Issue |
A&A
Volume 510, February 2010
|
|
---|---|---|
Article Number | A68 | |
Number of page(s) | 9 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912081 | |
Published online | 11 February 2010 |
The baryonic content and Tully-Fisher relation at z
0.6
M. Puech - F. Hammer - H. Flores - R. Delgado-Serrano - M. Rodrigues - Y. Yang
GEPI, Observatoire de Paris, CNRS, University Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
Received 16 March 2009 / Accepted 18 November 2009
Abstract
Context. Using the multi-integral-field spectrograph GIRAFFE
at VLT, we previsouly derived the stellar-mass Tully-Fisher Relation
(smTFR) at
for a representative sample of 63 emission-line galaxies. We found that
the distant relation is systematically offset by roughly a factor of
two toward lower masses from the local relation.
Aims. We extend the study of the evolution of the TFR by
establishing the first distant baryonic TFR in a CDFS subsample of
35 galaxies. We also investigate the underlying cause of the large
scatter observed in these distant relations.
Methods. To derive gas masses in distant galaxies, we estimate a
gas radius and invert the Schmidt-Kennicutt law between star formation
rate and gas surface densities. We consider the influence of velocity
dispersion on the scatter of the relation, using the kinematic tracer S suggested by Kassin and collaborators.
Results. We find that gas extends farther out than the UV light from young stars, a median of 30%. We present the first baryonic TFR (bTFR) ever established at intermediate redshift and show that, within an uncertainty of
0.08 dex, the zeropoint of the bTFR does not appear to evolve between
and z
= 0. On the other hand, we confirm that the difference between the
local and distant smTFR is significant, even considering random and
systematic uncertainties, and that accounting for velocity dispersion
leads to a significant decrease in the scatter of the distant relation.
Conclusions. The absence of evolution in the bTFR over the past
6 Gyr implies that no external gas accretion is required for
distant rotating disks to sustain star formation until z = 0
and convert most of their gas into stars. Finally, we confirm that the
larger scatter found in the distant smTFR, and hence in the bTFR, is
caused entirely by major mergers. This scatter results from a transfer
of energy from bulk motions in the progenitors, to random motions in
the remnants, generated by shocks during the merging. Shocks occurring
during these events naturally explain the large extent of ionized gas
found out to the UV radius in
galaxies. All the results presented in this paper support the ``spiral
rebuilding scenario'' of Hammer and collaborators, i.e., that a large
fraction of local spiral disks have been reprocessed during major
mergers in the past 8 Gyr.
Key words: galaxies: evolution - galaxies: kinematics and dynamics - galaxies: high-redshift - galaxies: general - galaxies: interactions - galaxies: spiral.
1 Introduction
The stellar-mass Tully-Fisher Relation (smTFR) has received increased attention over the past decade, both at low (e.g., Bell & de Jong 2001; Pizagno et al. 2007; Meyer et al. 2008) and high redshift (Atkinson et al. 2007; Flores et al. 2006; Cresci et al. 2009; Kassin et al. 2007; Conselice et al. 2005; Puech et al. 2008). However, the smTFR does not allow us to homogeneously characterize all galaxy morphologies together. McGaugh (2000) showed that dwarf galaxies, which have a larger gas content that more massive galaxies, fall downward the smTFR. Considering the baryonic (i.e., stellar plus gas) content of galaxies, he showed that all galaxies, including dwarves, define a common TFR (see also Begum et al. 2008), which by then was dubbed the baryonic TFR (bTFR). Because the bTFR appears to hold over five decades in mass (McGaugh 2000), it can therefore be considered as being somehow more ``fundamental'' than the smTFR.So far, the bTFR has been studied only in the local Universe (see
Avila-Reese et al. 2008; Trachternach et al. 2009; Stark et al. 2009; McGaugh 2004; Bell & de Jong 2001; Geha et al. 2006; Goruvich et al. 2004; McGaugh 2005; Meyer et al. 2008; Noordermeer & Verheijen 2007; Verheijen 2001; De Rijcke et al. 2007).
This was primarily because HI observations of galaxies are practically
limited to z < 0.3 (Lah et al. 2007). To estimate the gas content of more
distant galaxies, indirect methods were developed, such as the
inversion of the Schmidt-Kennicutt (SK) law (Kennicutt 1989), which
relates the gas surface and star formation rate (SFR) densities (e.g., Erb et al. 2006; Mannucci et al. 2009). Another difficulty in establishing a
reliable bTFR (or even a smTFR) at high z is that it is not always
very clear whether all distant samples are truly representative of the
luminosity or mass function of galaxies at those epochs. At
intermediate redshifts (i.e., z < 1), Flores et al. (2006) and Yang et al. (2008) assembled a representative sample of 63
emission-line galaxies observed by 3D spectroscopy for a project
called IMAGES. The kinematics of the entire galaxy surface is probed
by 3D spectroscopy, which allows us to classify them as a function of
their relaxation state. Rotating Disks (RDs) are galaxies that exhibit
regular rotation well-aligned along the morphological axis. Perturbed
rotators (PRs) have large-scale rotation with a local perturbation in
the velocity dispersion map that cannot be accounted for by rotation,
while galaxies with complex kinematics (CKs) do not exhibit
large-scale rotation, or have a strong misalignment between the
dynamical and morphological axes (Yang et al. 2008). Using this new
kinematical classification, Flores et al. (2006) showed that the larger
dispersion of the distant TFR was due entirely to galaxies with
non-relaxed kinematics, probably associated with mergers, as later
confirmed by Puech et al. (2008, hereafter P08) as well as Kassin et al. (2007)
and Covington et al. (2010). Restricting the smTFR to rotating disks (RDs),
for which the TFR can be confidently established, for the first time
P08 detected an evolution in zeropoint of the smTFR by
0.36
+0.21-0.06 dex between
and z = 0. However,
galaxies have a larger gas fraction than their local
counterparts (i.e.,
% at
-0.8), as
derived from the evolution of the gas-metallicity relation (Rodrigues et al. 2008). This led P08 to suggest that the bTFR, as opposed
to the smTFR, might not be evolving.
