X. Z. Zheng 1 - F. Hammer 1 - H. Flores 1 - F. Ass
mat 1 - D. Pelat 2
1 - GEPI, Observatoire de Paris-Meudon, 92195 Meudon, France
2 -
LUTH, Observatoire de Paris-Meudon, 92195 Meudon, France
Received 2 December 2003 / Accepted 17 March 2004
Abstract
Using HST/WFPC2 imaging in F606W (or F450W) and F814W
filters, we obtained the color maps in observed frame for 36 distant
(0.4 < z < 1.2) luminous infrared galaxies
(LIRGs,
m)
),
with average star formation rates of
100
yr-1.
Stars and compact sources are taken as references to align images
after correction of geometric distortion. This leads to an alignment
accuracy of 0.15 pixel, which is a prerequisite for studying the
detailed color properties of galaxies with complex morphologies. A new
method is developed to quantify the reliability of each pixel in the
color map without any bias against very red or blue color regions.
Based on analyses of two-dimensional structure and spatially resolved
color distribution, we carried out morphological classification for LIRGs.
About 36% of the LIRGs were classified as disk galaxies
and 22% as irregulars. Only 6 (17%) systems are
obvious ongoing major mergers. An upper limit of 58% was found for
the fraction of mergers in LIRGs with all the possible merging/interacting
systems included. Strikingly, the fraction of compact
sources is as high as 25%, similar to that found in
optically selected samples. From their K band luminosities, LIRGs are
relatively massive systems, with an average stellar mass of about 1.1
10
.
They are related to the formation of massive
and large disks, from their morphologies and also from the fact that
they represent a significant fraction of distant disks selected by
their sizes. If sustained at such large rates, their star formation
can double their stellar masses in less than 1 Gyr. The compact LIRGs
show blue cores, which could be associated with the formation of the
central region of these galaxies. We find that all LIRGs are
distributed along a sequence which relate their central color to their
concentration index. This sequence links compact objects with blue
central color to extended ones with relatively red central color,
which are closer to the local disks. We suggest that there are many
massive disks which have been forming a large fraction of their
stellar mass since z = 1. For most of them, their central
parts (bulge?) were formed prior to the formation of their disks.
Key words: galaxies: formation - galaxies: evolution - infrared: galaxies
The evolution of the cosmic star formation density (CSFD) shows the
history of the stellar mass assembly averaged over all galaxies. A
sharp decline of the CSFD since
1 has been found, whereas large
uncertainties still remain at higher redshifts, particularly due to
the uncertainties and biases regarding dust extinction (e.g. Madau et al. 1996; Hammer et al. 1997). Investigations of
global stellar mass density as a function of redshift indicate that
more than one quarter, probably up to half of the present day stars were
formed since
1 (Dickinson et al. 2003, and
references therein). This is in agreement with an integration of the
CSFD if the latter accounts for all the light re-radiated at IR wavelengths
(Flores et al. 1999). Hence the star-forming activities
since
1 still play an important role in the formation of
the galaxy Hubble sequence seen in the local universe.
Hubble Space Telescope (HST) observations show that the merger
rate increases significantly at
1, compared with that in the local universe (Le F
vre et al. 2000; Conselice et al. 2003). Such events were claimed to be related to dwarf galaxies while massive systems formed before redshift 1
(Brinchmann & Ellis 2000; Lilly et al. 1998; Schade et al. 1999). However, with Infrared Space Observatory (ISO) mid-infrared imaging, Flores et al. (1999) inferred that a substantial fraction of star
formation since
1 is associated with the LIRGs.
These objects are luminous star-forming
galaxies at intermediate redshifts (
0.5 to 1), different
from the faint blue galaxy population (Genzel & Cesarsky 2000;
Franceschini et al. 2003). It is widely accepted
that merger/interaction is very efficient in pushing gas into the nuclear
region and triggering violent star formation. Therefore LIRGs are suspected
to be merging systems and the evolution of these galaxies is
linked to the decline of the merger rate (Elbaz et al. 2002).
Although HST imaging showed that
most of the LIRGs are luminous disk/interacting galaxies (Flores et al. 1999), systematic investigation of their properties is
still required to understand their formation and evolution, as well as to link
them to counterparts in the local universe.
Morphological classification is essential to reveal the nature of distant LIRGs. However, at high redshifts, it becomes difficult to classify galaxy morphology securely because the images of the high-z galaxies suffer from reduced resolution, band-shifting and cosmological surface brightness dimming effects, compared with the local objects. With the HST Wide Field Planet Camera 2 (WFPC2), high resolution imaging in two or more bands with spatially resolved color distribution can be used to investigate the distribution of the stellar population, which is complementary to addressing the appearance in a single band. Furthermore, the star-forming regions and dusty regions can stand out in the color map. This is very important to the study of LIRGs, in which these regions are expected to be numerous.
The Canada-France Redshift Survey (CFRS) fields are among the most studied
fields at various wavelengths. Two CFRS fields 0300+00 and 1415+52
had been observed deeply by ISOCAM at 15 m (Flores et al. 1999, 2004)
and by HST (Brinchmann et al. 1998). To perform detailed analyses of
morphology, photometry and color distribution for distant LIRGs,
additional HST images through blue and red filters have been taken to
complement the color information of the two CFRS fields (PI: Hammer,
Prop. 9149). In this work, we present the preliminary results of the
color distribution of distant LIRGs. We correct additional
effects in HST images and recenter them accurately, which allows us to
access the color maps of complex galaxies. We also implement a method
to quantify the signal-to-noise (S/N) ratio of the color image to give a reasonable cut for the target area in color maps.
