A&A 458, 369-383 (2006)
DOI: 10.1051/0004-6361:20064996
D. Marcillac1,2 - D. Elbaz1 - S. Charlot3,4 - Y. C. Liang5,6 - F. Hammer6 - H. Flores6 - C. Cesarsky7 - A. Pasquali8
1 - DSM/DAPNIA/Service d'Astrophysique, CEA/SACLAY, 91191 Gif-sur-Yvette Cedex, France
2 -
Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
3 -
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
4 -
Institut d'Astrophysique de Paris, CNRS, 98 bis boulevard Arago, 75014 Paris, France
5 -
National Astronomical Observatories, Chinese Academy of Sciences, No. 20A Datun Road, Chaoyang District, Beijing 100012, PR China
6 -
GEPI, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
7 -
ESO, Karl-Schwarzschild Strase 2, 85748 Garching bei Munchen, Germany
8 -
Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany
Received 10 February 2006 / Accepted 26 May 2006
Abstract
Aims. We constrain the past star formation histories of a sample of 25 distant (
)
luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites.
Methods. We used high-resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200 000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies, based on analysis of Balmer absorption lines and the 4000 Å break.
Results. The locus of distant LIRGs in the diagram defined by H
and D4000 is roughly comparable to that of local LIRGs observed with IRAS, suggesting that no trend toward an evolution is detected between the local and distant LIRGs. We obtain similar results when using either the H8 or the H
Balmer absorption-line indices in combination with D4000.
By computing a birthrate parameter (
)
of
,
we confirme that the distant LIRGs are currently experiencing a major phase of star formation. The most likely duration of the bursts is 0.10
+0.16-0.06 Gyr, during which the LIRGs produce
5-10% of their current stellar mass. No evidence was found for successive starbursts on the scale of a few times 107 yr, such as those predicted by some numerical simulations of major mergers. However, the high number density of those galaxies suggests that they could have experienced between two and four LIRG phases until the present epoch. This scenario is not consistent with the formation of the
LIRGs through the continuous star formation characterizing isolated spiral galaxies as has been independently argued based on their morphology. Instead, minor mergers, tidal interactions, or gas accretion remain plausible triggering mechanisms for more than half of the distant LIRGs that do not harbor the morphology of major mergers.
Key words: galaxies: evolution - infrared: galaxies - galaxies: starburst
In a previous paper (Liang et al. 2004, hereafter Paper I), we presented an analysis of the emission line properties of the galaxies. The star formation rates (SFR), derived from the Balmer emission lines (H
and/or H
,
plus H
to derive dust attenuations) were corrected for dust attenuation and found to be consistent with the ones derived from the mid IR (MIR) using the technique described in Chary & Elbaz (2001). This study showed that LIRGs in general are not completely obscured by dust and that the use of high-resolution optical spectroscopy (
/
,
in the rest frame of the objects) could be used to derive intrinsic luminosities, hence SFR, in rough agreement with the IR-derived SFR, by minimizing the contamination by sky emission lines and allowing better corrections for underlying photospheric absorption lines. However, the consistent derivation of the signal-to-noise (S/N) ratio on the intrinsic luminosities lead to large uncertainties on the measured visual attenuation. Moreover, there is evidence of some completely obscured star formation as found in the most luminous objects studied in Flores et al. (2004) or in Cardiel et al. (2003). The limited statistics of those studies clearly call for an extention of the sample of distant LIRGs, with good S/N on the optical continuum and high spectral resolution, to robustly determine which fraction of the star formation taking place in LIRGs and ULIRGs is completely obscured by dust. However Hopkins et al. (2003) show that SFR([OII]), SFR(1.4 GHz), and SFR(FIR) are in very good agreement for a larger sample of local infrared galaxies detected with IRAS and spectroscopically observed with the Sloan Digital Sky Survey (SDSS).
In the present paper, we wish to address the problem from another angle: stars less massive, hence with longer lifetimes, than those responsible for the emission lines standardly used to derive the optical SFR of galaxies can escape their parent giant molecular cloud (GMC), their spectral signature might be used to derive key parameters concerning the starburst. The H
light used to derive an SFR is dominantly produced by the ionizing photons arising from stars more massive than
10
,
with lifetimes shorter than 3 Myr. Those stars never escape their parent GMC (average lifetime of 10 Myr) and the dense regions of very strong extinction, in contrast to the A and F stars, which are the main contributors to the Balmer absorption lines and the 4000 Å break.
We used these signatures of the optical continuum to compare distant LIRGs to nearby IRAS galaxies or synthetic spectra generated with the Bruzual & Charlot (2003) model. After finding a signature of the starbursts in the D4000-H
diagram we used it to derive the burst properties.