In this paper, we take advantage of the representative IMAGES sample
of intermediate-mass galaxies at
to establish the first
bTFR at high redshift, using the SK inversion method. The paper is
organized as follows: in Sect. 2, we present the sample and the data
used in this paper; in Sect. 3, we extend the previous analysis of the
smTFR by P08, obtaining new insight about its shift in zeropoint and
its scatter; in Sect. 4, we estimate the gas content of
intermediate-redshift galaxies and compare it to the stellar light
from young stars; in Sect. 5, we establish the first bTFR at
,
which is discussed in Sect. 6. Throughout, we adopt
H0 = 70 km s-1 Mpc-1,
,
and
,
and the
AB magnitude system.
2 Data and sample
2.1 The sample
We started from the 3D sample of P08. Galaxies were selected using
J-band absolute magnitudes as a proxy for stellar mass, such that
,
which roughly corresponds to
,
using a ``diet'' Salpeter IMF and Bell et al. (2003)
simplified recipes for deriving stellar mass from J-band luminosity.
Additional practical constraints were imposed on their rest-frame
[OII] equivalent width (i.e.,
)
and redshift (i.e.,
z between 0.4 and 0.75), by observing with the multi-integral-field
unit spectrograph FLAMES/GIRAFFE at the VLT. This provided a sample of
63 galaxies that represents the J-band luminosity function of galaxies
at these redshifts (Yang et al. 2008). In the following, we restrict our
study of the bTFR to galaxies within the CDFS (see Sect. 2.4). This
sample of 35 galaxies is still representative of the luminosity
function at
(Yang et al. 2008).
NIR photometry was derived from public images of the respective fields
where galaxies were selected (i.e., CDFS, CFRS, and HDFS). Stellar
masses
were estimated from
ratios
using the method of Bell et al. (2003), assuming a ``diet'' Salpeter IMF.
We estimated in P08 that this method, when applied to
galaxies, provides us with estimates of the stellar mass with an
associated random uncertainty of 0.3 dex, and a systematic uncertainty
of -0.2 dex. We note that this comparison takes into account the
influence of possible secondary bursts of star formation (Borch et al. 2006). In addition, the influence of TP-AGB stars on the
derivation of stellar masses could overestimate the stellar mass by up
to
0.14 dex (Pozzetti et al. 2007; Maraston et al. 2006) in a systematic way,
but constant with redshift. In Hammer et al. (2009b), it is shown that the Bell et al. (2003) method, when applied to
galaxies, provides us
with an upper limit to their stellar mass relative to the measurements
determined by other stellar-population-synthesis models and IMF
combinations, which is reflected by all systematic effects tending to
lower these estimates. Using these stellar masses to study the TFR is
therefore a quite conservative choice, leading to evolutionary trends
measured being perhaps lower than in reality. We refer the reader to
P08 and its Appendix for a clearer description of our adopted method,
as well as Hammer et al. (2009b).
We note that the IMF used to derive stellar masses (i.e., the diet Salpeter IMF following Bell et al. 2003) differs from the IMF used in the next section to derive star formation rates (i.e., the regular Salpeter IMF). However, in Hammer et al. (2009b), we argue that stellar masses derived using Bell et al. (2003) recipes and a diet Salpeter are roughly equivalent (in a one-to-one correlation and not only in statistical sense) to stellar masses derived using the models of Bruzual & Charlot (2003) and a regular Salpeter IMF. Therefore, in the following, we make no attempt to correct for this difference in IMF, since the current generation of stellar population synthesis models is simply not accurate enough for difference caused by these two IMFs to be significant.
Compared to P08, we modified the classification of two galaxies in the sample. J033245.11-274724.0 was studied in detail by Hammer et al. (2009a), who found in HST/ACS images from the UDF a dust-enshrouded disk that had not previously been seen in GOODS images because of their shallower depth (see a comparison between GOODS and UDF images in their Fig. 1). Accounting for this disk, the dynamical axis was found to be strongly misaligned with the morphological axis. Hammer et al. (2009a) argued that the morpho-kinematic properties of J033245.11-274724.0 can only be reproduced by a major merger event. Similarly, Puech et al. (2009) carried out a detailed study of J033241.88-274853.9, whose morpho-kinematic properties also appeared to be far more closely reproduced by a major merger model than that of a simple rotating disk. Therefore, we shifted the classification of both objects from RD to CK.
2.2 Star formation rates
We estimated the total SFR
of each galaxy, following
Puech et al. (2007b). Briefly, SFR
was taken to be the sum of
SFR
,
derived from the 2800 Å luminosity L2800, and
SFR
,
which was derived from Spitzer/MIPS photometry at 24
m using the Chary & Elbaz (2001) calibration between rest-frame 15
m
flux and total IR luminosity
.