This paper is organized as follows. Section 2 describes the HST imaging
observations and the archive data we adopt. In Sect. 3, we describe
the various effects which have to be corrected in aligning images in
different WFPC2 filters. In Sect. 4, we describe the method we use to
generate the color maps. In Sect. 5, we summarize the
morphological properties of the distant LIRGs. The results we obtained
of the LIRGs are discussed in Sect. 6. Brief conclusions are given in
Sect. 7. Throughout this paper we adopt
H0 = 70 km s-1 Mpc-1,
= 0.3 and
= 0.7. Unless specified, we exclude the PC chip and the
unit of pixel refers to that in WF chips. The bands B450,
V606 and I814 refer to HST filters F450W, F606W and F814W,
respectively. The Vega system is adopted for our photometry.
Ground-based spectroscopic redshift identification in the CFRS was
carried out with CFHT telescope for objects brighter than 22.5 mag
(
)
in five 10
fields (see Crampton
et al. 1995 for details). With improved data reduction,
Flores et al. (2004) present the updated
catalogs of the deep ISOCAM observations at 15
m for the two
CFRS fields 0300+00 and 1415+52 (see also Flores et al. 1999).
Using high resolution HST/WFPC2 imaging, the distant galaxies up to
1 can be spatially resolved. Three HST fields were observed in
the F606W and F814W filters in the two CFRS fields with ISOCAM observations (in Cycle 10, PI: Hammer, Prop. 9149), and two fields in
the F606W filter were observed to complement the observations in F814W
during Cycle 8 (PI: Lilly, Prop. 8162). The F606W and F814W filters
correspond to rest-frame U (3634 Å) and V (4856 Å) at a redshift 0.65. Those
fields were chosen to maximize the number of LIRGs contained in
each field. We also collected the HST imaging data with two band
observations in the CFRS fields. A detailed description of the previous
CFRS field HST imaging survey was presented in Brinchmann et al. (1998). The CFRS 1415+52 field partially overlaps
the Groth Strip Survey (GSS, Groth et al. 1994). We included
the GSS imaging data in our analysis.
Table 1 summarizes the HST imaging data used in this
analysis. The total exposure time is usually more than 6000 s,
in which the surface brightness corresponding to 1
above the
background is
25.5 mag arcsec-2. Note that the GSS is
relatively shallow except for one very deep field. Here we list the seven
of 28 GSS fields covering the CFRS 1415+52 field. The 28 GSS fields
are composed of 27 fields observed in Prop. 5090 (PI: Groth) and one very
deep field in Prop. 5109 (PI: West Phal).
Simard et al. (2002) carried out a detailed
morphological analysis on the GSS observations. They also provided
the physical scale and absolute magnitude for objects with
spectroscopic redshift identification using the Keck telescope. We
quoted these results directly and further details can be found in
their paper. The observation was divided into N exposures (Cols. 4
and 8 in Table 1) aimed at removing cosmic-rays and correcting
the bad/hot pixels.
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Figure 1:
Imitated HST/WFPC2
V606-I814 color maps for an elliptical galaxy at redshift 0.299 when the offsets between blue and red images are 0.0 pixel ( left panel), 0.15 pixel ( middle panel) and 0.30 pixel ( right panel). The color maps are 50 ![]() |
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Studies have been reported investigating color maps for
spheroidal galaxies at intermediate redshifts (e.g. Abraham et al. 1999; Ellis et al. 2001). In generating the
color map, a difficulty is to align two images accurately so as to
keep each pixel of the same object at the same position in two images. For
the spheroidal galaxies, generally the brightness peaks at
the galaxy center in different bands. It would be technically easy to
correct relative shifts between two images if the galaxy center is
used as a reference.
For the galaxies with complex morphologies, however, caution should be
applied in determining the relative shifts because their irregular
morphologies as well as contamination from star-forming regions
could easily effect efforts to find a reliable reference
point (e.g. the galaxy center in spheroidal galaxies). Such a
difficulty becomes more serious for galaxies at intermediate
redshifts when band-shifting effects become significant. The
uncertainty in aligning two images is required to be much smaller than
one pixel in addressing color distribution pixel-by-pixel. We model
the color map
V606-I814 to imitate the effect of the
alignment offset on the color map. We use the modeled HST/WFPC2 images of
an elliptical galaxy at redshift 0.299 with a de Vaucouleurs bulge+exponential
disk structure (B/T = 0.84) given by GIM2D (Simard et al. 2002),
to avoid contamination by the intrinsic color
fluctuation. Figure 1 illustrates that an offset
as small as 0.3 pixel will cause false structure "half-blue and
half-red'' in the color map (right panel), compared with the color map
with zero offset (left panel). Such an effect becomes marginal when the
offset decreases to 0.15 pixel (middle panel), which is the typical
uncertainty in our image alignment.
Instead of operating on individual objects, we dealt with the whole
images in data processing. Our method of data reduction to
align images is summarized below.
