Download Ultra-high-redshift galaxies from

Accepted to MNRAS (arXiv:1205.4270)
Rebecca Bowler, James Dunlop & Ross McLure
On this poster I describe how we found these extreme galaxies, the new data
that allowed us to do it and why this is interesting at all.
Redshift
(z)
0
1
2
3
4
5
6
7
8
9
10
Average Properties of Bright Galaxies at redshift 7
By stacking together the photometry of our four most robust candidates, we
gain in signal-to-noise and can therefore derive more accurate properties
from fitting a model. The stack images are shown above and the best fitting
SED is shown below, with the key galaxy properties highlighted:
Age of the
Universe
13.7 Billion years
5.9 Billion years
3.3 Billion years
2.2 Billion years
1.6 Billion years
1.2 Billion years
950 Million years
780 Million years
650 Million years
560 Million years
480 Million years
We found ten new galaxies in the
UltraVISTA data. Four of them shown here
in blue boxes are robust galaxies at z >
6.5. A further three galaxies shown here in
green boxes are highly likely to be at high
redshift, but could also be cool galactic
dwarf stars. The final three galaxies in our
sample are the least secure high-redshift
galaxies and are shown in red boxes, and
not only suffer from dwarf star
contamination but also that from lowerredshift galaxies.
What are we looking for exactly?
At the early epoch we observe at ultra-high-redshift, the galaxies are
young and highly star-forming. The spectral energy distribution
(the spectra or SED) can be modelled as a combination of many massive
young stars with a low dust and metal content. The young stars are hot
and blue, and so the spectrum peaks in the rest-frame hard UV.
However because of the neutral hydrogen throughout the universe at
this time, much of the high energy light is absorbed on the way to us.
The absorption gives us a strong spectral feature called the
Lyman-break that can be identified using photometric observations.
Photons here are
absorbed by
neutral hydrogen
in the IGM
Lyman-break (λrest = 1216Å)
Blue spectral slope is roughly flat
when plotted in magnitudes
CFHTLS
u*, g, r, i, (z)
~27
The Data = UltraVISTA Survey
HST/ACS
F814W (i)
Subaru
z’
~ 26.5
Galaxies at redshifts > 6.5 drop-out of the
optical imaging and so observation need to
be pushed into the near-IR data to detect
them. The UltraVISTA survey consists of
deep imaging over one square degree of the
sky, in four near-IR broad filters (Y, J, H and
Ks) using the wide-field near-IR imager on
the VISTA telescope in Paranal, Chile.
The diagram of the field to the right shows
the sources and characteristic depths (in AB
magnitudes) of the combination of
overlapping multi-wavelength data we used.
Selection Methods
What we observe
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the Moon
for scale
Area where multi-wavelength imaging overlaps
= one square degree on the sky
To form our sample of galaxies, we fit model SEDs to the measurements. A problem with selecting highredshift galaxies from coarse broad-band observations, especially with ground-based images blurred by the
atmosphere, is contamination by other astronomical objects. Cool galactic dwarf stars and dusty
galaxies at lower redshift can mimic a high-redshift galaxy when observed with broad filters.
Brown dwarf stars within our own galaxy can
contaminate our sample of high-redshift galaxies
An arrow
means the
galaxy is not
detected in
that filter
Magnitude here gives star-formation
rate estimate (SFR ~ 30 Msun/yr)
The luminosity function is a common way to characterise the galaxy population at different epochs. By
comparing the functions shape and overall normalisation at different redshifts, we can gain an insight into
the complex process of galaxy evolution. In particular, computer simulations can be tested by assessing
how well the predicted luminosity function matches observations.
Pre
di
Ob
ser
ved
cte
Bright galaxies are very rare compared to fainter galaxies, as
the luminosity function exponentially declines beyond a ‘knee’
or characteristic luminosity.
Luminosity
d
A prediction of the abundance of galaxies can be made by
considering the distribution of masses of dark matter
haloes in the very early universe, along with a prescription for
placing galaxies of a given luminosity within each halo (more
massive haloes contain more massive and hence luminous
galaxies).
Observations show a deficit of galaxies at the bright and faint
ends. The process of feedback is thought to be responsible,
where supernova in small galaxies and active nuclei in the larger
galaxies can quench star-formation and halt the growth of the
galaxy.
Our work, although based on a small number of galaxies, could
indicate that at less than one billion years after the big bang the
luminosity function shape looks more like that predicted from
the dark matter halo mass function.
Could we be seeing the galaxies before feedback from the central black hole
has halted their growth?
The Institute for Astronomy on the site of the Royal
The blue curve
The red curve show the best fit lower-redshift As the star is very cool the
shows the best fitting Observatory
Edinburgh
galaxy model , where the Balmer
or 4000Å-break spectrum of this T-dwarf
high-redshift model
Lyman-break (z = 6.98 ± 0.05)
Sets the redshift
The Luminosity Function
+ SCOSMOS
Spitzer IRAC
3.6μm/4.5μm
Model Galaxies and Contaminants
The points show the magnitude
of the galaxy in each filter
Stellar Mass (M★ ~ 6.98 ± 0.05 Msun)
old stars dominate the rest-frame
optical, observed here in the mid-IR
Also allows an
estimate of dust
reprocessing
(Av ~ 0.3)
UltraVISTA
Y, J, H, Ks
“deep”
Y+J ~ 25
Galaxies at high redshift are often detected using broad-band
photometry, which is in essence a low-resolution spectrum. A field of
the sky is observed in a combination of different colour filters and
the results from each filter are combined to obtain the maximum
information about the galaxies.
Given the relatively simple shape of the predicted galaxy SED at
redshift (z =) 7 shown above, when the galaxy is observed through
these filters no detection is seen in the optical wavelengths, but the
galaxies are bright into the near-IR. The image below shows the
transmission curves of the optical and near-IR broad-band filters used in
our search:
Spectral Slope (β = -2.0 ± 0.2)
probes rest-frame UV, hence the
young hot stars
Results
can be confused with the Lyman-break
peaks in the near-IR
Future Work
By incorporating galaxies found in a further one square degree of multi-wavelength data from the UKIDSS UDS
field, make the best determination to date of the bright-end of the z = 7 luminosity function.
© UK ATC, Royal Observatory, Edinburgh.
The squares placed onto this false colour image show the locations of some
of the most distant galaxies to be discovered from data taken with a
ground-based telescope. Although they might not look very impressive, these
galaxies are the most luminous and hence massive to be observed at such
an early time in the lifetime of the Universe. By studying the properties and
number densities of these early or ‘high-redshift’ galaxies, we can hope to
understand the subsequent evolution of galaxies into the majestic spirals
and giant ellipticals we see today.
As the light from these galaxies travels
towards us it is redshifted, moving the
high energy Ultra Violet (UV) photons
released from the hot young stars into the
near-Infrared (IR) by the time we measure it
here on Earth.
Number/Volume/Luminosity
Overview
Image Credits: UltraVISTA/Terapix/CNRS/CASU
Ultra-high-redshift galaxies from
UltraVISTA