Paper I indirectly confirmed the strong role played by LIRGs in the CSFR history with the derivation of gas metallicities in distant LIRGs twice lower that are the one measured in present-day galaxies of equal absolute B band magnitude. This result suggests that these galaxies have produced about half of their metals between
and today. It was also suggested that such a large metal production, as well as the large contribution of LIRGs to the CSFR and CIRB, could not result from a single burst phase in the galaxies harboring LIRG phases and hence that those galaxies must have experienced a series of LIRG phases in their lifetime. This possibility is tested in the present paper.
Finally we note that, based on optical spectra, no evidence was found in Paper I for a dominant contribution from active galactic nuclei (AGNs) in the sample of 76 distant LIRGs for which a spectroscopic redshift was determined. This confirmed the previous result from Fadda et al. (2001) that AGNs were contributing to less than 20% of the MIR light of distant LIRGs, as shown by their soft to hard X-ray radiation measured at the Newton and Chandra X-ray observatories.
In this paper, we study the stellar spectra of a sample of 25 LIRGs with a median redshift of
using high-resolution spectroscopy with the FORS2 instrument at the VLT (
/
,
equivalent to 2000 in the rest-frame of the galaxies). This sample is smaller than in Paper I because higher S/N are required to study the continuum emission in comparison to the emission lines. All galaxies are detected at 15
m with ISOCAM and the
11 galaxies located in one of the three fields are also detected at 24
m with the MIPS camera onboard Spitzer (Papovich et al. 2004; Elbaz et al. 2005). We show that both indicators provide consistent estimates of the total IR luminosity of the galaxies, hence also the SFR (see also Elbaz et al. 2005; Marcillac et al. 2005, for more detailed studies).
Section 2 presents the sample selection and the wavelet decomposition technique that we used to analyze the spectra. Section 3 describes the method used in this paper to study the SFR history of the galaxies, namely the Balmer absorption line H
(4101 Å) versus 4000 Å break position of the galaxies, as previously done by Kauffmann et al. (2003) for the Sloan Digital Sky Survey (SDSS).
We extended the method used in Kauffmann et al. (2003) to the high-order Balmer lines H8 (3889 Å) and H9 (3835 Å), which are easier to detect in distant galaxies due to the k-correction.
The comparison of local and distant LIRGs in this parameter space is discussed in Sect. 4, while the model used to generate Monte Carlo realisations of 200 000 spectra with different star formation histories is presented in Sect. 5. The results are presented in Sect. 6 and discussed in the last section.
Throughout this paper, we will assume H0= 75 km s-1 Mpc-1,
and
.
Table 1:
Description of the distant LIRGs sample. The technique used
to compute the equivalent widths of H,
H8, H9, H10 and
the 4000 Å break (D4000) is described in
Sect. 2.3.
and
SFR
were derived using the Chary & Elbaz (2001) technique.
The ISOCAM sources were selected to span the whole flux density
range of the three surveys whose 80
completeness limits are
150
Jy for the UDSF and UDSR and 250
Jy for the CFRS
3
,
while the detection limits are 50
Jy (UDSF, UDSR) and
170
Jy (faintest sources in the regime below completeness).
No optical selection was applied so that the magnitudes of the sources
range from
to 23.7. At about the same epoch (November 2003) as
the VLT-FORS2 observations, a 24
m survey was being performed with the MIPS camera
onboard Spitzer during the MIPS commissioning
phase (IOC/SV). This survey covers the whole UDSF field and all eleven ISOCAM-15
m sources were detected at 24
m, i.e. 45% of the sample.
The scan map AOT was used, with an
half-array overlap to cover about 1300 sq. arcmin with high redundancy
(20) and to get an integration time per sky pixel of about 230s
(Papovich et al. 2004). The data were reduced using the Spitzer
Science Center Pipeline and the BCD products (Basic Calibrated Data, Pipeline
version S10.0.3) were downloaded from the Spitzer
archive
.
The PSF-fitting photometry was performed using DAOPHOT
(Stetson 1987) with IRAF
.
From a total of 105 galaxies observed with FORS2 at the VLT (Paper I), we detected 3 stars,
13 galaxies were too faint for a redshift determination, and within the remaining list 13 other galaxies were not detected with ISOCAM. The resulting list of 76 galaxies with an MIR detection is divided into 34 "normal'' galaxies, 36 LIRGs, and 6 ULIRGs, where "normal'' galaxies designates all galaxies except IR luminous galaxies (i.e LIRGs and ULIRGs). The median SFRs associated to each total IR luminosity bin are 4, 54 and 196 yr-1. Because good quality spectra are required to study the stellar absorption lines (S/N > 3 on the continuum per resolution element), the final sample that we study in the present paper consists of 25 LIRGs with IR luminosities ranging between 1011 and 1012
.
The median redshift of the final sample is
.
