Where You Will Find Most Binary Star Mergers? Florida Astronomers Made Predictions (Astronomy)

Summary:

From fiducial model analysis, Jonah Rose and colleagues predicted that 50% of binary neutron star (BNS) mergers will occur in host galaxies with stellar mass between 10^10-10¹¹ M (which is in Milky Way mass scale).

While, they expect most BNS mergers to happen in somewhat lower mass systems for their fiducial Delay Time Distribution (DTDs) , they assume that high mass galaxies are more likely to host a BNS merger (on a per-galaxy basis) once they determine the true BNS DTD.


Rose and colleagues presented predictions for the host galaxy mass function and host galaxy specific-mass function for binary neutron star (BNS) mergers. Their predictions were generated by convolving a set of power law & BPASS (Delay Time Distribution’s) DTDs with the star formation histories from the IllustrisTNG cosmological simulation.

Figure 1. (left) The two fiducial DTDs shown as the BNS merger rate vs time. The BPASS DTD is split into two lines to show the range covered by the DTD as the metallicity of the host star changes. (right) The normalized BNS merger rate for the two fiducial DTDs as a function of the host galaxy stellar mass. The solid line shows the merger rate given the power law DTD with an exponent of s=-1 and tcut=0.01Gyr. The dashed line shows the merger rate given the BPASS DTD. Both merger rates have been normalized individually such that the total merger rate across the simulation for the given DTD is unity. The shaded bands show the mass range which contain 50, 68, and 95 percent of the mergers around the peak merger rate for the fiducial power law DTD. The arrow points to the host galaxy mass of the only BNS merger with a detected electromagnetic counterpart so far. © Jonah Rose et al.

From their fiducial model analysis, they predicted that 50% of BNS mergers will occur in host galaxies with stellar mass between 10^10-10¹¹ M (which is in Milky Way mass scale). This mass bin includes NGC 4993, the host galaxy of GW170817.

Their results showed that most BNS mergers do not happen in galaxies with highest mass, instead it happens in galaxies with masses between 10^10-10¹¹ M.


While the work we presented here continues our understanding of what we can learn from observations of BNS host galaxies, further investigations are necessary to fully understand how BNS form and evolve.

— said Jonah Rose, lead author of the study.

One example of such an investigation is to expand the set of DTDs examined using the methods in this current paper. The set of DTDs Rose and colleagues examined are broad, covering those most
commonly referenced (e.g. Safarzadeh & Berger 2019; Eldridge & Stanway 2016), but they do not exhaustively search the full range of DTDs proposed (e.g. Simonetti et al. 2019; Dominik et al. 2012). Also, their convolution of IllustrisTNG’s star formation rate histories (SFRHs) with their DTDs does not include any form of natal kicks. If these kicks are strong enough to dislodge the binary from smaller galaxies, it is possible their addition would weight the host mass functions toward higher mass galaxies.

Figure 2. The individually normalized BNS merger rates as a function of stellar mass for varied power law exponents (left) and varied tcut values (right). There is significant variation in the predicted host galaxy mass functions when the DTD is perturbed from the fiducial values. © Jonah et al.
Figure 3. The individually normalized BNS merger rates as a function of stellar mass per galaxy for varied power law exponents (left) and varied tcut values (right). While the host galaxy mass function (Figure 2) predicts most BNS mergers will happen in roughly Milky Way mass galaxies when averaged over the whole galaxy. population, this host galaxy specific-mass function (this figure) indicates that the rate of BNS merger rate is higher is higher in higher mass galaxies, when compared on an individual basis © Rose et al.

With a greater range of DTDs examined and a more detailed convolution, we will gain a clearer picture of where BNS mergers are exactly located, which delay times can be distinguished using the host galaxy mass function, and the most likely places they will be observed.

— said Bartos, co-author of the study.

Another way to incorporate a more complete set of DTDs would be to use a varied set of population synthesis models which cover a wide range of binary separations, kick velocities, initial mass functions, etc.

Including other star formation histories could also provide a more detailed look at the spread in possible host galaxy mass functions. While IllustrisTNG is broadly consistent with the cosmic star formation rate density and redshfit dependent galaxy stellar mass functions (Pillepich et al. 2018), its accuracy should not be over interpreted and different simulations will surely produce somewhat varied star formation histories that could impact their results.

However, the peak of the host mass function laying in the mass range M∗ = 10^10 – 10¹¹ M suggests that this method will miss, or take a longer to locate, most of the BNS mergers. Determining the true DTD would allow for more efficient electromagnetic follow-up by determining which observable: SFR, blue luminosity, or stellar mass, best correlates with BNS merger rate. In the long term, LIGO/Virgo/KAGRA will create a host mass function which can be used to determine the true BNS DTD.


While, we expect most BNS mergers to happen in somewhat lower mass systems for our fiducial DTDs, we assume that high mass galaxies are more likely to host a BNS merger (on a per-galaxy basis) once we determine the true BNS DTD.

— said Jonah Rose, lead author of the study.

Note: (i) in the near term: influence BNS merger event follow-up strategy by scrutinizing where most BNS merger events are expected to occur and (ii) in the long term: constrain the DTD for BNS merger events once the host galaxy mass function is observationally well determined.

Featured image: Artist’s impression of Binary neutron star merger © Gettyimages


Reference: Jonah C. Rose, Paul Torrey, K.H. Lee, I. Bartos, “Where binary neutron stars merge: predictions from IllustrisTNG”, ArXiv, pp. 1-11, 2021. https://arxiv.org/abs/2101.10343v1


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