To convert both
and
L2800 into SFRs, we used the calibrations of Kennicutt (1998),
which rely on a Salpeter IMF. Uncertainties were estimated by
propagating the flux uncertainty measurement using the SFR
calibrations. About half of the galaxies in the sample were not
detected by MIPS. In this case, we derived an upper limit to
SFR
as a function of redshift using Fig. 9 of Le Floc'h et al. (2005).
We assumed that SFR
is the mean of this limit and SFR
.
The corresponding uncertainty in this case is half the difference
between SFR
and the limit to SFR
,
which infers a mean
relative uncertainty of 50% for these objects (see Fig. 1 and
Table 1).
![]() |
Figure 1: SFR as a function of stellar mass in a subsample of 35 CDFS galaxies. RDs are shown as blue dots, PRs as green squares, and CKs as red triangles. The open blue circle corresponds to the RD+ galaxy for which the velocity measurement is more uncertain (see P08). Uncertainties in stellar mass are discussed in Sect. 2.1, and are not shown for reasons of clarity. |
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2.3 Gas mass estimates
To estimate the mass of gas within each galaxy, we used the SK
relation between SFR and gas densities of Kennicutt (1989), which is
given by
/yr/kpc2. Bouché et al. (2007) derived a new calibration of
the SK law, based on distant sources (
). They found that the
SK law holds up to
,
but has a quite different power-law
index (i.e., 1.7 instead of 1.4). For the CDFS sample, we find that
ranges from 0.004 to 0.46
/yr/kpc2. In
this regime, the two SK laws are relatively similar, the discrepancy
between the two relations being caused mostly by the higher
star-formation densities found in
starbursts (see Fig. 3 of
Bouché et al. 2007). This range corresponds to the star-formation
densities of local (U)LIRGs, starbursts, and star-forming disks
(Bouché et al. 2007). Finally, at z = 0.24, Lah et al. (2007) performed direct HI
measurements of distant galaxies, and found that there is no evolution
in the relation between HI mass and SFR. Therefore, we conclude that
it is more appropriate to rely on the local SK law rather than the one
at high-z to derive gas masses in a sample of
galaxies.
To do this, we first derived the SFR density
,
which was then converted into a gas
density
using the SK law. Gas masses were derived as
.
The same radius
,
which is defined in the next section, is assumed when
normalizing both densities, following Kennicutt (1989).
Uncertainties in
were derived using standard methods of
error propagation (see Table 1 and Fig. 2). It is
noteworthy that gas masses derived this way are independent of the
IMF, since the SK law and SFRs are derived using the same Salpeter
IMF
. Gas masses
and gas fractions in
galaxies are discussed further by
Hammer et al. (2009b). Of particular interest here, is that the gas masses
derived by inverting the SK law lead to a median gas fraction of 31%
in
galaxies. We note that gas fractions, in contrast to gas
masses, are not IMF independent. Strikingly, the same value is found
using a completely different data set and methodology (i.e., based on
the evolution of the mass-metallicity relation derived from FORS2
long-slit spectroscopy, see Rodrigues et al. 2008). It is therefore
very unlikely that the gas fractions derived at
could be
affected by any systematic effect, which makes us quite confident in
our estimates of gas masses.
![]() |
Figure 2:
Gas mass as a function of stellar mass in a subsample of
35 CDFS galaxies. RDs are shown as blue dots, PRs as green squares,
and CKs as red triangles. The open blue circle corresponds to the
RD+ galaxy for which the velocity measurement is more uncertain (see
P08). Uncertainties in stellar mass are discussed in Sect. 2.1, and
are not shown for reasons of clarity. The blue line represents equality
between the stellar and gas masses, while the blue dashed line
represents the local relation between these two quantities given by Schiminovich (2008). The median gas fraction is found to be 31%, or
|
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Table 1: Principle properties of the sample of 35 galaxies used in this study, in order of increasing RA (see Sect. 2.5).
![]() |
Figure 3:
Left: evolution of the stellar-mass TFR in the sample
of 64 galaxies of P08 (RDs are shown as blue dots, PRs as green
squares, and CKs as red triangles). The open blue circle correspond
to the RD+ galaxies for which the velocity measurement is more
uncertain, see P08). The two open red triangles correspond to the
two galaxies that were changed from RD to CK compared to P08 (see
Sect. 2.1). The black line is the local smTFR, while the blue
dash-line represent a linear fit to the
|
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2.4 [OII] gas radius estimates
To convert from density to mass, one needs to define and measure a gas
radius
.
To do this, we first constructed rest-frame UV
images by summing observed B and V bands. Given that not all galaxies
in the sample have homogeneous imaging (see Neichel et al. 2008), we
decided to restrict the study of the bTFR to galaxies with the highest
quality images. Therefore, we restricted the study of the bTFR to the
subsample of 35 galaxies lying in the CDFS with HST/ACS images, which
is still representative of the luminosity function at
(Yang et al. 2008). This allowed us to limit uncertainties in the
derivation of gaseous radii and therefore, gas masses.