The measured properties of the galaxies are summarized in Table 1.
The total (8-1000 m) IR luminosities,
,
were derived using
the library of template SEDs built by Chary & Elbaz (2001), as
in Elbaz et al. (2005). We also computed
from the Dale & Helou (2001) library following the technique described in Marcillac et al. (2005) and found a median value for
10% lower than with the previous library and with an rms of 17%, meanning that both techniques provide consistent luminosities.
We then compared the
derived with 15
m and/or 24
m flux densities for the 11 galaxies detected with both ISOCAM and MIPS. Both libraries of template SEDs provide consistent determinations of
using both measurements (with an rms of 30%, see also Elbaz et al. 2005). The median
derived from MIPS is 10% lower than the one derived from ISOCAM using both libraries, which suggests a possible variation of the MIR spectra of LIRGs as a function of redshift (see Marcillac et al. 2005).
The observations were performed during three nights with FORS2 on the
ESO-VLT with the combination of the grisms R600 and I600 (3 h per grism) to cover the
wavelength range 5000 to 9200 Å at a resolution of 5 Å (R=1200).
At the median redshift of the objects of
,
the resolution is
equivalent to 3 Å (R=2000).
Because absorption
lines are wider than emission lines, it is possible to increase the
S/N ratio on the absorption lines by working at a lower frequency
than the raw spectrum. The physical origin of the width of the absorption lines results from the complex combination of the internal dynamics of the stars and the global dynamics of the galaxy. In order to optimize the line extraction, we therefore decomposed the raw spectra into eight different wavelet scales (Table 2)
using the undecimated (keeping an identical sampling in
each wavelet scale) wavelet transform (à trous algorithm, Starck &
Murtagh 1994; Starck et al. 2002). We optimized the S/N ratio on the absorption line
features by selecting the best combination of wavelet scales. In the
wavelet space, the first scale (highest frequency), which we index on
scale 0, corresponds to features of size 0.7 Å, while features at
scale j have a size of
Å. Note that the
lowest frequency scale is equivalent to the raw spectrum smoothed by a
B-spline (equivalent to a Gaussian) of width 89.5 Å, i.e. the
baseline, while at higher frequencies, each wavelet scale "i'' is equal
to the difference between the raw spectrum smoothed at the scales "i''
and "i-1''. Hence the sum of all wavelet scales plus the baseline (here
at scale 7) is exactly equal to the initial raw spectrum.
Table 2: Definition of the wavelet scales used in the wavelet decomposition of the VLT-FORS2 spectra.
In order to determine the number of scales to take into account in the
decomposition, we started from the lowest resolution in the wavelet
space, equivalent to a spectral resolution of 89.5 Å. We then kept doubling the frequency level as long as the S/N ratio was increased.
This happened on the wavelet scale "4'' (equivalent to
11.2 Å), so we used the co-addition of the 4, 5, 6, and 7 wavelet
scales (from 11.2 to 89.6 Å, increasing the frequency by a factor 2
for each scale) in order to reconstruct spectra devoid of high-resolution noise. The steps of the decomposition are shown in
Fig. 1 for a LIRG, UDSR23 (z= 0.7094), located at the median redshift of the sample. The spectra resulting from the combination
of these four scales present the advantage of seeing the same spectral
resolution as the one used by Bica
Alloin (1986). The final result is compared to the raw spectrum in the
Fig. 2.
Note that the wavelet decomposition can dilute emission lines and that we have checked that absorption lines were not affected by a similar effect. Being wider, they are naturally less affected by this technique. However, we quantified this effect using some stellar spectra extracted from STELIB (Le Borgne et al. 2003) which resolution is about the same as the rest-frame one for the distant LIRGs.
We added a white noise to the stellar spectra to reach a S/N ratio of
on the continuum, interpolated the spectra to reach a resolution of 0.7
,
and applied the same wavelet decomposition to them as for the distant LIRGs. The equivalent widths of the Balmer absorption lines determined before and after the wavelet analysis differ by less than 4%. We included this weak difference in the equivalent-width uncertainties.
We used the definition of the H
pseudo-equivalent width indice as
defined in the Lick system (Worthey
Ottaviani 1997; see Table 3).
Since no Lick indices have been defined yet for the high order Balmer absorption lines, we adapted the windows defined by Bica & Alloin (1986) to the Lick index method for H8 and H9: for each line, the index continuum, the blue, and red bandpasses for each pseudo-continuum are summarized in Table 3.
The two lines present the advantage of being located at a lower
wavelength to be accessible to higher redshifts in the observed
optical range and to be less affected by the overlying nebular emission
lines at the same wavelengths.