The raw HST images were processed using the standard STScI pipeline. For HST/WFPC2 observations in integer-pixel dither mode with telescope relative offsets larger than a few pixels, camera geometric distortion will cause an additional shift (increasing toward the CCD corners, see HST/WFPC2 handbook for more details). We correct the geometric distortion for each individual exposure in a data set before combining them. Table 1 tabulates the dither offsets, usually twenty pixels (in which an additional shift of about half a pixel due to the geometric distortion is present at the WF chip corners) except for those in Proposal 5449 and GSS, in which consecutive exposures were taken at the same position and the distortion correction was not applied. The shifts between the exposures were obtained using two approaches, cross-correlation and point source reference. The cross correlation technique is to shift and/or rotate one image relative the other to maximize the cross-correlation between the two images, i.e. the best match. In the later approach, the shifts are derived from a comparison of the locations of a number of point and point-like sources in individual exposures. In our work, the point and point-like sources refer to the objects satisfying Full Width at Half Magnitude (FWHM) <2.5 pixels, 17 < m814 < 22, 17 < m450 < 23 and 17 < m606 < 23. These criteria exclude the extended objects and those saturated or faint. For each WFPC2 field, at least three reference sources are used to derive the relative shifts between the different exposures. Both approaches use real images/objects to derive the shifts and hence are free from guide star acquisition uncertainties. In general, the derived shifts are remarkably consistent with each other within 0.08 pixel and even better for crowded fields. For sparse fields and some fields with large dither offsets, the cross-correlation is not the best approach and measurement using the point source reference will be adopted. The cross-correlation is also not suitable to find the rotation angle and the shifts between images in different bands. The morphologies and brightness of the astronomical sources may differ in one wavelength window from those in another. The cross-correlation would be biased by sources with a real center offset between different bands, which is often seen in spiral and irregular galaxies. Geometric distortion correction, cosmic-ray removement and image combination are accomplished using the STSDAS/DITHER package (version 2.0, Koekemoer et al. 1995).
In some of the fields, blue images and red images were taken in different cycles, i.e. the HST telescope pointings in two bands were not the same, which results in different relative rotations and shifts between them. We have used point-like sources as references to determine the rotations and the shifts. In practice, we obtain the relative rotation angle from astrometric information recorded in the image header. The keyword ORIENTAT provides the position angle of the telescope pointing. Normally the uncertainty of the position angle determination is 0.003 degree, compared to a rotation deviation of 0.01 degree causing a 0.1 pixel offset at the corners of a WF chip. Table 2 lists the shift and the rotation angle of the blue image relative to the red one for the WF3 chip. Note that in image combination, the first image of the data set is always taken as the reference to stack images.
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Figure 2: Systematic offsets between V606 and I814 images in 27 GSS fields. Solid circle is I814 image and cross is V606 image. |
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Figure 3: Position offset for 178 point/point-like sources between two aligned images. The offsets are in the coordinates of WF3 chip. The median value in X is 0.006 pixel and the median value in Y is 0.004 pixel. |
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To show the advantages of the point source reference method,
a systematic shift exists between the V606 image
and I814 image in the observations of the 27 GSS fields in Prop. 5090.
The observation of each field was split
into 8 exposures in V606 and I814 bands alternately at the
same location, i.e. no offset between consecutive exposures and 4 exposures for each filter. For the 8 exposures of each field, we derive
the shifts relative to the first I814 exposure by
comparing the positions of point and point-like sources in
individual exposures. It is assumed that there is no relative
shift/rotation between individual chips during each exposure. At least
3 reference sources (usually 7 , or more than 10 for some fields) in
three WF chips are used to give median shifts in X and Y axes.
Figure 2 illustrates the distribution of the relative
shifts of the remaining 7 exposures to the first one for the 27 GSS fields. It reveals the systematic offset between V606 and I814 images. The shifts for each field are tabulated in Table 2. Combining the 27 fields, we get a median shift
of = 0.20 pixel and
= 0.08 pixel.
Figure 3 shows the distribution of the position offsets of
178 point/point-like sources in two aligned images for 11 CFRS fields
and 28 GSS fields. The median of the X offset is 0.006 pixel with a
corresponding semi-inter-quartile range (SIQR) of 0.036 pixel and the
median of the Y offset is 0.004 pixel with an SIQR of 0.070 pixel. The figure
shows that the systematic offsets have been corrected. The center
offset,
,
can be used to measure the
uncertainty of an alignment. For the 178 point/point-like sources,
the mean value of the center offset is 0.117 pixel.
This denotes that the images are well aligned and can be used for generating color maps.
Many sources do show a center offset in
different band images. Figure 4 shows the center offset
against the distance from the chip center for all sources (left panel)
with I814 brighter than 22 mag, compared to the distribution for
point/point-like sources (right panel). It is clear that a) the offset
is free of the position-dependent effect (e.g. geometric distortion);
b) a substantial fraction of sources show a large center offset (up to a few pixels), as shown in the histogram of the center offset (middle panel). In addition, we also plot the center offset distribution for
our ISOCAM-detected sample (see Sect. 5.1).
On average, ISOCAM galaxies present larger offsets than other sources,
which can be related to their intrinsic morphological properties. We
use the software Sextractor (Bertin & Arnouts 1996) to
extract source catalogs, including central positions and integrated
fluxes. For galaxies with complex morphologies, the central positions
given in Sextractor are not always their brightness peaks.
An aperture of 3
is adopted in our photometry. The updated
Charge Transfer Efficiency (CTE) correction (Dolphin 2002)
and updated photometric zeropoints are adopted in the WFPC2 photometry
calibration in the Vega system (Dolphin 2000).
To obtain the color map image of an extended source, a key point is to
quantitatively select the pixels in the color map with reliable
color determination. Instead of adopting a semi-empirical noise
model (Williams et al. 1996), a method is developed to
obtain the signal-to-noise (S/N) ratio of the color map image pixel-by-pixel.
