Fig. 2:
Total luminosity of the He II 4686 Angstrom recombination line
predicted for a starburst galaxy per unit stellar mass given ionization by
single degenerate progenitors alone (blue), and by the normal stellar
population (red). For the combined case (clearly dominated by single
degenerate progenitors), the predicted He II 4686 Angstrom emission is
outlined in black. Width of the lines denotes the uncertainty.
Type Ia supernovae (SNe Ia) have proven invaluable as cosmic signposts,
having revealed the accelerating expansion of the Universe. These
tremendously energetic events occur when a white dwarf undergoes a
thermonuclear explosion. But how do these explosions occur? The question
remains open, despite great effort and debate. However, scientists working
at the Max Planck Institute for Astrophysics have recently proposed a new
test that may soon shed light on this mystery.
There are currently two “standard” models for the progenitors of SNe Ia.
In the single degenerate scenario, a white dwarf accretes matter from a
co-orbiting companion star until enough mass has accumulated to trigger
an explosion. In the double degenerate scenario, a binary pair of white
dwarfs sheds angular momentum due to gravitational radiation and merges,
giving rise to a SN Ia. Observationally speaking, the clearest difference
between the two is that in the single degenerate scenario, the accreting
white dwarf must process a considerable amount of mass through steady
nuclear burning, making it a highly luminous source of X-ray and extreme
ultraviolet emission for up to a million years prior to the explosion.
Therefore, the most obvious way to distinguish between the two formation
channels is to look for some evidence of the existence of such hot,
luminous sources, allowing one to test the viability of the single
degenerate scenario. Past work has focused on looking for X-ray emission,
e.g. in the integrated X-ray luminosity of nearby galaxies [1]. However,
some type Ia supernova progenitor models predict that much of the
emission from accreting white dwarfs may be radiated in the extreme
ultraviolet, where it is totally absorbed by interstellar matter. In
order to move forward, an ideal test for the presence of a significant
single degenerate progenitor population would need to circumvent this
issue.
Rather than looking for emission from any putative single degenerate
progenitors directly, we can search instead for evidence of their effect
on the interstellar medium. For example, one could attempt to find
signatures of the gas ionized by such sources. In early-type galaxies
without ongoing star formation, we expect only post-asymptotic giant
branch stars (pAGBs) to be a significant source of ionizing radiation, at
least outside of the inner galactic nuclei. These stars likely power the
nebular emission-line regions now found in many ellipticals [2]. However,
in a recent paper, Tyrone Woods and Marat Gilfanov at the MPA have
demonstrated that if the single degenerate hypothesis is correct,
accreting white dwarfs should provide the dominant contribution to the
ionizing background in such galaxies, in particular for relatively young
stellar populations [3]. This is especially true for the ionizing
continuum beyond the second ionization edge of Helium at 54.4 eV.
For a given photo-ionized nebula, the total luminosity emitted in any
recombination line is roughly proportional to the incident flux of
ionizing photons. This suggests that one can confirm, or strongly
constrain, the presence of a significant contribution of single
degenerate progenitors to the SN Ia rate by searching for recombination
lines of ionized helium in the spectra of early-type galaxies. Performing
numerical calculations using the photo-ionization code MAPPINGS III [4],
the expected luminosity of the He II 4686 Angstrom line (the strongest He
II line seen in the optical) can be computed given reasonable assumptions
regarding the composition and distribution of the ionized gas. For a 1
billion year old starburst galaxy, the inclusion of the accreting white
dwarf population implied by a plausible single degenerate channel
increases the predicted He II 4686 Angstrom line luminosity by almost 2
orders of magnitude (see Fig. 2)!
At present, no line at 4686 Angstroms has been detected in the extended
emission-line regions of early-type galaxies. In part, this is because of
the intrinsic weakness of this line (though the far-ultraviolet He II
line at 1640 Angstroms is roughly 6 times stronger, and may be of use
here). However, if there exists a large population of accreting,
nuclear-burning white dwarfs in early-type galaxies which is consistent
with the single degenerate channel, then such a line should be detectable
by ongoing integral field spectroscopic surveys, such as CALIFA, or
through stacking analysis of available SDSS galaxy spectra [3].
For young, post-starburst galaxies, an upper limit on the He II 4686
Angstrom line luminosity of roughly 10^28 erg/s/solar mass would rule out
any high temperature population consistent with the single degenerate
scenario. Therefore, scientists working at the MPA hope that, in the very
near future, the SN Ia community will be able to confidently detect, or
place strong upper limits on, the presence of He II recombination lines
in early-type galaxies.
Tyrone Woods and Marat Gilfanov
References
1. Gilfanov M., Bogdan. A., 2010, Nature, 463, 924
2. Sarzi M., Shields J. C., Schawinski K. e. a., 2010, Monthly Notices of the Royal Astronomical Society, 402, 2187
3. Woods, T. E., Gilfanov, M., 2013, Monthly Notices of the Royal Astronomical Society, 1254
4. Groves, B. A., Dopita, M. A., Sutherland, R. S. 2004, Astrophysical Journal Supplements, 153, 9
2. Sarzi M., Shields J. C., Schawinski K. e. a., 2010, Monthly Notices of the Royal Astronomical Society, 402, 2187
3. Woods, T. E., Gilfanov, M., 2013, Monthly Notices of the Royal Astronomical Society, 1254
4. Groves, B. A., Dopita, M. A., Sutherland, R. S. 2004, Astrophysical Journal Supplements, 153, 9
