A&A 449, 67-78 (2006)
DOI: 10.1051/0004-6361:20054065
A. Tamm1,2 - P. Tenjes1,2
1 - Institute of Theoretical Physics, Tartu University,
Tähe 4, Tartu, 51010, Estonia
2 -
Tartu Observatory, Tõravere, Tartumaa,
61602 Estonia
Received 18 August 2005 / Accepted 22 November 2005
Abstract
Aims. We report on a photometric study of a sample of 22 disk galaxies in the Hubble Deep Field South NICMOS parallel field. The redshift range of the galaxies is
z=0.5-2.6.
Methods. We use deep NICMOS J and H band and STIS open mode images, taken as part of the HDF-S project, to construct rest-frame B-profiles and (U-V) color profiles of the galaxies. Before fitting isophotes, images are deconvolved with PSF. Derived surface brightness profiles are approximated by Sérsic luminosity distribution.
Results. Significantly large population of disks cannot be represented by an exponential disk, but this can be well done by Sérsic law, if n<1. This might be the same phenomenon which has earlier been referred to as truncation of disks. Parameter n does not vary significantly with redshift. Galactic sizes decrease with redshift as
.
The rest frame (U-V) color shows a clear decrease at
,
concordantly with the understanding of more intense star formation at earlier epochs. Color gradients
are small and roughly constant at z<2. At z>2, dominantly positive gradients appear, possibly indicating centrally concentrated star-formation. On the basis of (U-V) color and chemical evolution models, the disks observed at
have formed between z=3.5-7. Scale radii
of the galaxies correlate with the scale surface brightnesses
for the sample. None of the studied parameters shows clear dependence on absolute B luminosity for the galaxies.
Key words: galaxies: photometry - galaxies: fundamental parameters - galaxies: high-redshift - galaxies: spiral - galaxies: structure
Comparison of the structure of galactic disks at intermediate and high redshifts with the disks resulting from N-body simulations allows us to test the corresponding algorithms and to specify physical processes involved in disk formation. General properties of both elliptical and disk galaxies are rather well reproduced by modeling (e.g. White & Rees 1978; Navarro & White 1994; Abadi et al. 2003; Bell et al. 2003; Governato et al. 2004; Robertson et al. 2004; Nagamine et al. 2005). The initial shortcomings have been corrected by now and it has become possible to model the formation of extended, rotationally supported disks. On the other hand, good models for calculating the formation and evolution of galaxies involve a great number of free parameters (see e.g. Cole et al. 2000).
To test and constrain the simulations carried out at increasing
resolutions, detailed observational studies of individual high
redshift galaxies are indispensable. In a number of papers,
structural parameters of galaxies have been statistically studied
at a wide range of redshifts. Luminosity function has been derived
out to
by Chen et al. (2003) and Ilbert et al. (2005); to
by Poli et al. (2003)
and Giallongo et al. (2005); to
by Gabasch et al.
(2004). General morphologic studies and statistics out to
redshift
and even further have been conducted by
Bunker et al. (1999), van den Bergh et al. (2000),
Corbin et al. (2000), Bouwens et al. (2004), Cassata
et al. (2005) and many others.
Due to hierarchical clustering, galactic sizes should grow with time (e.g. Fall & Efstathiou 1980). An analysis of galactic size distribution allows us to constrain certain parameters in galaxy evolution simulations (feedback etc., see Cole et al. 2000). The observations do not clearly confirm this prediction as yet. For example, galaxy size evolution with redshift has been studied by Ferguson et al. (2004) and Trujillo et al. (2004, 2005). They show that, at a given luminosity, decrease of the sizes of galaxies with redshift is clearly present. On the other hand, Ravindranath et al. (2004) and Cassata et al. (2005) do not detect significant evolution. Serious sources of controversies are selection effects and difficulties in conducting photometric analysis of galaxies at high redshifts, both caused mainly by cosmologic dimming of surface brightness by a factor of (1+z)4.
Table 1: General information about the HDF-South NICMOS field observations.