From rest-frame UV images, we derived for each galaxy an axis ratio
b/a and a PA using SExtractor (Bertin & Arnouts 1996). For each galaxy,
these two parameters were used to generate a set of images with flat
ellipses of increasing radii using the IDL procedure DIST_ELLIPSE. To
account for the relatively coarse spatial sampling of the GIRAFFE IFU
(0.52 arcsec/pix, which roughly corresponds to 3.5 kpc at
),
these high-resolution ellipses were then rebinned to the GIRAFFE pixel
scale after determining as accurately as possible the position of the
GIRAFFE IFU on the UV image (see, e.g., Puech et al. 2007b). These
simulated [OII] GIRAFFE images were renormalized in terms of flux
using the observed IFU-integrated [OII] value, and pixels with a
resulting flux lower than the minimal [OII] flux detected within the
GIRAFFE IFU for a given galaxy were disregarded, to account for
GIRAFFE IFU pixels being selected in terms of signal-to-noise ratio
(i.e.,
,
see Flores et al. 2006). By identifying the
simulated [OII] map that most closely reproduces the observed one, it
is in principle possible to retrieve the underlying [OII] total-light
radius. However, the exact position of the IFU, as well as the precise
value of the seeing during the observations are subject to
uncertainties (e.g., Puech et al. 2007b,2008) that prevent us from
being able to determine the exact position of the IFU grid. The
relatively coarse spatial resolution of the GIRAFFE IFU also makes it
relatively difficult to retrieve precisely the underlying [OII] flux
distribution. To mitigate the uncertainty associated with these
effects, we did not use a classical chi-square minimization between
the simulated and observed [OII] maps, but used a spatial
cross-correlation to determine, for each galaxy, the subset of
simulated [OII] maps that maximizes the number of pixels illuminated
in both the observed and simulated [OII] maps. This subset of
simulated maps corresponds to a range of ellipse radii that are
deconvolved from the GIRAFFE coarse spatial sampling. We assumed that
the mean value of this range is a measure of the total [OII] radius
,
whose error-bars are equal to half this range. Finally,
were quadratically deconvolved from a 0.8 arcsec mean
seeing disk. The maximal value of the ratio of the errorbars to the
corresponding radii is found to be 51%, while their distribution
shows an average of 17% and a 1-
spread of 12%. The derived
radii and errorbars in kpc are shown in Fig. 4 and listed
in Table 1.
![]() |
Figure 4:
Comparison between the [ OII] and UV radii for the CDFS
sub-sample of galaxies. The two insets on the left (30-arcsec wide)
show MIPS imaging at 24 |
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3 Evidence that major mergers are responsible for the large scatter in the distant TFR
3.1 The z
0.6 stellar-mass TFR
The smTFR in the IMAGES sample was initially derived in P08 (see their
Appendix). We show in Fig. 3 (see left panel) a revised
version of this relation, in which the classification of two objects
has changed (see Sect. 2). We also show here all galaxies, including
non-relaxed ones. The dispersion in this relation was shown to be
caused entirely by non-relaxed systems by P08 (see next section).
Maintaining the slope at its local value, we found a shift in the
smTFR zeropoint of 0.34
+0.21-0.06 dex in stellar mass, or,
equivalently, of 0.12 dex in velocity, between
and
z = 0, consistent with the shift derived by P08. The errorbars account
for possible systematic effects, which were quantified in detail by
P08. Most of them are associated with the estimation of stellar mass.
As we noted in Sect. 2.1, all these effects would tend to decrease the
estimates of the stellar mass in
galaxies, since we
``maximized'' them by adopting the Bell et al. (2003) method. The only
systematic effects that could reduce the shift in zeropoint are
relatively insignificant (i.e., -0.06 dex), and are associated with
the derivation of rotation velocities (see P08). In P08, we found that
the shift of the smTFR with redshift cannot be interpreted as a pure
evolution along the velocity axis, and that only half, at most, could
be accounted for by a velocity shift caused by gas accretion within
the optical radius. We then concluded that most of the shift in the
smTFR reflects an increase in stellar mass in rotating disks over the
past 6 Gyr.
Koen & Lombard (2009) performed a permutation test to demonstrate that the
distant K-band TFR found by P08 was offset significantly from the
local relation. We repeated their analysis for the smTFR and found a
probability 1% that the two relations have identical slope and
zeropoint. To test the reliability of this result in terms of
systematic uncertainties, we shifted the distant smTFR in steps of
0.01 dex in stellar mass toward the local relation and repeated the
permutation test. We found that for this probability to exceed 10% we
would need to bias the stellar mass in the distant sample by +0.28
dex, which exceeds systematic uncertainties (see above). This means
that, within the systematic effects on the zeropoint shift identified
by P08, the two relations differ significantly, which secures the
results of P08.
3.2 Origin of the scatter in the z
0.6 smTFR
In Flores et al. (2006), we showed that the dispersion in the
smTFR was caused by non-relaxed galaxies. This result was confirmed by
P08, who showed that the dispersion in the distant smTFR, once
restricted to RDs (i.e., 0.12 dex), was similar to that in the local
relation, with 0.15 dex. The galaxies contributing to the scatter in
the distant smTFR exhibit strong perturbations in their kinematic maps
(e.g., Yang et al. 2008), which led us to claim that most of them were
probably associated with major mergers (see P08). This interpretation
is also supported by the large scatter seen in the specific angular
momentum versus rotation velocity plane, which could be produced by
these events, as demonstrated by Puech et al. (2007a).
Interestingly, Weiner et al. (2006) defined a new kinematic tracer
that combines rotation
velocity and velocity dispersion. They found that velocity dispersion
is an important component of the dynamical support in
intermediate-redshift galaxies, especially for morphologically compact
or disturbed systems. Their study confirmed the earlier finding of
Puech et al. (2006) that the majority of compact galaxies have motions in
which dispersion plays an important role (see also Puech et al. 2007a).
