Download Selection Criteria for LAEs at z=4.8

Preliminary Results on Stellar Populations of LAEs
at z=4.8
S. Yuma, K. Ohta, K. Yabe (Kyoto U.), K. Shimasaku, M. Yoshida (U. Tokyo), I. Iwata (OAO),
M. Ouchi (Carnegie), M. Sawicki (St. Mary’s U.)
ABSTRACT
We present the preliminary Spectral Energy Distribution (SED) fitting results of Lyman Alpha Emitters (LAEs) at z=4.8 in GOODS-N and its flanking fields. Observing
through the NB711 narrow-band filter [ eff  7126Å, FWHM = 73 Å] attached on Suprime-Cam, Subaru telescope, we obtained 33 Lyman Alpha Emitters (LAEs). 8 of them
are detected in mid-IR data obtained by IRAC camera on Spitzer Space Telescope (SST) toward GOODS-N and flanking field. The rest-frame UV to optical SEDs of 8
LAEs at z=4.8 are constructed. After fitting the observed SEDs with stellar population synthesis models by Bruzual & Charlot(2003), various physical properties of LAEs
are derived. In this work, we adopt the constant star formation history with fixed 0.2Zʘ metallicity. By taking into account the H emission line, the best fitted stellar
8
10
mass and age are in the ranges
10of  10 Mʘ and 1-550 Myr, respectively. These values are broadly consistent with other studies on LAEs at other redshifts (e.g. Gawiser
et al.2006&2007, Lai et al.2007, Nilsson et al.2007, Finkelstein et al.2007, and Pirzkal et al.2007). However, our derived star formation rates (SFRs) and dust extinctions
are significantly larger than those of LAEs by other studies. Comparing the derived mass and age with those of LBGs at z~5, it is found that our LAE candidates are
younger and less massive than LBGs.
1.Data and LAE Selection
3.SED Fitting Method
Broadband photometry of 8 objects are used to construct Spectral Energy Distributions
(SEDs). The SEDs consist of photometry from 4 bandpasses which are Ic and z’ bands
from Subaru data and IRAC channels 1 and 2. The observed SEDs are then fitted with
population synthesis models by Bruzual&Charlot (2003) as shown in the diagram
below:
Parameters used to build models:
Salpeter IMF (1955) + Star Formation History + Metallicity
 Data: From Ground-base to space telescope
The optical data are obtained by Suprime-Cam on Subaru Telescope using NB711 narrow
h
m
s
12
36
49
.4,
band and BVRIcz’ broadbands centering at Hubble Deep Field-North [RA(2000)=
Dec(2000)=  6212'58"]. The observations are operated in 2005 April.
Mid-IR data
GOODS-N field : There are publicly available data taken under SST Legacy Science
Program by IRAC camera on Spitzer Space Telescope (SST) in 4 bandpasses.
[http://ssc.spitzer.caltech.edu/legacy/]
 BC03 Padova evolutionary track(1994)
Stellar evolutionary track
(Padova 1994)
 Salpeter IMF 0.1-100 Mʘ
Model Spectrum
GOODS-FF: The flanking regions are also observed by using the same instrument (i.e.
IRAC on SST) in 2005 December and 2006 June. Note that the data observed by us are ~1
mag shallower than those in GOODS-N field.
Shift redward to z=4.8,
Calzetti extinction law(2000)
 Selection Criteria for LAEs at z=4.8
 0.2Zʘ metallicity
Convolve with filter
response function
Observed SEDs
Model SED
 2Minimization method:
2
 f i obs  f i mo 
2

   
obs

i 
i

1) Strong detection in narrow band: NB711 < 26.1 (3 at 2”.5 )
2) Large Ly equivalent width: RI-NB711 > 0.9 mag
RI : continuum brightness at Ly wavelength, (R+I)/2

This criterion corresponds to the observed EW more than 109 Å
Best fitted model with known population
(i.e. stellar mass, age, E(B-V), SFR)
3) Non-detection in B and V bands:

Figure 1. Plots of the observed
SEDs of LAEs at z=4.8 with
the best fitted model spectra.
The vertical error bars show
errors in photometry, while
bandwidths of each filter are
illustrated by horizontal ones.
33 objects are picked up as
LAE candidates at z=4.8.
V-Ic > 1.55, and V-Ic > 7.0(Ic-z’)+0.15
 Time runs from 0 to 1.2 Gyr (Age of the
universe at z=4.8) with equal logscale time
step of 0.1
 Constant Star Formation History (CSF)
 Adding H line to 3.6m bandpass
4.Results
B > 28.81 and V > 28.15, 2 limiting magnitude at 2”.5 diameter
aperture)
4) The continuum-break criteria as same as for LBGs at z~5 by Iwata
et al.(2007) to reject any possible low-redshift interlopers: Note
that these criteria of continuum break are applied to objects, Ic
and z’ photometry of which are brighter than 3 limiting
magnitude at 2”.5, 26.58 and 25.77 respectively.

 Calzetti dust extinction law (2000) with
E(B-V) = 0-1.0 (0.02 step)
GOODS-N
GOODS-FF
Because the observed fields of GOODS-N and its flanking fields are smaller than those observed by
Subaru Telescope, among 33 LAE candidates, there are 23 objects in the GOODS-N and its flanking
fields, 10 in GOODS-N and 13 objects in GOODS-FF. Requiring 2 detection in IRAC photometry
reduces the number of LAEs, which can be used to fit with models, to 8 objects: 5 in GOODS-N
Stellar masses: 10  10 Mʘ
8
10
AB Magnitude
Age: 1-550 Myr
and 3 objects in GOODS-FF.
2.Photometry
E(B-V): 0.00-0.58 mag
SFR: 20-3400 Mʘ/yr
Subaru Data: Use mag_auto output from SExtractor
IRAC data: Use aperture photometry with a 2”.4 diameter aperture and correct to
total magnitudes
B
V
R
NB711
Ic
z’
Observed Wavelength (Å)
An example of
Stamp pictures of LAEs
In all available bands
Figure2. A plot of IRAC ch1 magnitude
(3.6m) vs the star formation rates (SFR)
5.Comparison to LAEs at other z
Figure3. A plot between IRAC ch2
magnitudes (4.5m) against
derived stellar masses indicating
the relation of the stellar mass
and rest-frame optical magnitudes.
6.Comparison to LBGs at z~5
(a)
(a)
(b)
Histograms illustrate the distribution
of derived masses, ages, dust
extinction, and SFR of our LAE
candidates and those of LBGs at z~5.
(b)
Figure (a) indicates that ranges of the stellar masses of our sample are
comparable to those of LAEs at other redshifts except ones at z~5 by Pirzkal et
al.(2007). The difference in masses between our LAEs and those by Pirzkal et al.
may be explicable by the difference in rest-frame optical brightness between
these two samples. Pirzkal et al. used the upper limits for IRAC photometry, thus
they selected objects from the fainter population of LAEs at the redshift. The
fainter the rest-frame optical photometry, the less massive the stellar mass, as
seen in figure 3.
It is seen in figure (b) that our derived ages are broadly consistent with those of
LAEs at other redshifts. Most of our LAEs are young with ages in the order of
few Myr which agree well with those by Pirzkal et al. On the other hand, they
are higher than the ages of LAEs at z=3.1. It is however difficult to make a
strong constraint on age comparison due to large uncertainties in derived ages.
According to table 2, dust extinctions of LAEs at z=4.8 agree well with those of
LAEs at z=4.4 and 5.7, whereas the discrepancy can be seen if comparing them
with those at z~3 and z~5. SFRs derived in our work are hence different from
those at the redshifts.
(c)
(d)
The good comparison to our work
seems to be the stellar populations
derived by Yabe et al., using exactly the
same models as ours. Figure (a) shows
that the stellar masses of LAEs at z=4.8
are mostly distribute in the low mass
ranges compared to the distribution of
LBGs masses. In figure (b), Ages of
LAEs seem to be younger than those of
LBGs; however, they are comparable to
those by Verma et al..
Figure (c) indicates that the amount of
E(B-V) of LAEs is less than those of
LBGs if compared to those by Yabe et al.
Our derived SFRs are smaller than those
of LBGs from both studies as seen in
figure (d).