While there is a growing inflow of data concerning general
statistical properties of high redshift galaxies, just a few
papers have concentrated on the internal properties of distant
disk galaxies, e.g. kinematics, detailed surface brightness and
color distribution. For example, rotation curves for distant disk
galaxies ()
have been measured by Vogt et al.
(1996, 1997), Erb et al. (2003) and Böhm
et al. (2004). Moth & Elston (2002) have studied
HDF-N field and constructed rest-frame
(UV218-U300) color
profiles and rest-frame B surface brightness profiles for 83
galaxies at
z=0.5-3.5. They report color gradient
rising with redshift, which
suggests that star formation has shifted inwards during the
evolution of galaxies.
In Tamm & Tenjes (2003, 2005), we combined luminosity
distribution with rotation curves to construct self-consistent
mass distribution models for disk galaxies out to .
We
noted that for all the 7 galaxies studied, the usual exponential
disk assumption gave poor fit to the luminosity profiles; a more
steep cut-off at outer radii, provided by the Sérsic
(1968) index n<1, was needed. Was this because of
selection effects, evolution of disk parameters with time or
anything else? With this question in mind, in the present paper we
concentrate on the luminosity and color profiles of disk galaxies
at a redshift range as wide as possible, in an attempt to detect
evolutionary effects of several disk parameters with a particular
emphasis on the shape of the profiles. To achieve this, our main
task is to acquire surface brightness profiles as well as color
profiles for a sample of disk galaxies, introducing as few
selection effects as possible.
We have chosen a small field in the southern sky, the Hubble Deep Field (HDF) South NICMOS parallel field for the study. In addition to the information offered by the multi-color observations of this field, the choice of the southern sky provides comparison with the well-studied HDF-N region.
In the present work we take H0 =
and
To avoid generation of artificial evolutionary trends, galaxies at different redshifts should be observed in the same rest-frame waveband. This precaution eliminates possible effects of "morphological k-correction'', i.e. the dependence of the photometric properties of a galaxy on the rest-frame waveband. This phenomenon is especially frequent in the case of mid-type galaxies (Papovich et al. 2003). For comparison with near-by galaxies, rest frame optical passband would be the best choice. To study rest-frame optical properties of galaxies at high redshifts, observations made in near infrared are required. Presently, the best available resolution in near infrared is offered by the NICMOS camera aboard the Hubble Space Telescope (HST). However, the imaging properties of the NICMOS camera are still far behind those of the HST optical cameras.
During HDF South observations (Williams et al. 2000), the NICMOS camera was pointing in a slightly different direction, and in this way, a parallel field was created with NIC-3 camera through the J, H and K broad-band infrared filters. This field is also covered by WFPC2 camera I filter observations of Flanking Field 9 (Lucas et al. 2003). Additionally, the NICMOS parallel field has been observed with the HST STIS camera using the open mode, i.e. without any filter, which gives a very broad bandwidth with the central wavelength matching that of the standard V-band. Regrettably, the I and K band observations could not be used here - the former suffers from insufficient depth and the latter has too high noise and background level (the K images were taken during "bright'' time, i.e. when the telescope was pointed near the bright limb of the earth).
Observations conducted in the three remaining passbands - V, Jand H - have been used in the present study. The basic properties of these observations and the final images are given in Table 1.
We have used the fully calibrated, combined and dithered images of the HDF-S NICMOS parallel field, available via the homepage of the Space Telescope Science Institute. The lower limit of detectable surface brightness on these exposures is approximately 27.5 mag/arcsec2.
Table 1 shows that on NICMOS exposures the full-weight half-maximum (FWHM) of the point-spread function (PSF) is 0.23'', causing serious distortion of the galactic images. The width of the PSF as well as its slightly non-circular shape cause uncertainties in deconvolution of the images.
Unfortunately, spectroscopic redshifts of the galaxies in the HDF-S NICMOS parallel field have not been measured. Photometric redshifts, based on 9-band measurements, have been calculated by Yahata et al. (2000).