Kassin et al. (2007) later showed that using S in the smTFR leads to a
significant reduction in its scatter. In addition, Covington et al. (2010)
showed, using hydro-dynamical simulations of major mergers, that this
reduction in scatter is consistent with a (partial) transfer of energy
between the kinetic energy associated with bulk motions to that
associated with random motions. This transfer is driven by shocks and
collisions generated during the merger events (e.g.,
Montero-Ibero et al. 2006).
That the ``merger-hypothesis'' is the underlying cause of the large
scatter measured in the distant TFR can be tested by searching for
specific objects and verifying whether this transfer of energy between
bulk and random motions driven by interaction-driven shocks really
occurs. Peirani et al. (2009) studied one CK galaxy of the sample
(J033239.72-275154.7) in detail and they demonstrated that both its
morphology and kinematics were reproduced well by a 1:3 major merger
where the companion galaxy had been in a retrograde parabolic orbit
and there had been an inclination angle of 15 degrees between the two
orbital planes. This object is a perfect test-bed for the present
discussion, since it is measured to have
km s-1and
,
i.e., to be among the CK galaxies
in the distant smTFR that are the most deficient in rotation velocity
(see left panel of Fig. 3). Peirani et al. (2009) showed that,
because of the retrograde nature of interaction, the host galaxy can
lose angular momentum to its companion, resulting in the deceleration
of the main progenitor. This explains why the resulting merger remnant
appears to have systematically smaller velocities in the smTFR than
other objects. Shocks in the gaseous phase generated during the
interaction produce an increase in the velocity dispersion, similar to
that suggested by Covington et al. (2010) by their merger simulations (see
their Fig. 1). If the scatter in the smTFR is really caused by a
transfer of energy between bulk and random motions driven by merger
events, then this object should fall back onto the TFR, once its
velocity dispersion is taken into account.
To test this, we derived S in the IMAGES sample using
and
,
as defined by
Puech et al. (2007a). We show the resulting S-TFR in Fig. 3 (see
right panel). Using S results in the shifting of all RDs back onto
the local smTFR, so we propose to use the local slope in all future
investigations. We also note that several CK galaxies lie at
relatively high S compared to the bulk of the sample. These galaxies
were found to have large rotation velocity in the conventional smTFR.
However, these galaxies do not seem to be present in the
Kassin et al. (2007) smTFR, although Covington et al. (2010) managed to produce
some of these galaxies in their merger simulations (see their Fig. 3).
These differences seem to be related to different selection criteria
involving inclination. We did not apply any selection of this type
(see P08), while Kassin et al. (2007) removed galaxies with low
inclinations (i.e.,
deg) because of the larger associated
uncertainty, and those with high inclinations (i.e.,
deg), because of the effect of dust (see also
Covington et al. 2010).
The total scatter in the resulting S-TFR is significantly lower than
in the smTFR, since it decreases from 0.63 to 0.34 dex. This residual
scatter is still a factor of two larger than the residual dispersion
in the local relation, which is
dex (in mass).
Looking at the dispersion in more detail, the scatter remains roughly
constant in RDs and PRs (from 0.12 to 0.14 dex and 0.32 to 0.29 dex,
respectively), but strongly decreases from 0.83 to 0.41 dex among CKs.
That kinematically disturbed galaxies show a smaller but still large
scatter compared to the local relation suggests that part of the
remaining discrepancy is related to the greater uncertainty in
rotation velocity of CKs (see P08). The median uncertainty in the
rotation velocity of the sample is found to be 0.12 dex, which
translates into
0.3 dex in stellar mass, i.e., the same order of
magnitude as the residual scatter in the distant S-TFR.
As one can see in the right panel of Fig. 3, accounting for
velocity dispersion brings the merger-test J033239.72-275154.7 back
onto the S-TFR. This is the first direct observational evidence of
the transfer of energy described by Covington et al. (2010). As shown
above, this process can account for most of the scatter in the smTFR.
Given the still relatively large uncertainties, we cannot exclude
other process(es) playing a role in producing the scatter in the
distant relation, but these results clearly indicate that this
transfer of energy is the main underlying cause, and that mergers are
the main cause of the large scatter seen in the
TFR, as
initially argued by Flores et al. (2006; see also Kannappan & Barton 2004).
4 Gas content of intermediate-mass galaxies
4.1 Comparison between gas and stellar extents
We show the total-light [OII] radius
as a function of 1.9 times the UV half-light radius
in Fig. 4. For a
thin exponential disk, 1.9 times the half-light radius is equal to the
optical radius of the galaxy (Persic & Salucci 1991), which provides us with
a useful comparison point
. Galaxies that have
a
significantly lower than
are galaxies
for which the UV light extents farther than the IFU FoV (
arcsec2). Two examples of these galaxies can be seen on the right
side of Fig. 4. For these galaxies, it is clear that
assuming
to be a measure of the gaseous extent would lead
to underestimating the gas mass. Therefore, we defined
to be a conservative measure
of the gaseous extent. We note that for galaxies with non-relaxed
kinematics, this might still underestimate the gas radius. The
uncertainties correspond to the respective uncertainties in
or
depending on which radius was the
largest for a given galaxy.