Using this S/N ratio image, the color map area can be
constrained for the extended source. Adopting an approximation that
the Poisson noise distribution function in an HST image is close to a log-normal law, we can obtain that for two images with signals
,
and
noises
,
,
the noise of
the their color image satisfies
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(1) |
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Figure 4:
Left panel: center offset of the bright sources
(
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The S/N ratio image can give a quantitatively measurement of the
reliability of each pixel in the color map image. The S/N ratio image does not introduce any bias against very red or very blue color regions because it accounts for both the S/N of the two WFPC2 images. The S/N ratio image is shown with V606 and I814 images in three typical cases, elliptical galaxy (Fig. 5), spiral galaxy (Fig. 6) and merging system (Fig. 7). The resulting color map image with the S/N ratio image is displayed. The color bar next to the color map shows the color scheme in the observed frame. The color range is adjusted for best visualization. Each image is labeled top-left corner. In Figs. 5 and 6, each image has a size of 40
40 kpc while in Fig. 7 the size is 60
60 kpc.
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Figure 5: V606, I814, the color map S/N ratio, as well as color map images of elliptical galaxy 03.0037 at redshift 0.1730. ( See the electronic edition for a color version of this figure.) |
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Figure 6: Same as in Fig. 5 for disk galaxy 03.0046 at redshift 0.5120. ( See the electronic edition for a color version of this figure.) |
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Figure 7: Same as in Fig. 5 for merging system 03.1309 at redshift 0.6170. ( See the electronic edition for a color version of this figure.) |
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The 17 HST imaging fields cover about 87 of the 200 square arcminute
area of the CFRS fields 0300+00 and 1415+52. We obtained either
B450-I814 or
V606-I814 color maps for 265 galaxies
(
). Among them, 169 have spectroscopic redshifts
given in the CFRS redshift catalog or in the literature. ISOCAM observations detected 60 objects and 77 objects brighter than 300
Jy at 15
m with S/N > 5 in the CFRS fields 0300+00 and 1415+52, respectively. Of the 137 objects, a color map is available for 33 objects in the 0300+00 field and 26 objects in the 1415+52 field. Of
the 137 ISOCAM-detected objects, 82.5% (113) have optical
counterparts brighter than 22.5 mag and 94.2% (129) brighter than 23.5 mag in the
band. For objects fainter than
= 22.5 mag, the imaging sample points are limited for deriving reliable morphological parameters and color
distribution. Therefore they are not included in this analysis. Since the vast
majority of ISOCAM-detected objects are indeed optically bright, this
would not cause a significant bias. Note that the fraction of
ISOCAM sources imaged by HST (59 among 113
galaxies, 52.2%) is larger than the area fraction of HST imaging of the two CFRS fields (87 vs. 200 square arcminute, 43.5%), because our survey includes complementary data aimed at investigating ISOCAM source
morphologies.
In the 59 ISOCAM-detected galaxies, 53 objects have known redshifts, including 2 objects at z > 1.2 which have been removed from our sample.
Table 3 lists the complete sample of ISOCAM galaxies
imaged by HST in two bands. The objects are organized into
three groups, 6 with unknown redshift, 15 in the nearby (
)
and 36 in the distant (0.4 < z < 1.2) universe. Nearby galaxies are not IR luminous (
)
and can not be taken as the local counterparts of the distant LIRGs. The 36 distant
objects are used to reveal the morphological properties of the distant LIRGs. In each group, the objects are tabulated in the order of their
CFRS identifications. In Table 3, the apparent magnitudes m450 (Col. 3), m606 (Col. 4) and m814 (Col. 5) are given in HST Vega system. An aperture of 3
is adopted for the HST image photometry. The absolute B band magnitude (Col. 6) and K band magnitude (Col. 7) are provided in the AB system, using isophotal
magnitudes from the ground-based imaging (Lilly et al. 1995). The K-correction is calculated based on
ground-based B, V, I and K band CFRS photometry (see Hammer et al. 2001 for details). The IR luminosity is given in Col. 8. Three models are used to calculate the IR luminosity and
derive proper uncertainties (see Flores et al. 2004 for
details). Figure 8 shows the IR luminosity
distribution for the 36 distant LIRGs.
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Figure 8: Histogram of the IR luminosity for 36 distant LIRGs. |
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Based on HST imaging, two-dimensional fitting has been carried out to derive the structural parameters used to quantify the morphological features. With the structural parameters and information derived from color maps, the morphological classification was performed by two researchers independently.
The two-dimensional fitting is performed using the software GIM2D (see
Simard et al. 2002 for more details). Two components, bulge
and disk, are used to fit the surface brightness distribution. The
fractions of bulge luminosity in total B/T and
are listed in
Table 3 (from Cols. 9 to 14) . The parameter B/T is
correlated with the Hubble type, increasing for early-type galaxies.
In addition to the "quality'' parameter
,
the residual image,
which is the difference between the observed image and the modeled image, is
also used to estimate the quality of the fit.
Good fitting is characterized by a
close to unity and a residual image with little random-distributed residual emission. However, in the case of a spiral galaxy with visible arms, the residual image exhibiting regular arms will refer to the fitting as good even though the
value is biased to be different from unity due to the presence of arms. The inclination angle of the disk is also derived from the two-dimensional fitting.
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Figure 9:
V606-I814 (top) and
B450-I814 (bottom)
observed colors for LIRGs as a function of redshift. Similar to Fig. 6
in Menanteau et al. (2001), four solar metallicity,
formed at z = 5 galaxy models, including an Elliptical galaxy (single
burst), S0 galaxy (![]() ![]() |
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With information provided by color distributions, morphological classification can be improved dramatically. The appearance in a color map is free from the arbitrary adjustment in visualization of an image. Physical properties can be derived from the color map including whether some regions are dusty or star-forming. Apart from the usual morphological classification based on the brightness distribution, the color information provides a new way to compare distant galaxies with local galaxies in the Hubble sequence.
