The bimodality of type Ia Supernovae
F. Mannucci∗, N. Panagia† and M. Della Valle∗∗
∗INAF - IRA, Firenze, Italia
†STScI, USA; INAF - OAC - Catania, Italia; SN Ltd - Virgin Gorda, BVI
∗∗INAF - OAA - Firenze, Italia
Abstract. We comment on the presence of a bimodality in the distribution of delay time between
the formation of the progenitors and their explosion as type Ia SNe. Two "flavors" of such bimodality
are present in the literature: a weak bimodality, in which type Ia SNe must explode from both young
and old progenitors, and a strong bimodality, in which about half of the systems explode within
108 years from formation. The weak bimodality is observationally based on the dependence of the
rates with the host galaxy Star Formation Rate (SFR), while the strong one on the different rates
in radio-loud and radio-quiet early-type galaxies. We review the evidence for these bimodalities.
Finally, we estimate the fraction of SNe which are missed by optical and near-IR searches because
of dust extinction in massive starbursts.
Keywords: Supernova rates
PACS: 97.60.Bw
INTRODUCTION
The supernova (SN) rates in different types of galaxies give strong informations about
the progenitors. For example, soon after the introduction of the distinction between “type
I” and “type II” SNe [1], van den Bergh [2] pointed out that type IIs are frequent in
late type galaxies “which suggest their affiliation with Baade’s population I”. On the
contrary, type Is, are the only type observed in elliptical galaxies and this fact "suggests
that they occur among old stars". This conclusion is still often accepted, even if it is
now known not to be generally valid: first, SN Ib/c were included in the broad class of
“type I” SNe, and, second, also a significant fraction of SNe Ia are known to have young
progenitors.
THE WEAK BIMODALITY IN TYPE IA SNE
In 1983, Greggio & Renzini [3] showed that the canonical binary star models for type
Ia SNe naturally predict that these systems explode from progenitors of very different
ages, from a few 107 to 1010 years. The strongest observational evidence that this is the
case was provided by Mannucci et al. [4] who analyzed the SN rate per unit stellar
mass in galaxies of all types. They found that the bluest galaxies, hosting the highest
Star Formation Rates (SFRs), have SN Ia rates about 30 times larger than those in the
reddest, quiescent galaxies. The higher rates in actively star-forming galaxies imply that
a significant fraction of SNe must be due to young stars, while SNe from old stellar
populations are also needed to reproduce the SN rate in quiescent galaxies. This lead
http://arxiv.org/abs/0704.0877v2
FIGURE 1. SN rate per unit stellar mass as a function of the B–K color of the parent galaxy (from
Mannucci et al. [4]) showing the strong increase of all the rates toward blue galaxies
Mannucci et al. [4] to introduce the simplified two component model for the SN Ia rate
(a part proportional to the stellar mass and another part to the SFR). These results were
later confirmed by Sullivan et al. [5], while Scannapieco & Bildsten [6], Matteucci et
al. [7] and Calura et al. [8] successfully applied this model to explain the chemical
evolution of galaxies and galaxy clusters. A more accurate description is based on the
Delay Time Distribution (DTD), which is found to span a wide range of delay time
between a few 107 to a few 1010 years (Mannucci et al. [9]). The presence of a strong
observational result and the agreement with the predictions of several models (see also
Greggio [10]) make this conclusion very robust.
THE STRONG BIMODALITY IN TYPE IA SNE
Della Valle et al. [11] studied the dependence of the SN Ia rate in early-type galaxies
on the radio power of the host galaxies, and concluded that the higher rate observed
in radio-loud galaxies is due to minor episodes of accretion of gas or capture of small
galaxies. Such events result in both fueling the central black hole, producing the radio
activity, and in creating a new generation of stars, producing the increase in the SN rate.
This effect can be used to derive information on the DTD of type Ia SNe once a model
of galaxy stellar population is introduced.
The difference between radio-loud and radio-quiet galaxies can be reproduced by
the model of early-type galaxy shown in the right panel of figure 2: most of the stars
are formed in a remote past, about 1010 years ago, while a small minority of stars are
created in a number of subsequent bursts. A galaxy appears radio-loud when is observed
during the burst, radio-faint soon after, and radio-quiet during the quiescent inter-burst
period. The abundance ratio between radio-quiet and radio-loud galaxies, about 0.1 in
our sample, means that the duty cycle of the burst events is about 10%. As the duration
FIGURE 2. Left: (B–K) color distribution of early-type radio-loud (solid line) and radio-quiet galaxies
(dashed line) in three stellar mass ranges. The two groups of galaxies have practically indistinguishable
color distributions, meaning that the stellar populations are similar. Right: Model of early-type galaxies
reproducing both the dichotomy radio-loud/radio-faint and the similar (B–K) colors.
of the radio-loud phase is about 108 years, in 1010 years the early-type galaxies are
expected to have experienced 10 small bursts, i.e., 1 every 109 years and lasting for
about 108 years.
This model naturally explains the fact that radio-loud and radio-quiet early-type
galaxies have very similar (B–K) color, a sensitive indicator of star formation and stellar
age. This is shown in the left panel of Fig. 2, where the two color distributions are
compared. Only a small difference in the median of the two distributions might be
present at any mass, i.e., the radio-loud galaxies appear to be 0.03-0.06 mag bluer, and
this could be the effect of last on-going burst of star formation.