Galaxies with redshift z> 0.5 were selected for the further study. Discrimination between Hubble types is not always a simple task at high redshifts. Many objects may be in transition stages (protodisks or protospheroids) even at 1 < z < 2 (Conselice et al. 2004). Morphological structure of these objects was inspected visually on the basis of H-band exposures, all galaxies suspected for being disks were included in the sample. In some cases, also the high-resolution STIS V-band images were checked (see Table 1 for the comparison of PSFs and projected pixel sizes). Final confirmation for the sample to consist of disk galaxies comes from the luminosity profiles, which all exhibit Sérsic index n<2 and should thus have a late-type morphology (Andredakis et al. 1995; Ravindranath et al. 2004).
Galaxies showing significant asymmetry or irregular shape were excluded, because fitting ellipses to their isophotes would give rather scattered light profiles (light distribution is very sensitive to the galactic center position) and their interpretation would not be straightforward in the context of the present models and subsequent analysis. Thus possible starburst and interacting galaxies were rejected as photometrically incomparable to regular disks.
These selection criteria finally set a redshift limit at z=2.6, beyond which no disk galaxies could be distinguished with acceptable confidence. A final sample of 22 galaxies qualified for further analysis.
The choice of passbands described above allows us to determine rest-frame optical luminosity profiles, using STIS observations for redshifts z<1.0 and NICMOS J and H observations for 1.1<z<2.0 and z>2.1, respectively; the mean central rest-frame wavelength thereby becomes 420 nm, corresponding to Johnson B filter. Color information can be obtained, using STIS and NICMOS J observations at z<1.1, and NICMOS J and H bands at z>1.4 (no galaxies were found at 1.1<z<1.4). The mean central rest-frame wavelengths thus become 350 nm and 580 nm, allowing the derivation of (U-V) color distribution. Note that here STIS observations are used for U and B rest-waveband photometry for the same galaxies. This can be justified by the very wide "passband'' of STIS open mode, depending only on the detector sensitivity. A drawback of using such a wide wavelength range lies in the danger of suppressing possible color-features and trends, but it does not introduce or artificially amplify the evolutionary effects.
The STIS and NICMOS J-band images of the galaxies are presented in Fig. 1. STIS images are designated as V; x and y pixel coordinates of the galaxies on NICMOS images are used as galactic names; z is photometric redshift. Where necessary, the galaxies of the present sample have been encircled to avoid confusion. Some of the sample galaxies lie at the edge of the field of view of the STIS camera and the galaxy 380-1027 falls just outside it, thus no color-information could be acquired in the latter case.
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Figure 1:
Galaxies studied in the present paper as they appear on
STIS open mode (![]() ![]() |
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Figure 2: Rest-frame B-band luminosity profiles with Sérsic fits, and rest-frame (U-V) color profiles with linear fits. |
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Figure 2: continued. |
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To determine absolute magnitudes and especially to generate color profiles, it is vital to precisely estimate the background level for each galaxy. We measured background level at 2-4 empty-looking fields around each galaxy and found the uniformly subtracted background levels of both STIS and NICMOS final images to be slightly over-estimated. Individual background estimates for each galaxy were necessary due to considerable variations within each passband image, ranging from -0.00015 to 0 counts per second in the case of STIS and from -0.00005 to 0.00002 counts per second in the case of NICMOS images.
Before fitting ellipses to the galactic images with the STSDAS task ELLIPSE, we deconvolved images with PSF. Correct deconvolution is very important, if one wishes to conduct analysis of the luminosity and color profiles of faint objects. The bulge component of a disk galaxy often acts as a point source at high redshifts; wrong estimating of the spreading of its flux may cause significant drift of the parameters of the disk component.
In the case of the NICMOS images, the model PSFs were created using stellar images found in the same field. In the case of STIS, no unsaturated stellar images were found and the model PSF was created using the TinyTim package.
Ellipses were interactively fitted to the isophotes. Any object, which did not seem to be part of a given galaxy, was masked. Both fixed and free wandering values were tested for ellipse centers; the resulting differences in surface brightness profiles are included in the uncertainties of the profiles in Fig. 2. Ellipticity and position angle were kept fixed according to the outer isophotes of the galaxies.
For rest-frame photometry we transformed the observed passbands into rest-frame standard passbands, which needed minimal k-correction after redshifting. B-profiles and (U-V) color profiles were found to be most suitable (see Sect. 2).