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Figure 1:
Wavelet decomposition of the VLT-FORS2 spectrum of a distant
LIRG (UDSR23) located at a redshift of z=0.7094) and forming stars at
a rate of SFR
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The H
absorption line is surrounded by iron absorption lines that affect
both its red and blue pseudo continua and are responsible
for the negative EW measured for this line after a few Gyears, as discussed in the next section.
In order to avoid such pollution from neighboring lines, the red continuum of H8 was chosen to minimize the sensitivity to the CaII H (3933 Å) and K (3966 Å) lines, while the
3855-3865 Å region is not known to be affected by metallic lines. As a result this index is weakly affected by metallicity. Because no metallic line is located near it, H9 is the least polluted line but is somewhat fainter than H8 and is surrounded by the strong absorption of H8 and H10, which makes the two pseudo continua more difficult to define. We have primarily used H
and H8, which are better
determined for most of our spectra but also checked that
we obtained consistent results based on the H9 and H
lines when they were available.
We will discuss the results obtained with the Balmer absorption lines H
(4102 Å) and H8 (3889 Å) separately.
The advantage of this choice is that it provides two independent estimates of the parameters that we are deriving and can be used as a test of the robustness of the Bruzual & Charlot (2003) code that we are using.
There is an ongoing debate about possible misinterpretation of the equivalent width of the H
line because of metallicity ratios that could affect its neighboring regions, hence its associated pseudo-continua (see Thomas et al. 2004; Korn et al. 2005). On the one hand, H
has the advantage of allowing the comparison with studies of local galaxies such as SDSS galaxies (Kauffmann et al. 2003), while H8 is usually not available for local galaxies because it lies in a bluer region of the spectrum. On the other hand, the H8 line is not known to be affected by neighboring metallic lines and, as it is located in a bluer region of the spectrum, it is easier to measure for distant galaxies (less polluted by sky emission lines).
Before measuring these absorption
features, we corrected them from the overlying nebular emission
line whenever possible, as indicated in the paper. The nebular
emission lines are not detected directly from the spectra, because
they are too faint, but we computed their emission based on the
observed H
and H
emission lines (and H
for the low-z galaxies) assuming a line ratio corresponding to a case B recombination for electron densities
104 cm-3 and temperatures
(Osterbrock 1989).
The Balmer
emission line ratio was also used to compute the attenuation of these
lines before subtracting them from the absorption lines measurements.
The computation and values for these attenuations can be found in
Paper I.
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Figure 2:
Comparison of the VLT-FORS2 raw spectrum (grey line) of the
distant LIRG UDSR23 (z=0.7094, 51.8 ![]() ![]() ![]() |
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Following the same strategy as Kauffmann et al. (2003), we used the method explained in Bruzual (1983) and the blue and red bandpass definitions introduced by Balogh et al. (1999). The latter are narrower than the ones originally defined by Bruzual (1983) and present the advantage of being less sensitive to reddening effects.
The H8 and D4000 values for each distant LIRG are summarized in Table 1. The scientific interpretation is discussed in the following section.
In this section, we describe how Balmer absorption lines and the 4000
break trace the recent star formation history of galaxies. For this purpose, we have synthesized a single
stellar population (SSP) using the latest version
of the "GALAXEV'' code from Bruzual & Charlot (2003). For the moment, we do not include dust attenuation so as to simplify the discussion. Note, however, that the wavelength range over which the equivalent widths and the 4000
break are measured is small which implies a marginal correction due to dust attenuation. Nonetheless, we will include dust attenuation in the Monte Carlo realizations that we will compare to the LIRGs and discuss its effects on our findings.
This version of the GALAXEV code includes the spectral library
STELIB (Le Borgne et al. 2003) whose spectral resolution is 3
from
3200 to 9500
,
which is comparable to the present spectra in the
rest-frame of the galaxies. Figure 3 presents the
evolution with time of the high order Balmer absorption lines H8 and H9, as well as the H
absorption line and the 4000
break. The four lines in each plot correspond to four different metallicities with the following metal mass fractions (total mass in elements heavier than hydrogen and helium over the mass in hydrogen): Z= 0.004 (20% solar, dashed line), Z= 0.008 (40% solar, dash-dotted line),
Z= 0.02 (solar, solid line), Z= 0.05 (2.5 times solar, dotted line).
The D4000 is the sudden onset of stellar photospheric opacity shortward of 4000 .
It reflects the mean temperature of the stars responsible for the continuum: the metals located in the atmosphere of O and B stars are more ionized and produce a weaker opacity, hence a smaller
4000
break, than those in cooler stars (Bruzual 1983;
Poggianti & Barbaro 1997; Gorgas et al. 1999; Kauffmann et al. 2003). As a result D4000 keeps increasing as a function of the aging of the stellar population (see Fig. 3d). D4000 is sensitive to metallicity as is shown in Table 3d where it varies by more than 20% after a few billion years or when it is larger than 1.6. As see in the next section, the distant LIRGs have D4000 =
,
and therefore metallicity effects are negligible for them.
