Varia bles

Table 6.5: Multi-source stacking: targets with potential Hα lines which were identified sources from either the reference image or from a column flux profile peak. All are submillimetre-selected sources. See Section 6.4 for comments on each target.

Type / Redshift Observation Pointings

Object Field (z) ID to stack SNR

Sources with line detections

8C1909+722 HzRG HzRGs + 3.536 spec 1370153 1-10 3.1 8C1909+722 SMM1 HzRGs + 3.536 spec 1370154 1-10 10.0 8C1909+722 SMM2 HzRGs + 3.536 spec 1370155 1-10 2.4 4C60.07 HzRG HzRGs + 3.788 spec 1370162 1-10 2.5 SMMJ163627.94+405811.2 ELAIS N2 3.180 spec 1370027 1-6 ,8, 10 2.0

LAB10 SSA 22 3.090 spec 1370015 1-10 3.0

Subtotal 58 6.5

Sources without line detections

LESS-88 ECDFS 4.06 phot 1370019, 1371002 1-5, 7-9; 1-2 LOCK850.63 LH 4.73 phot 1370005 1-5, 7-10 SMMJ131201.17+424208.1 SSA 13 3.405 spec 1370026 1,3,4,6,7,9,10 SMMJ141813.54+522923.4 EGS 3.484 spec 1370025 1-10 SMMJ123712.05+621212.3 GOODS-N 2.914 spec 1370033, 1371008 1-5; 1-5 SMMJ14009+0252 Abell Cluster 1835 2.934 spec 1370012 1-3, 5-10 8C1909+722 SMM3 HzRGs + 3.536 assoc 1370156, 1371006 1-10;1-3 8C1435+635 HzRG HzRGs + 4.261 spec 1370157 1-10 4C41.17 HzRG HzRGs + 3.792 spec 1370163 1-7

Subtotal 85 2.9

Total pointings 143 4.0

pointings taken in each case have been co-added after noise-weighting, and the result is shown in Figure 6.6. This shows a clear Hα detection for the 6 sources with line detections, with an SNR of 6.5. There is also a peak for the stack of all potential Hα sources, for which the SNR is 4.0.

Stacking of Pa-α sources

For targets with potential Pa-α detections, we have identified the sources in 10 of the 13 sources targeted (including both submillimetre-selected and Spitzer-selected), with tentative lines detected in two of them (see Table 6.6 for details). The number of pointings involved was low, however, and no Pa-α line was seen in the stacked spectrum of all the sources.

6.7

Discussion and conclusions

This was an ambitious project to observe the spectra of well-known sources in a wave- length region which had not been observed spectroscopically since ISO-SWS, and which will not be accessible again until JWST spectroscopy in about five years’ time. These

6.7 Discussion and conclusions

Table 6.6: Multi-source stacking: targets with potential Pa-α lines which were iden- tified sources from either the reference image or a column flux profile peak. *The Spitzer source labelled Lockman 63 is not the same as the Lockman 63 targeted as a submillimetre source.

Field / Redshift Observation Pointings

Object Type (z) ID to stack SNR

Sources with line detections

SSG1 Bo¨otes 1.05 spec 1370021 1-10 2.0 70Bootes1 Bo¨otes 0.501 pah 1371702 1-6 2.5

Total pointings 16

Sources without line detections

SMMJ141742+523025 EGS 0.661 spec 1370091 1-8 EGS70-41 EGS 0.450 spec 1371713 1-2 EGS70-126 EGS 0.420 spec 1371740 1, 4, 5

EGS55 EGS 0.670 spec 1371767 1-2

EGS1-5 EGS 0.530 spec 1372277 1-2

LOCK850.05 Lockman Hole 1.2 phot 1370003 1-6, 9-10 Lockman 63* Lockman Hole 1.15 phot 1371955 1-2 SXDF850.36 XMM-LSS-SXDF 0.92 phot 1370010 1-10

Total pointings 27

observations were taken in the AKARI warm phase, when the quality of observations had been degraded by factor of two (and more in the later stages of the project), which meant that only the strongest emission lines were observable. The quality of the spec- tra obtained was in most cases extremely poor. Sub-frame multi-pointing drizzling was also tried as an alternative way to reduce the noise, and one further detection was achieved with this. The most significant exception to this was the detection of Hα emission in two high-redshift radio galaxies and associated submillimetre galaxies, which is described in more detail in the next chapter.