No reliable k-corrections existed for the transformations from neither the STIS open mode nor the NICMOS passbands and we had to calculate them. In our calculations we relied on the synthetic spectra of redshifted Sb galaxies (as a mid-way between S0 and Sc galaxies) constructed by Bicker et al. (2004) according to their chemical evolution models. In these spectra, effects of evolution and redshifting had already been taken into account (therefore the correction actually includes also evolution correction). Our share was to calculate the relation between the observed flux and the redshifted standard U, V or B filter flux for each set of cameras, filters and redshifts. The throughput curves of the NICMOS filters and STIS clear imaging were taken from NICMOS and STIS Instrument Handbooks, respectively. Due to the available choice of filters, the final k-corrections for both rest-frame B-band and (U-V) color remained rather modest, typically around 0.4-0.7 mag.
The final, k-corrected surface brightness profiles in rest-frame B-color with estimated error bars are presented in the upper panels of Fig. 2. In the lower panels, k-corrected rest-frame (U-V) profiles and corresponding error bars are given. To calculate the extent of the error bars, inaccuracy of our Hubble type classification and the population synthesis models were taken into account. The latter was difficult to estimate precisely because no actual uncertainties of chemical evolution models are available. Thus we made a rough estimate on the basis of deviations between synthetic spectra calculated by different authors. These uncertainties are amplified by different widths and shapes of the filter passbands. The uncertainties of the synthetic spectra at higher redshifts are larger than at lower redshifts, but the estimated errors remain comparable ( 0.25-0.45 mag). This is because of the spectra being stretched as (1+z) with respect to the filter passband on one hand, thus enabling more exact determination of the flux, and the usage of the wide STIS imaging throughput at the tricky UV-region for lower redshift galaxies on the other hand, which can not be very accurate. The step of determining actual flux according to the synthetic spectra dominates in the estimates of the uncertainties of color measurements in Fig. 7.
The usage of different filters and detectors for imaging at different redshifts would cause errors in absolute photometry and introduce artificial trends with respect to redshift, if calibrations or k-corrections were calculated improperly. Considerable errors of this kind would be seen as a jump in magnitude-redshift plots at the redshift of the filter change, e.g. between z=1.1 and z=1.4 in our case of (U-V) vs. z graph in Fig. 5. The graph of absolute B magnitudes MB vs. z was also checked for this effect; no jumps were found.
Although in some cases (usually at smaller redshifts), the bulge component was distinguishable from the disk, no attempt was made to split the luminosity profile into two components, because this would have given systematically different disk parameters at different redshifts.
Luminosity profiles were fitted by a Sérsic surface density
distribution (Sérsic 1968):
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Figure 3: Normalizing parameter bn in Sérsic formula (1) at low n values. Comparison of our approximation (2) with numerically calculated actual values, together with approximations by Capaccioli (1989) and Ciotti & Bertin (1999). |
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As a tool for fitting surface brightness distribution, the
Sérsic law has a purely empiric background. Starting from a
spatial density distribution law and projecting it along the line of
sight would be a more physical method, also allowing a
straightforward comparison with kinematic data and thereby enabling
the construction of self-consistent models for mass distribution.
Thus, fitting of a surface density distribution, deduced from the
space density distribution law of the very general form
Comparison of these two distributions at different n values is
given in Fig. 4. Surface brightness value
and radius r are given in the same relative units for both distributions.
We stress that no rescaling has been conducted for matching the
distributions; the fit has been achieved by keeping the total
luminosities equal. Note that a perfectly exponential
distribution, corresponding to the Sérsic law with n=1 cannot be attained with distribution (3).
For galaxies in the present sample, no additional kinematic data are available at present. For this reason, we only present here the parameters resulting from fitting of the Sérsic law (1) to the brightness profiles. This also allows more direct comparison with other studies.
Table 2: General galactic parameters.
The rest-frame B luminosity profiles, fitted to the Sérsic
law, and the (U-V) color profiles with linear fits are shown in
Fig. 2. The main properties of the galaxies and the results
of the photometric analysis are given in Table 2.