Even if the slope of D4000 versus age is less flat than for young population, i.e. 7 Gyears, it is flat enough to provide uncertain stellar ages if used alone.
In order to trace back the recent star formation history of galaxies, it is therefore necessary to use
another tracer of stellar age such as the Balmer absorption lines, which exhibit a steep slope as a function of stellar ages in this range of ages (see Figs. 3a-c). Balmer absorption lines are mainly produced by the atmosphere of A
to F stars. However, O and B stars, which do not exhibit strong absorption lines, indirectly affect them by increasing the continuum level and therefore diluting them, which explains the flat values for the equivalent widths of H8, H9, and H
in the first few million years (lifetime of O and B stars). The rapid increase that follows is produced by the dominant role of A and F stars, which then disappears after
0.5 Gyear producing the rapid decline of the equivalent widths in Figs. 3a-c. Here again, it is worthwhile noticing the marginal role played by the metallicity in the evolution of the Balmer lines EW with time. The EW varies by less than 20% a few billion years after the burst as a function of metallicity.
Table 3:
Definition of the pseudo-equivalent width indices for the
H
(H
in Worthey
Ottavianni 1997) and for the
two high order Balmer absorption lines. For the two last lines,
we followed the same principle as for Lick indices while using
windows defined Bica
Alloin (1986).
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Figure 3:
Time evolution of the H8, H9, and H
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Figure 4:
H
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A sample of 401 local (
)
LIRGs detected with IRAS with optical spectra from the SDSS and emission lines typical of star forming galaxies, as opposed to AGNs, was identifed by Pasquali et al. (2005). The locus of the D4000 and EW (H
)
measured for the distant LIRGs is compared to that of local LIRGs and of the field SDSS galaxies in Fig. 4. First, note the concentration of both LIRG populations at low D4000 and high H
,
which suggests that local and distant LIRGs share a similar recent star formation history. We could not produce similar figures for high-order Balmer absorption lines for this comparison since they are too blue to be accessible in the SDSS spectra.
Their position in the D4000-H
diagram indicates that the young stellar population that is producing the large IR luminosity is not completely obscured by dust in the optical because the median D4000 value of 1.2 for these galaxies correspond to stellar ages lower than 1 Gyear, which is much below the ages of these galaxies (see Table 4).
This already suggests that even in these dusty galaxies the optical spectral signatures can be used as a tracer of the recent star formation history. The quasi absence of distant LIRGs above
D4000 = 1.25, where half of the local LIRGs lie, suggests that the distant LIRGs are younger than the local ones.
The relative proportion of young and old stellar populations can be studied in these galaxies by comparing the stellar masses, derived mostly from the old stellar population dominating in the near infrared range, with their D4000 values. Massive galaxies are generally older as shown by Fig. 5, where the stellar mass of SDSS galaxies increases with D4000. The position of the local LIRGs in Fig. 5 suggests that they are massive galaxies that were located on the right side of the plot before the burst and were shifted to the left during the burst, which decreased their D4000 value. This figure reinforces our interpretation that the low value of D4000 for the LIRGs is due to the addition of a young population on top of an older population. The comparison of field SDSS galaxies with IRAS LIRGs in four bins of D4000 from 1 to 1.4 (bin size 0.1, 68% error bars) shows that local LIRGs exhibit systematically lower D4000 values for their stellar masses.
The incompleteness of the photometric data for our sample of galaxies
prevented the determination of their stellar masses. However, another
sample of mid IR selected LIRGs at
located in the
Hubble Deep Field South was studied by Franceschini et al. (2003), who
computed their stellar masses using a Salpeter IMF and a combination
of single stellar populations with ten different ages, to fit their
UV-optical-NIR spectra of IR luminous galaxies. From their Table 6, a
total of 14 LIRGs possess a spectroscopic redshift between z=0.4 and 1.2, and 7 more have a photometric redshift in this range. After
converting Franceschini's values to H0= 75 km s-1 Mpc-1,
we find a median stellar mass of
10
for
the 14 galaxies with a spectroscopic redshift. Including the less
robust photometric redshifts only changes this value to
10
.
Combined with a median D4000
1.2, this places the distant LIRGs in a very similar locus as the local
LIRGs (see Fig. 5). Their low D4000 values are therefore
also representative of the young stellar population of the burst
superimposed on top of an older stellar population dominating the
stellar mass of the galaxies.
Table 4:
Median and 68% dispersion (around the median) of the D4000 and H
indices for local and distant galaxies.