We obtain color map for each LIRG in the observed frame. Instead of
applying K-correction and deriving the color in the rest frame, we compare
the observed color with the modeling color. Figure 9
illustrates the modeled color-redshift curves for
V606-I814and
B450-I814. Using GISSEL98 (Bruzual & Charlot
1993), the observed colors in the HST Vega system are given at
different redshifts for four galaxy models, corresponding to
elliptical (single burst), S0 ( Gyr), Sbc (
Gyr) and
irregular galaxies (constant star formation rate with a fixed age of
0.06 Gyr). We assume a formation epoch at a redshift of z=5.
Distant LIRGs are compared with the models using their integrated
colors. Almost all LIRGs are redder than an Sbc galaxy. Using the modeled
color-redshift relations, we investigate the colors of specified
regions. We refer to the region in the outskirts as red as or redder than
an elliptical galaxy as a dusty region and the region off the central
area bluer than an Sbc galaxy as a star-forming region.
By visually examining galaxy morphologies, we have tried to label each
target with its "Hubble type''. For the galaxies well fitted by
a bulge+disk two-dimensional structure, we divide them into five
types in terms of the fraction of bulge luminosity in total: E/S0 (0.8 < B/T
1), S0 (0.5 < B/T
0.8), Sab (0.15 < B/T
0.5), Sbc (0 < B/T
0.15) and Sd (B/T = 0). We also introduce three additional types to describe the
compact (C) galaxy which is too concentrated to be decomposed,
irregular (Irr) galaxy and "tadpole'' (T) galaxy. A quality factor is
provided to represent our confidence of our classification: 1 -
secure, 2 - possibly secure, 3 - insecure and 4 - undetermined.
We also provide a classification for galaxies that show signs of
interacting or merging: M 1 - obvious merging, M 2 - possible merging,
I1 - obvious interaction, I2 - possible interaction and R -
relics of merger/interaction.
The experienced eye is essential to classify the objects differing significantly from the adopted models. With information from the two-dimensional structure fitting and the color map, the visual examination is carried out by two of the authors F.H. and X.Z.Z. independently, to reduce arbitrariness. The classifications are consistent with each other for most objects. After fully discussing a few objects, a final morphological classification for the LIRG sample is made. In Table 3, galaxy type (Col. 15), quality factor (Col. 16) and Interaction/merging type (Col. 17) are listed.
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Figure 10:
I814 band and color map images of distant LIRGs.
For each target, the name and redshift are labeled top-left and top-right in the I814 image. The color bar ranges from -1 to 3 for the
V606-I814 color map and 0 to 4 for the
B450-I814 color map. The blank in I814 image is due to the target imaged close to the chip border. The size of each image is 40 ![]() ![]() |
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Figure 10: continued. |
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Figure 10: continued. |
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In Fig. 10, color maps (right) of the 36 LIRGs are shown, along with I814 negative greylevel images (left). The images are given in order of CFRS identification (from left to right, top to bottom). The color bar in each color map shows the color range. The same color ranges over -1 to 3 and 0 to 4 are applied to all objects in V606-I814 and B450-I814, respectively. For the 36 LIRGs, the morphologies as well as the color distributions are remarkably different. A description of each target is present in turn.
From Table 3, we can derive the global morphological
properties for distant LIRGs. Of the 36 galaxies, 13 (36%) are
classified as disk galaxies with "Hubble type'' from Sab to Sd. For
these disk galaxies, their morphological classifications are secure (Q < 2).
The object 03.1522 is an extremely red edge-on disk galaxy. This galaxy is
so red that almost no light is detected in the B450 band, which leads
to few pixels available in its color map. We label this galaxy simply as
"Spiral'' due to the large uncertainty in determining the bulge
fraction. Of the 13 disk galaxies, 4 of them are very edge-on.
Morphological classification shows that 25% (9 in 36) of the LIRGs
are compact galaxies as they show concentrated light distribution
(see the definition in Hammer et al. 2001). Such a high
fraction is strikingly similar to that derived from optical samples
(Guzm
n et al. 1997; Hammer et al. 2001). Their
two-dimensional structure fitting suffers from large
uncertainties.
Of the 36 galaxies, 8 (22%) are classified as irregular galaxies
which show complex morphology and clumpy light distribution.
In the 36 LIRGs, only 6 cases (17%) are major ongoing mergers
showing multiple components and apparent tidal tails. Five (either
compact or irregular) LIRGs are possibly linked to merging (labeled
by M 2) and one spiral LIRG is probably an interacting system
(labeled I2). Signs of merging or relics of interactions (possibly) occur in 9 LIRGs (labeled R and R?). Accounting for them and for major mergers, the total
fraction of merging/interacting systems is estimated to be 58% (21 of 36) of the LIRGs.
For galaxies at redshift 1, the cosmological dimming effect
becomes significant and the morphological classifications might have
some uncertainties since faint features are barely detected at the
detection limits of our WFPC2 imaging data. Some irregular galaxies
could be spiral galaxies. The objects classified as spiral
galaxies and major mergers are free from those uncertainties because
their main structural properties have already been determined. Hence,
in the present LIRG sample, the fraction of spiral galaxies is well
estimated or slightly underestimated if there are spirals
misclassified as irregular galaxies. HST I814 band imaging is
available for all LIRGs. Thus the compactness of each galaxy can be
well determined and the fraction of the compact galaxies is reliable
(see e.g. Hammer et al. 2001).