Most RDs (see blue dots) fall relatively close to the equality line,
which instills confidence in the estimate of
in dynamically
relaxed systems. To first order, one can assume that, at least on
large spatial scales and for relaxed systems, the spatial distribution
of UV light emitted by young OB stars is correlated with the emission
from the [OII] ionized gas detected by GIRAFFE. However, in more
disturbed galaxies, there is a clear trend in which the gas is more
extended than the UV light. We find a median [OII]-to-UV radius ratio
of
(1-
bootstrapped uncertainty), which is
consistent with the mean ratio of
1.2 of emission line to B-band
scale-lengths found by Bamford et al. (2007) in a sample of field galaxies
at similar redshifts. Using a Student t-test, the probability that
this median ratio is equal to one (i.e., that the total [OII] and UV
radii are statistically equivalent) is found to be 6%. This trend is
even more pronounced for more compact systems: galaxies with a UV
radius lower or equal to the median in the sample have a median ratio
of
1.7, while those having a UV radius larger than the median
value, have a ratio
0.9. We also examined the GOODS/MIPS (DR3)
images of the four galaxies with a UV radius smaller that one GIRAFFE
pixel. All were detected, although only three at a significantly high
level for their flux to be measurable. Interestingly, two of them were
resolved by MIPS (see the two insets on the left side of Fig.
4), which confirms that the gas content can be
significantly more extended than the UV stellar light.
4.2 What causes the large gaseous extent in
galaxies?
It is interesting to investigate what causes this discrepancy between the extents of the ionized gas and the UV ionizing stellar light from OB stars. Most of the ionizing stellar light is assumed to be emitted by stars earlier than B2 (Strömberg 1939), so to first order, one would expect conversely that the ionized gas is confined to within the UV radius. What mechanisms can explain our actual findings?
Supernovae (SN) and AGN feedback are two mechanisms that are known to
be able to drive outflows in galaxies. If these mechanisms were
responsible for ejecting ionized gas to the UV radius, we should
detect offsets between absorption and emission lines in a large
fraction of galaxies in the sample. For 20 galaxies in the GIRAFFE
sample, we were able to retrieve FORS2 integrated spectra (see
Rodrigues et al. 2008) that allowed us to measure absorption lines.
Among these galaxies, we found systematic shifts in only three or
possibly four of them (at a 100 km s-1 level in
J033224.60-274428.1, J033225.26-274524.0, J033214.97-275005.5, and
possibly in J033210.76-274234.6). Hence, we conclude that outflows,
regardless of their powering mechanism, cannot explain why gas extends
farther out than the UV stellar light.
If ionized gas is not pushed out of the UV disk by outflows, then the
gas must be ionized by a mechanism other than the radiation of OB
stars. Interestingly, for one object in the sample, Puech et al. (2009)
observed a region that is completely devoid of stars, where ionized
gas was detected. They concluded that this gas was ionized by shocks
induced by a major merger. These interaction-driven shocks could be an
important mean of ionizing gas in
galaxies.
Hammer et al. (2009b) showed that the morpho-kinematics of most galaxies in
the sample can be reproduced by the simulations of major mergers. We
note that gas in all galaxies in the sample have lower
ratios than their local counterparts, which might be a dynamical
signature of these shocks (Puech et al. 2007a). In particular, most PRs
show off-center velocity dispersion peaks (see Yang et al. 2008),
which strengthens this interpretation.
5 The baryonic Tully-Fisher relation at z
0.6
5.1 The local relation
We used the local sample gathered by McGaugh (2005) to derive our
local reference bTFR. McGaugh (2005) used
and (amongst
others) the Bell et al. (2003) method to estimate stellar mass, so their
sample is particularly well suited to compare with our high-z bTFR.
However, they used the B-band luminosity to derive stellar mass, which
is a less accurate tracer than K-band luminosities. Hence, we
rederived stellar mass from K-band luminosities by cross-correlating
the McGaugh (2005) sample with the 2MASS database. Stellar masses
were derived following the method outlined in P08 for their local
sample, i.e., by accounting for k-corrections and extinction. We found
that
has a residual scatter of
dex. In the
following, we checked that by instead using the McGaugh (2005)
stellar masses derived from the B-band luminosity (see his Table 2
with
= 1) would not change significantly our
results.
Stark et al. (2009) established a new calibration of the local bTFR using gas-dominated galaxies, which has the advantage of being less sensitive to a given estimator of the stellar mass. However, Stark et al. (2009) averaged the bTFR produced by various stellar mass estimators. Therefore, we chose to keep our own derived relation, which has the advantage of resulting from the same methodology used to derive the smTFR, both at low and high redshift, as well as the bTFR at high redshift, which minimizes possible systematic effects. We nevertheless note that our slope and zeropoint are consistent with their calibration. In contrast to the smTFR, whose calibration depends on the galaxy populations analyzed, the bTFR is found to be independent of galaxy type (Stark et al. 2009). We therefore do not expect any bias to be caused by the choice of a particular local sample and its level of representation in the local population.