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Figure 5:
Stellar mass as a function of D4000 for the SDSS galaxies (black
points) and distant LIRGs of the present sample (large filled blue
circle). The median stellar mass for the distant LIRGs was derived
from Franceschini et al. (2003) as discussed in the text. The
sub-population of SDSS galaxies detected with IRAS and without an AGN
signature in their optical spectra (star-forming galaxies) are marked
with orange filled circles. The median and 1-![]() |
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Figure 6:
Influence of the attenuation, velocity dispersion, and
metallicity for a typical star formation history synthesized with the
GALAXEV code (Bruzual ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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We used the Bruzual & Charlot (2003) model to synthesize a series of 200 000 Monte Carlo realizations including various star formation histories. Although the technique is similar to the one used in Kauffmann et al. (2003), it presents two differences related to the populations of galaxies that we are studying here, i.e. dusty starbursts. The priors of the model were set to include a range of dust attenuations and to include a larger fraction of starbursting galaxies. We will discuss the effects of both modifications in the following.
Each star formation history was modeled with eight parameters:
Table 5: Description of the range of values used as priors for the simulations.
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Figure 7:
Location of the 200 000 Monte Carlo realizations using the parameters of SIM1 (see Table 5) in the H8 versus 4000 Å (D4000) break diagram.
Light blue points ( lower-left): starbursting galaxies.
Dark blue points ( upper part):
post-starburst galaxies (galaxies having experienced a recent starburst which ended less than 2 Gyear ago). Black points: galaxies with continuous
star formation in the two past Gyears. Bold red line: track
followed by an individual galaxy with continuous star formation
(numbers in squares = age in Gyear) generated with GALAXEV (![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Figure 8: Same as in Fig. 7 for the SIM2 Monte Carlo simulation of 200 000 model galaxies. The distant LIRGs are represented with green points with error bars. |
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Figure 9:
Location of the 200 000 Monte Carlo realizations using the parameters of SIM2 (see Table 5) in the H
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75% of the distant LIRGs are located within the solid rectangle in Fig. 7, hence at the location of galaxies experiencing a starburst in the simulation. The remaining 25% of the distant LIRGs lie in the dashed rectangle that includes the region of post-starbursts but also galaxies with successive or longer starbursts. Hence, the results of the simulation from SIM1 simulation show that the observed galaxies are mainly coherent with being starbursting galaxies.
As a second step, we designed a new simulation, with 80% of the galaxies experiencing an ongoing starburst (SIM2), to sample the locus of the distant LIRGs better and therefore better study the properties of the starbursts themselves. A third and last simulation (SIM3) was generated to quantify the probability that the distant LIRGs experienced a previous starburst during the past 2 Gyear. Half of the galaxies in SIM3, i.e. 100 000 galaxies, have experienced a previous starburst during the last 2 Gyear, which ended before the onset of the ongoing starburst.
The individual positions of the distant LIRGs are compared to the SIM2
realizations in Figs. 8 and 9 in
the H8-D4000 and H
-D4000 diagrams, respectively.
UDSR09, which is lying at the bottom of the simulation, is a clear outlier. Liang et al. (2005) studied this object carefully and its optical spectrum shows strong metal absorption lines, such as Ca H K, G-band, Mg H, Na D lines, but weak Balmer absorption lines. The X-ray emission of this object, in addition, is mainly associated to an AGN. As a consequence, an AGN can partially contribute to the MIR luminosity.
Note that
the observed galaxies are not identical in both figures neither in
numbers nor in identity because both H8 and H
cannot both be measured for all individual galaxies. The dots correspond to the same
simulated galaxies in both figures and in both cases. Note that the
observed LIRGs do lie below the continuous star formation regime in
both figures, although in slightly different locations. These
differences will be discussed in Sect. 6.5 when we will present
the resulting PDFs. Note also that the distant LIRGs are distributed
in two populations in Fig. 8, one located at the
bottom of the diagram and a second in the upper-left. The second
population corresponds to simulated galaxies that experienced a
succession of two starbursts during the past 2 Gyears. Such histories
are expected in the framework of major mergers of spiral galaxies with
several encounters between two galaxies.
The PDF obtained for the Scalo parameter of a typical galaxy with
SFR/
(1-
)
is represented in Fig. 10a. In about 60% of the distant LIRGs, the PDF converge towards a determination of the Scalo parameter. For 10 out of 17 galaxies for H
(see Table 6), we compute a median value of
(1-
). For 12 out of 22 for H8 (see Table 7) we find
(1-
). These two results are consistent with each other, as illustrated by Figs. 11a and c. The fact that the H8-D4000 diagram provides the tightest constraint on the Scalo parameter is due to the fact that the sky background is lower in the bluer wavelength range of H8 than in H
.
We therefore use that range of values for the Scalo parameter in the next set of computations.