The amount of mass in younger stars can be estimated from the (B–K) color, that
is consistent with the value of (B–K)∼4.1 typical of old stellar populations. By using
the Bruzual & Charlot [12] model, we obtain that no more than 3% of stellar mass
can be created in the 10 bursts (0.3% of mass each) if we assume negligible extinction,
otherwise the predicted color would be too blue. The maximum mass in new stars can
reach 5% assuming an average extinction of the new component of AV = 1. More details
will be given in a forthcoming paper.
This model predicts that traces of small amounts of recent star formation should be
present in most of the local early-type galaxies. This is actually the case: most of them
show very faint emission lines (Sarzi et al. [13]), tidal tails (van Dokkum [14]), dust
lanes (Colbert et al. [15]), HI gas (Morganti et al. [16]), molecular gas (Welch & Sage
[17]), and very blue UV colors (Schawinski et al. [18]).
Using this model with a total fraction of new stars of 3%, we derive the results shown
in figure 3. We see that the theoretical models by Greggio & Renzini [3] and Matteucci
& Recchi [19], while giving a good description of the rates displayed in figure 1, predicts
too few SNe in the first 108 years (about 11%) to accurately fit figure 3. The observed
rates can be reproduced only by adding a “prompt” component (in this case modeled
FIGURE 3. Left: The two DTD studied here, from Greggio & Renzini [3] (GR83) and Mannucci et
al. [9] (MDP06). The latter is the sum of two exponentially declining distributions with 3 and 0.03 Gyr
of decay time, respectively, each one containing 50% of the events. Right: the solid dots with error bars
show the type Ia SN rate as a function of the radio power of the parent galaxy. The dashed line shows the
results of the GR83 model, the solid one those of MDP06.
in terms of an exponentially declining distribution with τ =0.03 Gyr) to a “tardy”
component (an other declining exponential with τ =3 Gyr), each one comprising 50%
of the total number of events.
It should be noted that this strong bimodality is based on a small number of SNe (21)
in early-type galaxies, and the results of oncoming larger SN searches are needed to
confirm (or discard) this result.
EVOLUTION OF THE SN RATE WITH REDSHIFT
A related issue is how the rates measured in the local universe and discussed above
are expected to evolve with redshift. The usual approach is to start from the integrated
cosmic star formation history and obtain the rates by using some assumptions on pro-
genitors (for core-collapse SNe) and on explosion efficiency and DTD (for SN Ia, see
Mannucci et al. [4] for a discussion). Near-infrared and radio searches for core-collapse
supernovae in the local universe (Maiolino et al. [20], Mannucci et al. [21], Lonsdale et
al. [22]) have shown that the vast majority of the events occurring in massive starbursts
are missed by current optical searches because they explode in very dusty environments.
Recent mid- and far-infrared observations (see Pérez-González et al. [23] and references
therein) have shown that the fraction of star-formation activity that takes place in very
luminous dusty starbursts sharply increases with redshift and becomes the dominant star
formation component at z≥0.5. As a consequence, an increasing fraction of SNe are
expected to be missed by high-redshift optical searches. By making reasonable assump-
tions on the number of SNe that can be observed by optical and near-infrared searches
in the different types of galaxies (see Mannucci et al. [24] for details) we obtain the re-
sults shown in figure 4. We estimate that 5–10% of the local core-collapse (CC) SNe are
out of reach of the optical searches. The fraction of missing events rises sharply toward
FIGURE 4. Evolution of the rates of type Ia (two left-most panels) and core-collapse SNe (two right-
most panels), from Mannucci et al. [24]. In the first and third panels, the dashed line shows the total
rate expected from the cosmic star formation history, the light grey area the rate of SNe that can be
recovered by the optical and near-IR searches, and the dark grey area the rate of SNe exploding inside
dusty starbursts and which will be missed by the searches. The second and forth panels show the fraction
of missed SNe.
z=1, where about 30% of the CC SNe will be undetected. At z=2 the missing fraction
will be about 60%. Correspondingly, for type Ia SNe, our computations provide missing
fractions of 15% at z=1 and 35% at z=2. Such large corrections are crucially important
to compare the observed SN rate with the expectations from the evolution of the cosmic
star formation history, and to design the future SN searches at high redshifts.
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http://arxiv.org/abs/astro-ph/0702355
	Introduction
	The weak bimodality in type Ia SNe
	The strong bimodality in type Ia SNe
	Evolution of the SN rate with redshift

  We comment on the presence of a bimodality in the distribution of delay time
between the formation of the progenitors and their explosion as type Ia SNe.
Two "flavors" of such bimodality are present in the literature: a "weak"
bimodality, in which type Ia SNe must explode from both young and old
progenitors, and a "strong" bimodality, in which about half of the systems
explode within 10^8 years from formation. The "weak" bimodality is
observationally based on the dependence of the rates with the host galaxy SFR,
while the "strong" one on the different rates in radio-loud and radio-quiet
early-type galaxies. We review the evidence for these bimodalities. Finally, we
estimate the fraction of SNe which are missed by optical and near-IR searches
because of dust extinction in massive starbursts.

Introduction
	The weak bimodality in type Ia SNe
	The strong bimodality in type Ia SNe
	Evolution of the SN rate with redshift