Distribution of the parameter n, the effective radius ,
the
(U-V) color distribution, and color gradient
are shown in Figs. 5-7 as a function of
redshift. The three different brightness levels of the data points
indicate three absolute rest-frame B magnitude-bins of the
galaxies: white points stand for galaxies with MB < -20 mag, grey
points for
MB=-19 ...-20 mag and black points for MB > -19 mag.
For equal treatment of the galaxies at different redshifts, absolute
brightness has been calculated within rest-frame surface brightness
mag/arcsec2.
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Figure 4:
Comparison of the Sérsic surface brightness
distribution (solid lines) and a distribution, derived from the
space density distribution of Eq. (3) (dashed lines) at
different n values. Surface brightness value ![]() |
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Figure 5: The Sérsic luminosity profile curvature n as a function of redshift. Open circles stand for galaxies with absolute luminosity MB < -20 mag, gray circles for -19 mag < MB < -20 mag and black circles for MB < -19 mag. |
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As is evident from the present sample, nearly half of galaxies
cannot be fit to an exponential disk model, while for the rest,
works well out to the highest redshifts
(Fig. 5). A similar result was obtained by Moth & Elston
(2002) for the HDF North field. As suggested in Tamm &
Tenjes (2005), the more curved profiles might indicate the
effect known as disk truncation, discovered by studies of local
edge-on disk galaxies (de Grijs et al. 2001; Pohlen et al.
2002) and more recently detected also at higher redshifts
(Pérez 2004; Trujillo & Pohlen 2005). Such a
luminosity distribution is sometimes fitted by a double-exponential
profile (Pohlen et al. 2002; Pérez 2004). The
physical background of disk truncation has not become clear as yet
(see e.g. Sasaki 1987; and de Grijs et al. 2001, for
more discussion about the origin of this phenomenon).
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Figure 6:
The
Sérsic luminosity profile effective radius ![]() ![]() ![]() ![]() ![]() |
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Figure 7:
Galactic
parameters - rest frame (U-V) color a) and
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In contrast to these truncated disks, some galaxies in the present sample have flattening profiles (e.g. 607-767, 708-695, 380-1027, 353-444). Similar profiles have been measured near z=1 for gamma-ray burst-selected disks (Conselice et al. 2005b) and out to z=3 for large disks in HDF-South primary field (Labbé et al. 2003). Erwin et al. (2005) have found a significant proportion of local barred S0-Sb galaxies with flattening profiles, which they call "anti-truncated'' disks. They claim that, at least in several cases, this effect might be caused by interactions. Observations of interacting galaxies (Chitre & Jog 2002) and numeric simulations (Bournaud et al. 2005) have demonstrated the possibility of mergers to smear out the outer parts of luminosity profiles. In the case of our sample, visual companions are present for all four galaxies with flattened profiles, thus interactions may have a role in the development of this flattening. The companions have been masked prior to ellipse fitting, therefore the luminosity of the companions cannot be responsible for the effect.
The 7 galaxies studied in Tamm & Tenjes (2003, 2005) have significantly low n values compared to the present sample. This is most likely a result of the small size of the samples or selection effects, favoring luminous galaxies with regular kinematics.
In Fig. 6, evolution of the size of the present sample is
shown. Error bars of sizes were derived by varying
while
fitting the Sérsic law within the surface brightness profile error
bars. The slight degeneracy between parameters n and
has
also been taken into account.
Starting from the locally observed Universe and modeling galactic
evolution backwards, Bouwens & Silk (2002) derived a
scaling relation
r(z)/r(0) = 1-0.27z for the B-band radius. The
long-dashed line in Fig. 6 shows this relation. The
least-squares linear fit to our sample gives
(solid line). Here, the median value of Sérsic half-light radii
kpc for late-type galaxies from the
SDSS survey (Shen et al. 2003) has been used as the value of
(considering the median absolute luminosity of the present
sample). It is seen that the model developed by Bouwens & Silk
(2002) fits our data rather closely. Our result is
consistent with the small
values found for UDF galaxies by
Elmegreen et al. (2005).