The fact that the LIRGs are found to produce stars at a rate that is four times higher than their averaged past SFR confirms that they are experiencing a major phase of star formation in their lifetime. This result is consistent with the large
and SFR(IR).
As noted above, the H8-D4000 diagram provides the tightest constraint on the Scalo parameter with
.
Combined with the ongoing SFR measured from the MIR emission for the distant LIRGs (quoted in Table 1) for individual galaxies and with a median value of:
yr-1, the Scalo parameter allows us to compute the averaged past SFR of the distant LIRGs:
yr-1.
If we assume that the progenitors of the distant LIRGs formed stars
at a constant rate equal to the averaged past SFR, a median stellar
mass of
(see Sect. 4) is
assembled in about
Gyear (light-weighted age), implying that
the first dominant stellar populations formed at a redshift
.
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Figure 10: Example of PDF obtained for 1 galaxy: a) the Scalo parameter (Sect. 6.2); b) burst duration (Sect. 6.2); c) burst fraction (Sect. 6.2); d) effective burst fraction (Sect. 6.2). |
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However, we can still derive the effective burst stellar mass fraction,
,
which is equal to the mass of stars produced during the burst, i.e.
,
divided by the total stellar mass of the galaxy, i.e.
.
Since
years and
yr-1, we obtain an effective burst stellar mass fraction of
%. Note that a burst producing 10% of a
galaxy, will convert
of molecular gas into stars, which is consistent with the mass of molecular gas observed in local LIRGs and ULIRGs (Sanders & Mirabel 1996).
Table 6:
Results concerning the burst duration, the Scalo ratio
,
and the burst fraction (
)
when using the H
data.
Table 7:
Results concerning the burst duration, the Scalo ratio
,
and the burst fraction (
)
when using the H8 data.
A sub-sample of the distant LIRGs lies in the upper-left part of the
H8 and H
versus D4000 diagrams
(Figs. 8, 9).
This region is populated by Monte Carlo realizations of galaxies for which the ongoing burst of star formation was superimposed on a previous burst that ended less than 2 Gyear ago. In order to quantify the probability of such an occurrence, we generated a third simulation, SIM3, in which half of the realizations experienced two successive bursts during the Past 2 Gyear. Using the same Bayesian statistics, we then computed for each galaxy a probability for various possible durations between the two bursts. Two representative numbers are assigned for each galaxy in Tables 8 and 9: t20 and t50. These values correspond to lookback times associated with a 20 and a 50% chance of finding a previous burst that ended t20 and t50 Gyear before the onset of the ongoing one. For example, in the case of UDSR20, we obtain
t20= 0.4 Gyear and t50= 1 Gyear from the H8-D4000 diagram (see Table 8), which implies that there is a 50% chance that a previous starburst occurred 1 Gyear before the onset of the present one and a 20% chance that the delay was only a 0.4 Gyear. Hence UDSR20 is a good candidate for two successive bursts. We did not consider higher probabilities or longer timescales because of the limited constraints that we can set on those parameters and because after about 1.5 Gyear, the memory of the previous burst is lost with this technique.
Local LIRGs and ULIRGs are known to be predominantly triggered by major mergers (Borne et al. 1999; Sanders & Mirabel 1996), and numerical simulations of such mergers predict that the two galaxies cross each other several times, potentially inducing a series of bursts separated by a few tens of million years (Mihos & Hernquist 1996). However, in distant LIRGs, the probability that a previous starburst occurred less than 0.5 Gyear ago is nearly always lower than 20%. This result must be considered together with recent evaluations of the morphological properties of distant LIRGs, which also suggest that most of them are not produced in major mergers (Bell et al. 2005; Zheng et al. 2004; Elbaz & Cesarsky 2004). Bell et al. (2005) suggest that distant LIRGs could either be non-triggered phases in isolated spirals with larger gas masses, possibly experiencing some infall, or minor mergers, where the dwarf galaxy responsible is not detected. Our determination of a burst duration of 0.1 Gyear and a Scalo parameter of 4, seems to rule out the possibility that distant LIRGs are isolated spirals forming stars at a constant rate over a long duration. The starbursts may instead be triggered by tidal effects and minor mergers in regions of the universe where the local density of galaxies is enhanced, as suggested by Elbaz & Cesarsky (2003) or by the infall of intergalactic gas (Combes 2005). Further kinematical studies of distant galaxies (see Flores et al. 2006; Puech et al. 2006) will help to distinguish between the various scenarios (mergers, gas infall) discussed here.
Comparison of distant LIRGs, selected from ISOCAM and MIPS onboard ISO and Spitzer, to local LIRGs, selected from IRAS and the SDSS, shows that both populations present similar spectral features and therefore suggests that they are experiencing comparable starburst phases. Half of the local LIRGs present D4000 values that are higher than the maximum D4000 of distant LIRGs indicates that the dominant non bursting stellar population is younger for distant LIRGs, as expected.
