHS 11.27 11.00 65.9 4.5 0.16 -7.5
J 11.00 10.77 67.3 5.0 0.17 -5.2
Ks 11.08 10.68 72.9 6.7 0.19 +3.5
±5×10−4 ±0.07 ±3×10−3 ±4×10−5 ±5×10−3
Table 1. Outputs from GALFIT parametric Sérsic fits performed on the ugrizY DECam coadd images and Y JKs VHS data. The fit was joint
across bands, allowing the effective radius, re (in pixels), Sérsic index, n, ellipticity,  = 1−b/a, and position angle, θ, to vary with wavelength.
One pixel corresponds to 0.2636′′. The final row lists indicative errors based on the single band analysis.
Figure 2. Concentration versus asymmetry for NGC4993 (in blue),
compared to a sGRB hosts sample (Conseliece et al., in prep.) in
red, and field galaxies (black dots) with stellar mass within ±0.2
dex of NGC4993 value and redshift z < 0.2. The lines separate
different Hubble types as shown in Conselice (2003).
ical for an early-type galaxy. In Figure 2 we compare these
values to field galaxies of similar masses (within 0.2 dex of
NGC4993) and redshifts (z < 0.2) from the GAMA survey,
and to a sample of sGRB hosts (Conselice et al. in prep.)
taken in F814W imaging from HST. NGC4993 stands out as
peculiar with respect to other GRB hosts: such objects tend
to lie on the more highly asymmetric side of late-type galax-
ies.
The results from the single Sérsic fit across all bands are
summarized in Table 1 (the band-by-band fits give broadly
consistent results). We find an increase in Sérsic index to-
wards redder bands and a rotation in the position angle. This
rotation of bluer versus redder bands suggests there could be
two superimposed stellar populations with differing orienta-
tions. This may have arisen during the course of the galaxy’s
secular evolution but could also be caused by a minor galaxy
merger, as indicated by the presence of shells.
The middle panel of Figure 1 shows DECam r-band resid-
uals from GALFIT and the position of the transient. At least
four shell structures are clearly visible. Closer inspection
with HST data (right panel in Figure 1) reveals a possible
further broad inner shell, on which the transient seems to lie,
and obvious dust lanes (visible also as a negative residual in
the DECam version). The r-band absolute magnitude from a
4 sq.arcsec region around the transient location in the galaxy-
subtracted template image is −10.65. This luminosity implies
a rather high stellar density in the locale of the BNS coales-
cence. In summary, we find compelling evidence for a recent
minor galaxy merger in NCG4993, and the location of the
kilonova event with respect to the shells leads us naturally to
ask whether there is a causal connection between the two.
From Figure 2 we see that clear major galaxy mergers
are unusual amongst sGRB hosts. Furthermore, the other
sGRBs are at cosmological distances and thus are mostly un-
dergoing extensive galaxy formation through star formation
or merging. If the hosts have to be related by some common
features, this is an indication that NGC4993 has undergone
some merging activity, but a minor merger such that the bulk
morphology is still elliptical. We thus explore the possibility
that the kilonova was a result of a recent galaxy merger in
NGC4993.
4. PHOTOMETRIC AND SPECTROSCOPIC SED
4.1. SED fitting methods
We use pPXF (Cappellari & Emsellem 2004; Cappellari
2017), for the spectral fitting. It enables extraction of the
stellar kinematics and stellar population from absorption line
DECAM GW170817 HOST GALAXY FOLLOW UP 5
Figure 3. Left panel: spectroscopic fit of the 6dF optical spectrum. The black line is the observed spectrum,the red line is the PPXF fit for the
stellar component, and the orange line is the best–fit including ionized gas emission lines. The zoomed panels show Hβ , OIII, NII and Hα lines.
The green points at the bottom are the fit residuals, while the purple line is the gas-only best–fit model spectrum. Right panel: BPT diagram
for NGC4993 (red star) and the other galaxies (black points) in the galaxy group with AAT spectra available. The dashed lines represent the
Kewley et al. (2006) classification method for AGN, star–forming (HII) and composite (Comp) galaxies. Many of the group galaxies have very
weak AGN or LINER-like emission. Error bars represent 1σ error from the propagation of fit errors on line strengths.
spectra of galaxies, using a maximum penalized likelihood
approach. We use the Miles stellar libraries, and fit over the
wavelengths 4000−7409 Å, excluding the range 5500−5600
Å of the 6dF spectrum, where a strong sky line contaminates
the flux.