Galactic size evolution has also been predicted from simple models
of the hierarchical structure formation scenario and an approximate
scaling has been derived with disk formation time:
,
where in the case of
,
(Mo et al. 1998). In a
study of galaxies at a redshift range
z = 1.5-5, Ferguson et al.
(2004) found the scaling of
to give a
good fit to their sample, while Trujillo et al. (2005)
achieved a better fit with
for galaxies at
z
= 0.3-2.5. The relations
and
,
scaled with
kpc, are also shown in
Fig. 6, as dotted and short-dashed lines, respectively. The
former shows a decrease significantly faster than that of the
present sample. However, it must be kept in mind that the
theoretical relation uses the disk formation redshift instead of the
observed redshift for scaling. Further more, in contrast to the
present sample and the literature referred to above, Ravindranath et al. (2004) and Cassata et al. (2005) did not detect any
significant evolution of galaxy sizes with redshift in a thorough
study of galaxies at
.
Figure 6 shows that the trendline fitted to the present
sample is quite sensitive to the data point corresponding to the
largest galaxy with
kpc at z=0.6. Its extensive disk
is obvious also on Fig. 1 and its light profile is a very
regular one, thus there is no doubt that this galaxy is clearly a
large disk galaxy. The only reason for this data point to be
significantly offset could be a wrong photometric redshift estimate.
In Fig. 7a, rest frame (U-V) color as a function of
redshift is presented. The (U-V) color shows a mild decrease until
,
followed by a notable drop of roughly 0.5 mag by
.
At lower redshifts, the dependence can be compared to
the Deep Groth sample studied by Weiner et al. (2005), for
which a very slight, if any, decrease of (U-B) was detected for
late-type galaxies in the redshift range z=0-1.5. The notable drop
of the (U-V) values at
cannot be due to miscalibration
- a uniform set of filters and calibrations has been used for the
redshift range
z=1.4-2.6 (see Sects. 2 and 5.5). The drop can
rather be related to a major star formation peak at z>2 (see
below). A decrease of the rest-frame (U-V) color with increasing
redshift up to
was derived also by Kajisawa & Yamada
(2005). More detailed comparison with our results is not
possible at present because in their study, different morphological
types are presented together. Our result is quantitatively close
also to the result obtained from the analysis of the synthetic
spectra of Bicker et al. (2004). Absolute (U-V) values are
rather sensitive to k-correction, which, in turn, is uncertain. This
may cause a constant vertical shift of (U-V) values. However,
trends in Fig. 7 should be independent of these
uncertainties.
Figure 7b shows that at redshifts ,
there are small
or no color gradients. Gradients begin at z>2 and are dominantly
positive. A similar rise with redshift of the rest-frame color
(U218-U300) gradients was discovered by Moth & Elston
(2002), suggesting that star-formation was more centrally
concentrated at higher redshifts.
Recent hydrodynamical simulations of galaxies in CDM cosmology by Robertson et al. (2004) indicate that star
formation in disks peaks between redshifts z = 2-4. Studies of the
Fundamental Plane of early-type galaxies at redshifts
have also shown that the last epoch of major star formation peaks at
(see Holden et al. 2005, and references
therein). A jump in star formation beyond redshift z = 2 matches
well with the jump in (U-V) color in Fig. 7a (see above).
Smaller color gradients at lower redshifts z = 0.5-2 are due to smoothing by interactions in disk evolution (Conselice et al. 2005a) and large scale gas motions (Samland & Gerhard 2003).
Positive color gradients also exist in local disk galaxies
(MacArthur et al. 2004). Positive color gradients may
indicate disk formation from inside out. For the Milky Way
protogalaxy such formation scenario was suggested by van den Bergh
(1993). In the case of high-redshift galaxies, direct
interpretation of color gradients may be rather complicated. Color
gradients are determined by radial distribution of initial
metallicities, stellar ages, star-formation rate, gas accretion
details etc. Dependently on model details, both outside-in and
inside-out formation of disks can be simulated (Sommer-Larsen et al. 2003; Robertson et al. 2004).