K band luminosity is widely used to estimate the stellar mass. For
our 36 sample LIRGs, K band luminosity is available for 24 of them.
Following Hammer et al. (2001), we assume a unity
mass-to-luminosity ratio in the K band and estimate the stellar masses for
the 24 LIRGs. The derived stellar masses range from
1.4
1010 to 2.9
10
,
compared to 1.8
10
for the Milk Way mass.
Note that extinction and age of stellar populations are not considered in
estimating the stellar massdue to the numerous related
uncertainties (e.g. those associated with the assumed stellar
population mixing and the dust extinction modeling). We notice that
the stellar masses we obtained are, systematically, from 50% to 100% of the values
derived by Franceschini et al. (2003), after a careful examination of galaxies with similar K band luminosities. Indeed,
Franceschini et al. (2003) have derived stellar
masses using spectral synthesis modeling of the overall optical-IR
continuum. Further discussion about uncertainties in the mass estimate can be found in Berta et al. (2004). Adopting
= -21.82 for
H0 = 70 (Glazebrook et al. 1995), LIRGs are systems
ranging from 0.3
to 6.7
with median value
2.2
.
These LIRGs were undergoing violent star formation at
rates of the order of
100
yr-1. They may add a
significant mass contribution over a short time. For a galaxy, the
ratio of stellar mass to SFR is the time scale for the formation of the
bulk of the stars. If star formation is sustained at the observed rate, LIRGs
could duplicate themselves within 108 to 109 yr
(Fig. 11).
![]() |
Figure 11: Stellar mass versus time scale to duplicate the bulk of stars at a rate of the observed value. The stellar mass is derived from K band luminosity. The size of the point is scaled by the SFR. Nearby galaxies listed in Table 3 with available K band luminosity are shown with open circles for comparison. |
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A large fraction of LIRGs are classified as disk galaxies. In
Table 3, the disk scale length
is also listed
for the LIRGs classified as spiral galaxies. We can see that all the
LIRG disks are large galaxies with
2.9 kpc
(corresponding to 4 kpc at H0 = 50) except for the object 14.1190,
which is a face-on Sd galaxy from the GSS fields. We suspect that its
disk scale length has been severely underestimated due to the shallow
detection (
24.5 mag arcsec-2 in the I814 band, compared
to
25.5 mag arcsec-2 in other fields).
We compare the LIRG disks with the size-selected disk sample using the
distribution of the disk central surface brightness. Lilly et al. (1998, L98 hereafter) presented a detailed study of a
large disk (
4
h50-1 kpc)
sample, which is homogeneous and essentially complete.
Similar to L98 (see their Fig. 12), we plot the LIRG disks in the diagram of
central surface brightness versus redshift. The observed
I814 band central surface brightness of the disk components
derived from the two-dimensional structure fit is used to obtain the
rest frame
band central surface brightness. Cosmological
dimming and K-correction are corrected following L98. We use the
central surface brightness color
V606-I814 (or
B450-I814) to calculate the K-correction except for LIRG disks 03.0085, 03.0445, 03.0932, and 14.0393, whose integrated HST colors
are adopted because the central surface brightness is either available
only in one band or not well estimated in the B450 band. The edge-on
LIRG disk 03.1522 is not included because of the heavy extinction.
A spectral energy distribution is chosen
to match the observed color from theoretical ones at different ages
for a solar abundance galaxy with e-folding time
= 1 Gyr (see
Hammer et al. 2001 for more details). As shown in
Fig. 12, the central surface brightness of 8 LIRG disks is
consistent with that of the large disk galaxies in L98 at similar
redshift (LIRG disks 03.0085, 03.0445, 03.0932, 03.1522 and 14.0393
are absent because of no secure structural parameters available in one
band). This confirms that LIRG disks belong to the large disk galaxy population.
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Figure 12:
Central surface brightness of the disk components as a
function of redshift for LIRG disks in our sample (solid square),
compared with the large disk galaxies in L98 (open circle). The
long-dashed line is the selection criterion (
![]() |
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We also investigated if the size-selected sample of distant disk
galaxies includes LIRGs. In L98, 5, 14 and 5 objects were selected
from the CFRS 0300+00 and 1415+52 fields in redshift bins 0.2 < z < 0.5,
0.5 < z < 0.75 and 0.75 < z < 1.0, respectively. After a
cross-identification with ISOCAM observations, 6 of 19 disk galaxies
in 0.5 < z < 1.0 are found luminous in the IR band with IR luminosities
ranging from 1.6
to 12
(the median
value is 5.3
). No galaxy in the redshift
bin 0.2 < z < 0.5 is identified as a nearby IR luminous or starburst
galaxy. Of the 6 IR luminous large disk galaxies, 5 are included in
our HST sample, including 3 morphologically classified as spirals, one
as a bar-dominated compact galaxy (03.1540), although it shows evidence
of an extended faint disk, and one as a merger (14.1139). This indicates
that in the large disk galaxy population at redshifts ranging from 0.5
to 1.0, about 30
12% are infrared luminous. This confirms that
LIRG disks are large (massive?) disks.
LIRGs are systems with an average stellar content of 1.4 ,
and they are intimately linked to large disks - size-selected disks include
a significant fraction of LIRGs.
We believe that the star formation in large disks (derived from
UV measurements) in L98 is severely underestimated.