5.2 The baryonic TFR at z
0.6
We show the first bTFR obtained at high redshift so far, in Fig. 5. Overall, the bTFR has the same structure as the smTFR, with
non-relaxed galaxies associated with the greatest amount of
dispersion. Holding the slope constant, we found a distant zeropoint
of
,
i.e., a shift of +0.06 dex in baryonic mass between
the z = 0 bTFR and
RDs. This shift translates into a -0.02dex shift in rotation velocity. The scatter in the distant bTFR for
RDs is found to be roughly similar to that in the local bTFR, with
0.14 dex, which suggests no evolution in slope. In comparison with the
distant smTFR, the scatter in RDs is found to be roughly equivalent,
with 0.14 dex in the bTFR instead of 0.12 dex in the smTFR (i.e.,
bootstrapping reveals that the 1-
spread of the scatter in RDs
in the smTFR is
0.03 dex). In contrast, the total scatter is
found to be significantly larger, with 0.83 dex in the bTFR instead of
0.63 in the smTFR. We used Monte-Carlo simulations to investigate
whether the larger uncertainty associated with the gas mass, compared
to the stellar mass, is responsible for the increase in scatter in the
bTFR. To do this, we simulated 1000 smTFRs by moving the galaxies
within the errorbars in their baryonic mass. We then refitted the
resulting smTFRs, and found that the total scatter (as well as the
scatter for RDs) does not increase significantly. Therefore, it
appears that the bTFR has a significantly larger scatter than the
smTFR, at least for the total galaxy population.
![]() |
Figure 5: Evolution of the baryonic TFR in the CDFS subsample (see text). The black line is the local bTFR of McGaugh (2005), while the blue-dashed line is the fit to the distant relation. All panels have similar limits to ease the comparison between the different relations. |
Open with DEXTER |
In assessing whether there is an evolution in the intercept of the
bTFR, it is important to account for possible systematic effects,
which are usually more significant than random effects in the TFR (see
P08 and Sect. 3). In the following, we express all systematic
uncertainties in terms of their influence on the evolution of the
zeropoint of the bTFR between
and z = 0. Systematic effects
on the velocity estimates in the distant sample were found to be
0.02 dex, which corresponds to
0.08 dex once converted into
.
Regarding the stellar mass, we identified in P08 two
possible systematic effects, which are an evolution in the IMF (+0.05
dex), and an evolution in the K-band mass-to-light ratio (+0.1 dex)
with redshift (see Appendix of P08). Finally, concerning gas masses,
the only possible systematic effect could be an evolution of the SK
law with redshift, since we argued in Sect. 2.3 that this quantity is
derived independently of the IMF. As stated in Sect. 2.3, there is no
evidence of an evolution in the SK law between
and z = 0,
based on a comparison between their star-formation densities and those
of local starbursts. Therefore, the main systematic effect that could
affect the baryonic mass would be that related to stellar mass, which
translates into a possible impact on the evolution of the bTFR
zeropoint shift between
and z = 0 by +0.15 dex (0.05+0.1, see
above). In conclusion, we find a bTFR zeropoint shift between
and z = 0 of
(random, see above)
+0.23-0.08 (systematic).
Using the permutation test of Koen & Lombard (2009), we find a probability of
62% that the local and distant relations have the same slope
and intercept. Given that there are only six RDs in the distant bTFR,
we also tested whether the local and distant intercepts are different,
assuming that the slope does not evolve. To do this, we used a Welch
t-test (Koen & Lombard 2009), and found that the probability that they are
equal is 91%. We also found that this probability drops below 10%
when we systematically increase the local bTFR zeropoint by at least
0.77 dex: this means that the local and distant bTFR zeropoints are
not statistically different within systematic uncertainties (see
above), assuming no evolution in slope. As stated above, the residual
scatters of the local and distant relation are similar, which suggests
that assuming there is no evolution in slope is a reasonable
hypothesis. Therefore, we conclude that we do not detect any
significant shift in the bTFR between
and z = 0.
6 Discussion and conclusions
There is now little doubt that the large scatter found in the
TFR is caused by non-relaxed systems associated with major
mergers (Covington et al. 2010; Flores et al. 2006; Kassin et al. 2007; Puech et al. 2008; see also
Kannappan & Barton 2004). Mergers provide a natural and coherent frame
for interpreting the morpho-kinematics properties of
galaxies (see Hammer et al. 2009b). In this paper, we have reported
that ionized gas extends farther out than the ionizing light from OB
stars: we indeed found a probability of 6% that the median ratio of
the gas to stellar radius is equal to one. This can be seen as a
natural consequence of shocks produced during these events. It is even
difficult to figure out how gas lying significantly farther out than
any radiation source could be ionized by any other process, given that
a very negligible fraction of objects in the sample are found to
contain evidence of gas outflows, which might spread ionized gas out
to the UV radius.
Returning to the TFR, we confirm that, once restricted to rotating
disks, the
smTFR appears to be shifted toward fainter
masses by 0.34 dex. This shift is found to be very significant within
the random and systematic uncertainties. In contrast, by
including the gas fraction of the baryonic mass in the TFR, we found
that, within the uncertainties, the
relation is consistent
with the local relation derived for the McGaugh (2005) sample. This
implies that star formation in rotating disks is mostly fed by gas
that is already gravitationally bound to galaxies, otherwise we would
have detected significant evolution in the bTFR. This does not mean
that there is no external gas accretion: given the still relatively
large associated uncertainties, there could be room for external gas
accretion, at a level of up to roughly one third of the local baryonic
mass. However, there is presently no need for this external gas
accretion.
