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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 s t age Im s age no n Im y is o lter l i t a tic alax ere tec J fi Op he g ed h De and d) - t tect (Y cke de sta s age Im is IR y ar- alax ere Ne he g est h - t ight br s s age xie Im gala here R d-I ive ed Mi ass tect m - e de ar 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