Indeed from their IR luminosities, LIRG disks were forming stars at a
high rate, ranging from 18 to 210
yr-1 averaged
to 110
yr-1. This is contradictory to the modest star
formation rate of about 3-10
yr-1 reported by L98.
This indicates that UV and [OII] luminosities are poor tracers of the star formation rate.
The number density of LIRGs is much larger at
1 than at
the present day by a factor of more than 40 (Elbaz et al. 2002).
From the above discussions, we believe that they significantly
contribute to the large and massive disks population at z = 0.5-1.
Our result is contradictory to that of Brinchmann & Ellis
(2000), who claimed that dwarf galaxies, rather than
massive systems, were responsible for the star formation activity since
1. As an example, we assume that the L98 SFR of
7
yr-1 apply to the large disks which have not been
detected by ISOCAM (which is somewhat unrealistic since the ISOCAM detection
limit at z = 0.75 is
40
yr-1). Assuming the SFR
derived from the IR luminosity for the ISOCAM-detected disks in L98, the
average SFR at redshift
0.75 would be on average
40
yr-1, or 6 times
larger than the L98 average value. Further investigation will address
the contribution of the massive galaxies to the CSFD (Hammer et al. 2004).
![]() |
Figure 13: Central color versus concentration index. The distributions (large crosses) of normal elliptical, Sab and Sbc galaxies are derived from a nearby galaxy sample (Frei et al. 1996). The modeling colors for the galaxies E, S0, Sbc and Irr at this redshift are 2.05, 1.88, 1.10 and 0.38 respectively (see Sect. 5.2.2). We assume that Sbc galaxies are bluer by 0.1 mag than E/S0 galaxies and Scd galaxies are 0.1 mag bluer than Sbc galaxies in the bulge color (de Jong 1996). A central color of 2.05 is adopted for E/S0 galaxies. The CFRS ID is labeled for each data point. A Spearman rank-order (S-R) correlation analysis reveals a correlation coefficient of -0.42 with a probability of 0.026 that the null hypothesis of no correlation is true. |
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We have noticed that for most compact LIRGs (e.g. 03.0523, 03.0603, 03.0615 and 03.1540), the color maps have revealed a central region strikingly bluer than the outer regions. These blue central regions have a size similar to that of bulges and a color comparable to that of star-forming regions. Since the bulge/central region in local spirals is relatively red, such a blue-core structure could imply that the galaxy was (partially) forming the bulge. This is consistent with the scenario proposed by Hammer et al. (2001) that luminous compact galaxies are counterparts of the bulges in local spiral galaxies at intermediate redshifts. From color maps, the central regions (bulges?) in formation are revealed directly.
Let us assume that luminous compact galaxies are progenitors of the
spiral cores/bulges and that disk components are formed later. Then a
correlation is anticipated among star-forming systems (LIRGs: galaxies
forming disks and/or bulges) between the color of their central region
and the galaxy compactness. Star formation would first occur in the
center (bulge) and would gradually migrate to the outskirts
(disk), leading to redder colors of the central regions as the disk
stars were forming. We have investigated the relation between the
central color and the compactness for the LIRGs excluding the six major
ongoing mergers, for which no central color is available due to their
two separated components, and two objects whose structural
parameters are not available. To reduce the contamination from the
surrounding disk, a circle aperture with radius 1 kpc centered on the
I814 band brightness peak is adopted to
determine the central color. PHOT in IRAF is used to do aperture photometry. This aperture
includes 5 pixels for objects at redshift 1 and the random fluctuation
is marginal due to the high S/N in the central regions. We convert the
observed central color
V606-I814 or
B450-I814 to
the observed color
V606-I814 at a median redshift of 0.7434 (see
Hammer et al. 2001). At this reference redshift, The centroid
wavelengths of the HST filters I814 and V606 correspond roughly
to the B band (4596 Å) and U band (3440 Å). The same method
described in the previous subsection is used to estimate the K-correction.
We use the concentration index (
= 0.1) defined in Abraham et al. (1994) to measure the compactness. It is provided as
an output of GIM2D in the two-dimensional structure fitting.
We also compute the same parameters for a population of local
galaxies from Frei et al. (1996). The central color of the local
galaxies has been assumed to be that of local ellipticals (for E and S0) or local bulges (for Sab to Sbc), and has been transformed to the
B(4596 Å) - U(3440 Å) color system using the GISSEL98 model with
= 1 Gyr. Figure 13 shows the investigation of the
concentration index from the I814 band as a function of the central
color. In this
diagram, the LIRGs are distributed along a sequence from galaxies with
blue central color and compact morphology to galaxies with relatively
red color and extended light distribution. Almost all distant LIRGs
are discrepant from the sequence delineated by local spirals and
ellipticals. There are two extreme cases, 03.0603 and 03.1522. The
object 03.0603 could be contaminated by AGN because of its very blue
point-like core. As mentioned in Sect. 6.1, the object 03.1522 is a
very dusty edge-on spiral galaxy. Its very red central color is
related to a (dust-screen) extinction. The
sequence still exists when we replace the central color with the color
contrast, which is defined as the difference between the central color
and the integrated color (3
aperture).
The compact galaxies with a blue central color are suggested to be the galaxies forming their cores (bulges). Figure 13 clearly shows that these compact galaxies are located at one end of the sequence. The other end is occupied by extended galaxies with a concentration index and central color closer to those of local galaxies. It appears that such a sequence linking compact LIRGs to extended ones hints at a formation/assembly scenario for at least part of the present-day spiral galaxies: the bulge formed first and dominated a galaxy with a compact morphology and a relatively blue central color, and later the disk was assembled around the bulge, resulting in an extended light distribution which ultimately resembles those of local galaxies.