At higher redshifts, Cresci et al. (2009) found an evolution of 0.41 dex in
the zeropoint of the smTFR between
and z = 0, i.e., of
similar amplitude (within uncertainties) to the evolution found in
this paper between
and z = 0. This would imply that the smTFR
does not evolve significantly between
and
,
while
it evolves significantly between
and z = 0. To try to
understand this surprising result, we refitted the smTFR found by Cresci et al. (2009), but assumed that the local slope of reference is that
used in the present study. The choice of the slope in the local
relation is crucial for deriving the evolution in zeropoint as a
function of redshift (see Sect. 5.1 of P08 for details). While the
distant sample used by Cresci et al. (2009) seems to be representative of
galaxies, at least for galaxies with stellar masses higher
than
(Förster Schreiber et al. 2009), they used
the local relation derived by Bell & de Jong (2001), which relies on the
sample of Verheijen (2001) that was shown to be biased toward an
excess of low-mass, gas-rich galaxies by Hammer et al. (2007). In P08, we
derived a local smTFR using a representative subsample of the SDSS
from Pizagno et al. (2007). In this sample, the slope is found to be
smaller, i.e., 2.8 (see Appendix of P08) instead of the value of 4.5
used by Cresci et al. (2009). By refitting the relation of Cresci et al. (2009)
using the slope derived from this representative local sample, we find
an evolution in zeropoint of
0.6 dex, instead of
0.4 dex
found using the Bell & de Jong (2001) slope. We conclude that the shift found
by Cresci et al. (2009) probably underestimates the evolution in zeropoint
between
and z = 0.
Finally, Cresci et al. (2009) interpreted the shift of the
smTFR
as the result of gas accretion onto the forming disks in filaments and
cooling flows, as suggested by theoretical and numerical models. This
gas accretion process could also play a role in feeding the outer
regions of
galaxies with fresh gas, to within a limit of
30% of the local baryonic mass, as discussed above. However, it would
remain unclear what the ionization source of this gas is (see Sect.
4). These models also clearly predict that these cold flows are
strongly suppressed in the massive,
haloes that the
galaxies studied in this paper inhabit (e.g., Dekel et al. 2009). In a
companion paper (Hammer et al. 2009b), we explore another possibility that
a large fraction of local spirals could have rebuilt their disk
following a major merger at
.
This scenario, dubbed as
``spiral rebuilding disk scenario'', was proposed by Hammer et al. (2005),
owing to the remarkable coincidence of the evolution of the merger
rate, morphology, and fraction of actively star-forming galaxies. In
this scenario, major mergers expel gas in, e.g., tidal tails, which is
later re-accreted to rebuild a new disk (Hopkins et al. 2009; Barnes 2002; Robertson et al. 2006). It is tempting to associate a
significant part of the gas reservoir with this process, which can
therefore provide us with an evolutionary framework in which to
intepret the non-evolution of the bTFR with redshift, and the finding
that ionized gas extends farther out than the stellar UV light. A
dynamical imprint of this accretion might already have been detected
in terms of the lower
found in distant gaseous disks
compared to local spirals (Puech et al. 2007a). Therefore, different
physical mechanisms could be driving galaxy evolution and the shift in
the smTFR, depending on redshift, which would complicate the
interpretation of the evolution in the smTFR from very high redshift
to z = 0.
This study made use of GOODS data in the CDFS field (MIPS and HST/ACS images), as well as of the 2MASS and the CDS databases. We thank X. Hernandez and L. Chemin for useful discussions about the subject of this paper.
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Footnotes
- ...
IMF
- The IMF affects the SFR calibration in such a way that both
the SFR and the numerical factors in the SK relation change and cancel
eachother. It is possible to derive a direct relation between UV and IR
luminosities and gas densities, in a similar way to Erb et al. (2006) using the
H
luminosity, therefore eliminating the SFR density, which is the IMF-dependant quantity.
- ... point
- For a thin exponential disk, this radius equals 3.2 times the disk scale-length, which defines the total stellar-light radius, and is statistically equivalent to the isophotal radius R25, see Persic & Salucci (1991).
All Tables
Table 1: Principle properties of the sample of 35 galaxies used in this study, in order of increasing RA (see Sect. 2.5).
All Figures
![]() |
Figure 1: SFR as a function of stellar mass in a subsample of 35 CDFS galaxies. RDs are shown as blue dots, PRs as green squares, and CKs as red triangles. The open blue circle corresponds to the RD+ galaxy for which the velocity measurement is more uncertain (see P08). Uncertainties in stellar mass are discussed in Sect. 2.1, and are not shown for reasons of clarity. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Gas mass as a function of stellar mass in a subsample of
35 CDFS galaxies. RDs are shown as blue dots, PRs as green squares,
and CKs as red triangles. The open blue circle corresponds to the
RD+ galaxy for which the velocity measurement is more uncertain (see
P08). Uncertainties in stellar mass are discussed in Sect. 2.1, and
are not shown for reasons of clarity. The blue line represents equality
between the stellar and gas masses, while the blue dashed line
represents the local relation between these two quantities given by Schiminovich (2008). The median gas fraction is found to be 31%, or
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Left: evolution of the stellar-mass TFR in the sample
of 64 galaxies of P08 (RDs are shown as blue dots, PRs as green
squares, and CKs as red triangles). The open blue circle correspond
to the RD+ galaxies for which the velocity measurement is more
uncertain, see P08). The two open red triangles correspond to the
two galaxies that were changed from RD to CK compared to P08 (see
Sect. 2.1). The black line is the local smTFR, while the blue
dash-line represent a linear fit to the
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Comparison between the [ OII] and UV radii for the CDFS
sub-sample of galaxies. The two insets on the left (30-arcsec wide)
show MIPS imaging at 24 |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Evolution of the baryonic TFR in the CDFS subsample (see text). The black line is the local bTFR of McGaugh (2005), while the blue-dashed line is the fit to the distant relation. All panels have similar limits to ease the comparison between the different relations. |
Open with DEXTER | |
In the text |
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