We use LEPHARE (Arnouts et al. 1999, Ilbert et al. 2006)
for the broadband Spectral Energy Distribution (SED) fitting.
We add a 0.05 systematic uncertainty in quadrature to the
magnitudes. The simple stellar population (SSP) templates
used are Bruzual & Charlot (2003), with two metallicities
(Z and 2.5Z), a Chabrier (2003) Initial Mass Function
(IMF) and a Milky Way (Allen 1976) extinction law. The
SFH chosen is lognormal:
Ψ(t, t0, τ ) =
1
t
√
2piτ 2
e−
(ln t−ln t0)
2
2τ2 , (1)
as it is the most representative family of models with only
two parameters (Gladders et al. 2013). Here t0 and τ are the
half–mass–time and width.
Motivated by our morphological analysis, we allow for an
additional burst of recent SF. This is modeled as a Gaussian
centered at tburst with width of 10 Myr and peaking at a frac-
tion 0.4− 0.1 of the peak of the log-normal SFH (as no evi-
dence for strong late SF is found).
The same templates are used to perform spatially–resolved
SED fitting across DES+VHS coadded images within 10×
10 pixels, including the galaxy dust extinction. The other
sources in the field are masked out using the segmentation
map output by SEXTRACTOR.
4.2. SED fitting results
Figure 3 shows the best fit model of the 6dF spectrum,
which results in a reduced χ2 of 1.22. An analysis of the
mass fraction in age shows that part of the core galaxy stel-
lar population has a supersolar metallicity, but the weighted
mean value 〈[M/H]〉 = −0.012±0.010 is marginally consis-
tent with solar metallicity. The mean age is 11.298± 0.054
Gyr, and the mass-to-light ratio is 5.23±0.15 in r-band.
The stellar model fit reveals the existence of weak ionized
gas emission lines. However, the line ratios from the fit sug-
gests they are produced by a harder ionizing source than star-
formation, formally lying in the AGN region of the Bald-
win, Phillips & Telervich (BPT; Baldwin et al. 1981) dia-
gram. Blanchard et al. (2017) argue that there is a weak AGN
present in the core of the galaxy on the basis of radio and X-
ray emission, and so we conclude there to be no evidence
of recent star-formation from the 6dF spectrum, irrespective
of the highly uncertain O[III]/Hβ ratio. A comparison of
galaxies in the group using AAT spectra and classification by
Kewley et al. (2006) is shown in Figure 3.
Given the evidence of dust presence in the HST study,
we estimate the dust content using the Balmer decrement
(Berman 1936) observed from the spectrum. The redden-
ing is E(B −V ) = 0.12± 0.50 in the case of I(Hα)/I(Hβ) =
3.1, which is expected in the case of AGN activity. We
therefore restrict our dust models to have reddening values
0.1,0.2,0.3,0.4,0.5 in the photometric fits.
The photometric best–fitting template has a solar metallic-
ity, a quickly declining log-normal SFH with t0 = 3Gyr and
τ = 0.1. A low reddening E(B−V ) = 0.1 is preferred, and the
stellar mass is (2.95± 0.65)× 1010M. The inclusion of a
late SFH burst is disfavored by the fitting apart from inter-
mediate apertures.
Previous work found that the presence of dust lanes may
bias the galaxy stellar mass from unresolved galaxy SED fits
to lower values (Sorba & Sawicki 2015). The total stellar
mass from fitting over the SEXTRACTOR segmentation map
of NGC4993 is (3.8±0.20)×1010M, more than 1σ higher
6 PALMESE ET AL.
Figure 4. Upper panel: sSFR map resulting from the pixel SED
fitting of DES+VHS bands. Other objects have been masked out.
x and y axes correspond to the map pixels. One pixel corresponds
to a physical size of 0.526 kpc at the galaxy redshift. Lower panel:
pixel color-magnitude diagram for the pixels covering NGC4993
from DECam g and r single epoch exposures taken previously to
the BNS event. The core of the galaxy is shown in red, while the
cyan points represent the 1.5′′ around the location of the BNS event
(10.6′′ from the center).