In trying to make sense of the sequence, we point out that intense star formation could happen in processes of both bulge and disk formation. In our color maps we see evidence that for LIRG spirals, star formation spreads over all their disks. A star formation history with several episodes of massive starbursts is suggested for LIRGs according to their composite stellar populations (Franceschini et al. 2003). A star formation history with multiple starburst episodes is also implied by the morphological classification of LIRGs such that massive starburst (luminous infrared phase) can happen in the spiral/irregular phase, major merger phase or compact phase. The major mergers in LIRGs would most likely result in spheroidal systems with violent starbursts, and become similar to the compact LIRGs with blue central colors such as those described in this paper. The spiral LIRGs could have undergone a compact stage before the observed epoch.
However, a simple evolutionary sequence does not account for all LIRGs. For example, among 12 LIRG disks with known bulge fraction, 3 are classified as Sd, i.e. with a very small bulge, and 8 LIRGs are classified as irregulars. Examination of these 11 Sd and Irr reveals the presence of a well-defined central component in most of them (03.0085, 03.0533, 03.9003, 14.0711, 14.0725, 14.0914, 14.0998 and 14.1190). On the other hand, such a component might not be a bulge (or a forming bulge), because it could be elongated or have peculiar shapes (14.0814 has an S-shape and it could be a giant bar). It is beyond the scope of this paper to evaluate the relative importance of each physical process (major and minor merging, bars, disk formation) that drive galaxy formation.
The scenario suggested by the sequence in Fig. 13 for
disk galaxy formation is also supported by metallicity investigation
of LIRGs by Liang et al. (2003). They found that, on average,
the metal abundance of LIRGs is less than half that of the local disks with
comparable brightness. They suggested that LIRGs form nearly half
of their metals and stars since ,
assuming that they eventually
evolve into the local massive disk galaxies.
It should be borne in mind that contribution from active galactic nuclei (AGNs) might strengthen this sequence since AGNs usually appear blue with respect to stellar populations (the situation is opposite if the AGNs are obscured severely by dust) and the existence of bright AGNs will bias the galaxies to the compact ones. However this concerns only a small fraction of our compact LIRGS (2 among 9). On the other hand, dust extinction will redden the color and smear the sequence.
Specific efforts were made to obtain the color maps for galaxies with
complex morphology, including the accurate alignment of the blue and
red band images and a new method to quantitatively determine the
reliability of each pixel in the color map. These efforts allow us to
access the spatially resolved color distribution of the distant LIRGs,
which often have complex morphologies, relating to
interactions/mergers. In two 10
10
CFRS fields 0300+00 and 1415+52, HST WFPC2 imaging in F606W (or F450W) and F814W filters is available for an 87 square arcminute area. From these fields, we select a representative sample of 36 distant (0.4 < z < 1.2) LIRGs detected in deep ISOCAM observations. A two-dimensional structure
analysis was carried out using GIM2D software. With structure
parameters and color distribution, a careful morphological
classification was performed for the distant LIRGs. We find that
about 36% LIRGs are spiral galaxies and about 25% LIRGs show compact
morphology. About 22% LIRGs are classified as irregular galaxies,
showing complex and clumpy structures. Among 36 LIRGs, only 6 (17%)
of them were undergoing a major merger episode, revealed by a
distinctive close galaxy pair with distorted morphology and apparent
tidal tails. The fraction of mergers could reach 58% if all of the
possible post-mergers/pre-mergers are included.
Inspection of their stellar masses derived from K band absolute magnitude shows that LIRGs are massive systems. The LIRGs classified as disk galaxies belong to the large disk galaxy population, and become a significant fraction of large distant disks selected by their sizes.
We find that LIRGs are distributed along a sequence in the central color versus compactness diagram. The sequence links the compact LIRGs with relatively blue central color to extended LIRGs with central color and compactness close to those of the local normal galaxies. The compact LIRGs showing blue central color are suggested to be systems forming their bulges, in agreement with the suggestion of Hammer et al. (2001). We argue that the sequence suggests that distant compact LIRGs would eventually evolve into spiral galaxies in the local universe.
Acknowledgements
We are grateful to Francoise Combes for helpful discussions and help in our morphological classification. We thank Jun Cui for his help in improving this manuscript. We thank the referee Dr. B. Mobasher for his helpful comments. X.Z.Z. gratefully acknowledges financial support from the Ministry of National Education of France. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.
Table 1: HST imaging with two band observations in CFRS fields 0300+00 and 1415+52.
Table 2: Offsets and rotation between blue and red images.
Table 3: Catalog of the deep ISOCAM-detected sources in CFRS fields 1300+00 and 1415+52.
A color image is a difference of two images in a logarithm function
(here we ignore the scaling constant which will not affect the final results).
Given one HST image in the blue band with flux FB and another one
in the red band with flux FR, the color image is defined as
![]() |
(2) |
In logarithms, the blue image B =
and the red image
R =
satisfy a Normal law.
A difference between two Normal law images is also a Normal law image.
This property is the main advantage of using the log-normal law to approximate
the Poisson noise.
The signal
and noise
of the color image are given by
![]() |
(3) |
![]() |
(4) |
![]() |
(5) |
![]() |
(6) |
![]() |
(7) |
![]() |
(8) |
With
=
,
we obtain the signal and noise for the color image as
![]() |
(10) |