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Figure 8:
Effective surface brightness
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According to the analysis of their disk galaxy formation model, Westera et al. (2002) have found that luminosity from Uto V mainly indicates star formation, and the effects of age and metallicity are negligible. Thus the positive color gradient most possibly refers to intensive star formation in central parts of galaxies at z>2.
Relying on (U-V), we may roughly estimate the disk formation
time. According to our photometry, the disks at
have
On the basis of simple stellar population
chemical evolution models (Worthey 1994; Bressan et al.
1994), these disks have ages 1-2 Gyr and have thus formed
at z=3.5-7.
Coenda et al. (2005) have shown that disks obey similar
photometric scaling relations as spheroids and elliptical galaxies.
Degeneracy exists between the effective radius, effective surface
brightness, the parameter n and absolute magnitude. For the
current sample, a trend can be shown for the effective surface
brightness
to diminish with larger effective radius
,
presented in Fig. 8. Least squares fit gives
mag, which, regarding the small size of the
sample, is remarkably close to the dependence found by Coenda et al.
(2005)
mag.
Small sample sizes and selectional biases are critical topics for most of the studies of high-redshift galaxies. The present sample of 22 galaxies can not offer reliable statistics and cautions against jumping into severe conclusions.
Morphological identification of these disk galaxies was obtained by visual inspection of H-band exposures, all galaxies suspected for being disks were included in the sample. In some cases, also the high-resolution STIS V-band images were checked. Some of the galaxies of the present sample have been studied on the basis of NICMOS images by Rodighiero et al. (2001), and classified as E-S0. We would like to point out that for classification and accurate photometry it is important to carefully deconvolve the PSF of NICMOS images. Even in the case of good restoration of NICMOS images, additional observations with other detectors, preferably with higher resolution (e.g. STIS in the present case) still prove beneficial. For example, visual inspection of the STIS image of the galaxy 419-572 reveals its nature of a late-type (at least Sb) galaxy.
Usually, the cosmological dimming as (1+z)4 favors the selection of more luminous galaxies at higher redshifts. For our sample, Figs. 5-8 reveal no clear dependence of any of the parameters on the absolute luminosity of the galaxies; this fact allows us to consider the sample to be sufficiently complete for the conducted analysis.
The selection of the sample on the basis of H-band imaging
(corresponding to B-band at z>2) might cause a slight bias
towards bluer galaxies at higher redshifts. On the other hand, the
change of the spectral energy distribution is slow between B and R passbands and we consider this possible selection effect not to
be a reason of a color jump at
in Fig. 7.
For calculating k-corrections, all galaxies were assumed to be Sb morphological types. This approximation causes additional scatter of absolute magnitudes and colors. However, according to our estimates, the resulting uncertainties remain within 0.2 mag even in the worst cases.
Among possible sources of errors and uncertainties, the ever
insufficient imaging depth and spatial resolution should be
considered. With the currently available NIR imaging a dilemma
remains, whether to consider faint patches around galaxies to belong
to the galaxy and include them in photometry or to mask them as
extraneous. This may slightly affect the luminosity distribution at
the outer regions of a given galaxy and the values of the
photometric parameters, especially the shape parameter n. In the
present case, we have followed a rather conservative masking
strategy, excluding most of the "suspicious'' patches. A more
liberal treatment would give slightly higher n and
values
and lower absolute magnitudes. The shift of these values would be
rather systematic.
Insufficient spatial resolution influences the interpretation of surface brightness profiles in the central parts of galaxies. In the present study we decided not to decompose galaxies into the bulge and disk components. Otherwise, rather serious systematic differences would appear in handling of low-redshift and high-redshift galaxies. In general, decomposing light profiles into a share of a bulge and a disk would decrease the disk radius.
Acknowledgements
We would like to thank the anonymous referee for useful comments and suggestions helping to improve the paper. We acknowledge the financial support from the Estonian Science Foundation (research grant 6106). This paper is based on NASA/ESA Hubble Space Telescope NICMOS and STIS observations of the Hubble Deep Field South obtained from the data archive at the Space Telescope Science Institute. This research had made use of the NASA/IPAC extragalactic database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the NASA.