than the unresolved SED fitting. The specific SFR (sSFR)
map from our pixel SED fits is shown in Figure 4, where
the shell structure is clearly visible, suggesting that the sSFR
is slightly more accentuated in the stellar halo compared to
the inner parts. Younger ages (by ∼ 2 Gyr) are also pre-
ferred in the outer regions, though we still do not find ev-
idence for a star formation burst at late times, and explain
our results by the stripping of stellar populations from the
lower-mass galaxy in a minor dry merger. A dust model
with E(B−V ) = 0.1 is preferred in the inner few kpc, while
E(B−V ) = 0 is found outside. Despite the presence of dust
lanes, an analysis of the HST photometry and a comparison
with extinction models suggests that the effect of dust is not
extreme, with reddening values that are consistent with 0.1 in
the core. We therefore believe that the dust obscuration does
not play a significant role in our SFR estimates.
4.3. Pixel Color Diagrams
In Figure 4 we show a color-magnitude diagram for all the
pixels within the field of view of the DECam data near the
galaxy. The image has been cleaned of stars and other con-
tamination, thus all points come from the galaxy itself. The
position of the GW source, 10.6′′ offset from the center, is the
cyan colored pixels, while the center of the galaxy is shown
in the red points. This galaxy is well represented by a pixel
“main sequence” that is bluer at fainter levels, which is typi-
cal of early–type galaxy color gradients (e.g., Lanyon-Foster
et al. 2007). We conclude that there is no significant dif-
ference between the transient position and other outer light,
although it is bluer than the core region. This further sup-
ports the scenario in which the BNS formation is not related
to some particular recent star formation event in this region.
5. DISCUSSION AND CONCLUSIONS
5.1. BNS formation and delay time under the hypothesis of
galaxy merger
In the most accepted shell formation scenarios the shells
are stellar debris coming from the less massive, stripped
galaxy, and the arcs form at the apocenter of the orbits of
the infalling material (Quinn 1984).
Based on our results, we believe that NGC4993 experi-
enced a dry minor galaxy merger with still visible signs. The
shells are expected to be washed out within a time that de-
pends on the velocity dispersion at their position. We esti-
mate the shell survival time in two ways, based on the veloc-
ity dispersion of the galaxy as well as the velocity dispersion
of the shell itself. From the 6dF spectrum the line–of–sight
central velocity dispersion is σv = (160.0± 9.1)kms−1. We
estimate its value at the position of the transient. The ve-
locity dispersion of early type galaxies drops from its cen-
tral peak value at larger radii, and observations show that the
maximum drop to the outer parts of ellipticals near the ef-
fective radius is ∼ 40% of the central value (Emsellem et al.
2004). Based on the distance of the shell from the center,
R≈ 4 kpc, we estimate that the dynamical time at this radius
is tdyn ≡ R/σv ≈ 60 Myr (the line-of-sight velocity is relevant
here, given the shell’s geometry, but e.g. if we assume a 3D
isotropic velocity dispersion it would reduce the dynamical
time by
√
3).
So far we have no measurement of the shell’s velocity dis-
persion, but estimates from the literature suggest for similar
shells in other galaxies σv ≈ 20 km/s (Quinn 1984). This
would give a dynamical time scale of tdyn ≈ 192Myr. De-
tailed simulations of shells in other galaxies suggest that sur-
DECAM GW170817 HOST GALAXY FOLLOW UP 7
vival time could be even larger that 1 Gyr, depending on the
assumed scenario (Pop et al. 2017).
The survival time of the shell could be used as an upper
limit for the time the minor merger took place, i.e tdyn ≥ tmerg,
so we estimate tmerg . 200 Myr.
If the BNS was formed as such in a shell, then we would
have expected to see evidence for recent star formation, but
we find no indication of this. In the absence of star forma-
tion it is plausible that the BNS coalescence was triggered
by a dynamical process, e.g. NS-NS capture or the desta-
bilization of a pre-existing wide-separation binary. These
processes will be quite sensitive to the stellar density which,
given the Sérsic index and the luminosity from the residual
image found in section 3.2, is high in the center of NGC4993
and around the transient position. If this dynamical hypothe-
sis is true, then the delay time ∆tNSM between the BNS for-
mation and coalescence is . 200 Myr. On the other hand,
Blanchard et al. (2017) find a median delay time of 11+0.7−1.4Gyr
under the assumption that the binary was formed through sec-
ular SF.
5.2. Galaxy environment
If the binary formation is related to dynamical processes
in galaxy merging as we are investigating here, then this is
most likely to happen in galaxy groups and low mass clusters.