The first important result of this study is the identification of an optical signature for the presence of a starburst in these galaxies, in spite of their large dust attenuation. While continuous star formation follows a line along decreasing Balmer EW and increasing D4000, a burst superimposed on this population produces a loop that first decreases D4000 and then increases the Balmer line EW. However, after about 1.5 Gyear, the memory of the burst is lost and the galaxy behaves like others that did not experience a starburst. As result, we are limited to study only the averaged past star formation history for lookback times shorter than 1.5 Gyear. The burst characteristics were derived from probability distribution functions (PDF) using a Bayesian statistics as in Kauffmann et al. (2003).
The median ratio of present over averaged SFR, the so-called Scalo parameter, for the distant LIRGs is
(we used the H8 line for which a larger sample of galaxies is available and the PDFs present a sharper peak), which indicates that these galaxies are experiencing an atypically intense phase of star formation in their lifetime. A median SFR of 52
+34-33
yr-1 for the ongoing starbursts was derived from their MIR luminosities; Hence, their mean SFR averaged over their lifetime is
yr-1. Knowing the median stellar mass for LIRGs of equivalent luminosity and redshift range (from Franceschini et al. 2003), we derived an age for those distant LIRGs of
Gyear, suggesting that they formed at
.
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Figure 11:
Distribution of the Scalo parameter and the burst duration;
a) the Scalo parameter in the H8-D4000 diagram;
b) the burst duration in the H8-D4000 diagram;
c) the Scalo parameter in the H
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Figure 12: The lines are the same as in Figs. 7 and 8. The black circles are proportional to t20 defined before, while the grey ones to t50. |
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Figure 13: The lines are the same as in Figs 7 and in 8. The black circles are proportional to t20 defined before, while the grey ones to t50. |
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Table 8: Values of t20 (black circle) and t50 (grey circle) obtained for the 22 galaxies in the H8-D4000 diagram, to quantify the probability of two successive starbursts (see text).
Table 9:
Values of t20 (black circle) and t50 (grey circle) obtained for the 17 galaxies in the H
-D4000 diagram, to quantify the probability of two successive starbursts (see text).
For the bursts themselves, we computed a median duration of
years, during which the galaxies produced
% (the error bar includes 68% of the galaxy sample) of their stellar mass. This corresponds to a mass of molecular gas of about
,
which is consistent with that observed in local LIRGs and ULIRGs (see Sanders & Mirabel 1996).
We note that all simulations produced in this paper assume the same fixed IMF for both the underlying star formation and the burst of star formation. Some evidence that the formation of low-mass may be less efficient in the environment of active star formation in the solar neighborhood were suggested in the past (Larson 1986; Scalo 1986; Maeder 1993). A top-heavy IMF could also account for the enhanced ratio of light elements to iron in massive early-type galaxies (Worthey et al. 1992) and for the relative enrichment of oxygen to iron in the intra-cluster medium (Arnaud et al. 1992). In our study, the occurrence of a top-heavy IMF in the burst episode would only weakly influence the derived burst duration timescales, which are set by the spectral signature of massive A to F stars. However, a top-heavy IMF during the burst phase would imply a much lower contribution to the total galaxy mass by longer-lived, low-mass stars.
Finally, we discussed the possibility that the distant LIRGs had experienced a previous starburst prior to the ongoing one during the past 1.5 Gyear. While most galaxies are not consistent with a merger scenario where two galaxies merge in several phases producing a series of bursts separated by a few ten million years (Mihos & Hernquist 1996), the majority present more than 50% chance of having experienced a previous burst in the past 1.5 Gyear, i.e. since .
If these properties are typical of LIRGs between z= 1 and z= 0, then this suggests that the population of galaxies experiencing LIRG phases experienced on average 2 to 3 LIRG phases since z= 1 and up to 4 since their birth around z= 4-5, as also suggested by Hammer et al. (2005).
This scenario is not consistent with the formation of distant LIRGs
through the continuous star formation characterizing isolated spiral
galaxies as has been argued independently based on their
morphology. Instead, minor mergers, tidal interactions, and gas
accretion all remain equally plausible triggering mechanisms for more
than half of the distant LIRGs that do not harbor the morphology of
major mergers.
Acknowledgements
We wish to thank the anonymous referee for constructive remarks that helped improving the paper, in particular on the wavelet analysis. We also wish to thank Anna Gallazzi, Nicolas Gruel, Emmanuel Moy, and Jean Luc Starck for helpful discussions and comments, and Emeric Le Floc'h for technical support with the MIPS data. S.C. thanks the Alexander von Humboldt Foundation, the Federal Ministry of Education and Research, and the Programe for Investment in the Future (ZIP) of the German Government for their support.