According to the 2MASS catalog (Tully 2015), NGC4993
resides in a group, of which we analyze the remaining 7
galaxies. A spectral analysis shows that NGC4993 is not
the only galaxy showing AGN activity (see Figure 3), but it
is peculiar in terms of age, metallicity and mass-to-light ra-
tio. It shows an older stellar population (the mean age of the
other 13 galaxies is Log(Age) = 9.56±0.17), lower metallic-
ity (mean: M/H = −0.31± 0.11), and higher M/Lr (mean:
2.41± 0.45 ) than the average. The group has a projected
virial radius of Rvir = 0.36 Mpc and a line-of-sight velocity
dispersion σv = 143kms−1 (Tully 2015). The crossing time is
therefore tcr ∼ Rv/(
√
2.5σv)∼ 1.6 Gyr.
If galaxy mergers are correlated to BNS coalescence, fu-
ture GW studies could possibly concentrate on galaxy groups
(but note that these are crowded regions and therefore match-
ing candidates to a host could be difficult). In order to have
precise measurements of H0, one needs to identify the host
galaxy redshift clearly. When the match is clear, the proper-
ties of the type of host galaxy found could help future studies
to select the right host galaxy or create galaxy catalogs of
likely hosts for GW EM follow-up and untriggered kilonova
searches (Doctor et al. 2017). In fact, large photometric sur-
veys such as DES, LSST or WFIRST are expected to observe
kilonova events at redshifts beyond the sensitivity of GW ex-
periments, where the angular separation between galaxies de-
creases (Scolnic et al. 2017).
5.3. BNS merging constraints
We derive a constraint on neutron stars merging rate at time
t by using:
RNSM(t) = αRNS(t′) , (2)
where α is the fraction of neutron stars which are in binaries,
t′ = t −∆tNSM and the fraction of mass of formed stars that
are NS is:
RNS(t′) =
∫
dM?Φ(M?)Ψ(t?)ΘNS(M?) , (3)
withΦ(M?) being the IMF,Ψ(t?) is our best fit SFH,ΘNS(M?)
is 1 for star mass ranges of 8M < M < 20M, zero other-
wise. We drop the metallicity dependence inΘNS because we
only consider a solar metallicity for the galaxy, as a result of
our spectroscopic fit. t? is the time when the progenitor of the
NS was formed, therefore satisfying t′ = t? + tlife, with tlife be-
ing the lifetime of the progenitor before becoming a NS. We
assume a tlife = 0.02 Gyr, but our calculation is insensitive
to this choice as the typical lifetime of these massive stars
(∼ 0.01− 0.03 Gyr) is much shorter than the timescale over
which the SFH found for NGC4993 is changing at late times.
We assume a Chabrier IMF, but this choice is not relevant as
we are only exploring the high mass end of the IMF. Assum-
ing α = 0.002 and the distribution of ∆tNSM from Vangioni
et al. (2016) (their Figure 3 for solar metallicity), and our best
fit SFH from Eq. 1 with t0 = 3Gyr and τ = 0.3, we get a NS
formation rate of RgalNS = 3.6
+28
−3.6×10−5 yr−1 and a BNS merger
rate of RgalNSM = 5.7
+0.57
−3.3 ×10−6 yr−1 for the whole galaxy. Er-
rors reflect the uncertainty on the SFH, which dominates our
errors: they represent the two central quartiles of the rates
distribution computed with the SFHs of the pixel SED fitting
over the galaxy.
Given the sensitivity of the BNS merger event rate to
the recent SFR of a galaxy, it is somewhat surprising that
GW170817 occurred in an old, early type galaxy. We there-
fore ask what is the probability of observing such an event
in any early–type galaxy within the LIGO–detectable vol-
ume. To make this estimate we integrate the stellar mass
function of early–type galaxies from Weigel et al. (2016) and
scale the per–solar–mass rate from Eq. 2 to the mass con-
tained within the LIGO detectable volume (radius 80 Mpc).
We find RearlyNSM = 23
+2
−14 yr
−1Gpc−3 resulting in 0.038+0.004−0.022 ex-
pected events. This calculation assumes that the SFH of
NGC4993 is representative of local early–type galaxies. In
fact much of the mass will be contained in more massive,
and on average older and less star–forming, galaxies. We
contrast this with a similar calculation for all galaxy types,
using the cosmic SFR density from Gladders et al. (2013),
finding RallNSM ≈ 270 yr−1Gpc−3 and ∼ 0.5 expected events.
This result shows that it is unlikely that we observed one
such BNS merger with LIGO over the combined nine months
of operations in an early–type galaxy. The assumptions in
8 PALMESE ET AL.
the calculation include the fraction of NS that form in bina-
ries (α = 0.002) and the delay time distribution, both coming
from binary star models (where the progenitors of the BNS
were already a bound system) and satisfying Milky Way con-
straints. If the BNS formation mechanism is via dynamical
interaction, our result could point to a higher value of α or a
shorter ∆tNSM for systems that recently underwent a galaxy
merger, more so for those that have high stellar density (such
as early–type galaxies). It is therefore of interest to know the
fraction of galaxies similar to NGC4993 that show similar
signs of a galaxy merger in the form of visible shells. We se-
lect galaxies from the first year of DES data with size, surface
brightness and Sérsic index within 10% of the best fit val-
ues for NGC4993. We find 1100 such galaxies, and visually
inspect them to identify shell galaxies. Only 15% of these
objects display shells, and so NGC4993 is unusual amongst
early-type galaxies. This is far from conclusive evidence for
a merger origin of BNS events. However, the coincidence of
evidence for a recent merger in a galaxy for which a BNS
event was otherwise improbable is compelling.
Funding for the DES Projects has been provided by the
DOE and NSF(USA), MEC/MICINN/MINECO(Spain),
STFC(UK), HEFCE(UK). NCSA(UIUC), KICP(U. Chicago),
CCAPP(Ohio State), MIFPA(Texas A&M), CNPQ, FAPERJ,
FINEP (Brazil), DFG(Germany) and the Collaborating Insti-
tutions in the Dark Energy Survey.
The Collaborating Institutions are Argonne Lab, UC
Santa Cruz, University of Cambridge, CIEMAT-Madrid,
University of Chicago, University College London, DES-
Brazil Consortium, University of Edinburgh, ETH Zürich,
Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE
Barcelona, Lawrence Berkeley Lab, LMU München and the
associated Excellence Cluster Universe, University of Michi-
gan, NOAO, University of Nottingham, Ohio State Univer-
sity, University of Pennsylvania, University of Portsmouth,
SLAC National Lab, Stanford University, University of Sus-
sex, Texas A&M University, and the OzDES Membership
Consortium.
Based in part on observations at Cerro Tololo Inter-
American Observatory, National Optical Astronomy Obser-
vatory, which is operated by the Association of Universities
for Research in Astronomy (AURA) under a cooperative
agreement with the National Science Foundation.
The DES Data Management System is supported by
the NSF under Grant Numbers AST-1138766 and AST-
1536171. The DES participants from Spanish institutions
are partially supported by MINECO under grants AYA2015-
71825, ESP2015-66861, FPA2015-68048, SEV-2016-0588,
SEV-2016-0597, and MDM-2015-0509, some of which in-
clude ERDF funds from the European Union. IFAE is par-
tially funded by the CERCA program of the Generalitat
de Catalunya. Research leading to these results has re-
ceived funding from the European Research Council un-
der the European Union’s Seventh Framework Program
(FP7/2007-2013) including ERC grant agreements 240672,
291329, and 306478. We acknowledge support from the
Australian Research Council Centre of Excellence for All-
sky Astrophysics (CAASTRO), through project number
CE110001020.
This manuscript has been authored by Fermi Research Al-
liance, LLC under Contract No. DE-AC02-07CH11359 with
the U.S. Department of Energy, Office of Science, Office of
High Energy Physics. The United States Government retains
and the publisher, by accepting the article for publication,
acknowledges that the United States Government retains a
non-exclusive, paid-up, irrevocable, world-wide license to
publish or reproduce the published form of this manuscript,
or allow others to do so, for United States Government pur-
poses.
AP, WH and OL are supported by ERC Advanced Grant
FP7/291329.
Based in part on data acquired through the Australian Astro-
nomical Observatory.
Based on data obtained from the Mikulski Archive for Space
Telescopes (MAST). STScI is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA
contract NAS5-26555. Support for MAST for non-HST data
is provided by the NASA Office of Space Science via grant
NNX09AF08G and by other grants and contracts.
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