A significant number of targets did not show in the reference image, although continuum emission was present in most of the pointings, suggesting that real fluxes were being observed from the sources (as well as noise). Some sources could nevertheless be targeted for spectroscopy in the aperture if they showed a clear peak in their column flux profile. I could not obtain spectra for a significant number of sources: 7 of the 22 Hα sources and 5 of the 13 Pa-α sources could not be identified at all. Only Hα and Pa-α lines could realistically be identified, given the quality of the observations. (No targets with Hβ / [OIII] as potential strongest potential lines were targeted.) Lines from targets with relatively weak lines like Brackett lines in the observed wavelength region were not observed. With only a single convincing emission line detected for any source, this work did not yield any new redshifts (other than the two discussed in the next chapter).

6.7 Discussion and conclusions

For the submillimetre-selected sources, 6 (tentative) Hα line identifications were made. Although most were at modest SNRs, the stack of all 6 gave an SNR of 6.5, and the stack of all 15 potential Hα sources (i.e. those with clear targets in the reference image and with the appropriate redshift) showed a line with an SNR of 4.0. This confirms that submillimetre galaxies are active with starburst and/or AGN activity. This is discussed in more detail in the next chapter for the most interesting detections. Table 6.4 shows that three of the successful identifications were the three most luminous objects observed, and the average luminosity of successful identifications was over twice that of non-detections. For these distant targets, dust obscuration was clearly a factor in non-detection.

The results for the Spitzer-selected sources, taken in the later stages of the project when the instruments were even warmer, and with fewer pointings, were almost non- existent.

For the 10 sources whose Pa-α line would fall in the observed wavelength range, only 2 (tentative) line identifications were made, and the stack including non-detected sources did not show a result for Pa-α.

A more detailed discussion of the physical interpretation of these results will be given in the next chapter, including their relation to high-redshift [CII] lines.

Chapter 7

Detection of Hα emission from

z > 3.5 submillimetre luminous

galaxies with AKARI-FUHYU

spectroscopy49

This chapter presents tentative Hα emission line detections of four submillimetre- detected galaxies at z > 3.5: the radio galaxies 8C1909+722 and 4C60.07 at signal- to-noise ratios (SNRs) of 3.1 and 2.5, and two submillimetre-selected galaxies (SMGs) near the first of these at SNRs of 10.0 and 2.4, made with the AKARI Space Telescope as part of the FUHYU mission program. These are the highest-redshift Hα detections in such galaxies, made possible by AKARI’s unique near-infrared spectroscopic capa- bility. The two radio galaxies had known redshifts and surrounding structure, and I have detected broad Hα components indicating the presence of dust-shrouded quasars. I conclude that powerful AGNs at z > 3.5 occur in peaks of the star-formation density fields, supporting a close connection between stellar mass build-up and black hole mass assembly at this redshift. I also show that 4C60.07 is a binary AGN. The Hα detections of the two SMGs are the first redshift determinations for these sources, confirming their physical association around their companion radio galaxy. The Hα-derived star forma- tion rates (SFRs) for the SMGs are lower than their far-infrared derived SFRs by a factor of ∼10, suggesting a level of dust obscuration similar to that found in studies at redshifts ∼ 1 < z < 2.7.

7.1

Introduction

As discussed in Section 2.5, finding the explanation for the close connection between active galactic nuclei (AGNs) and star formation in galaxies is a key question in cos-

49_{This chapter is based on research published in Sedgwick et al. [2013]. A full list of the}

7.2 HzRGs and associated submillimetre sources: targets observed

mology. The discovery of high-redshift submillimetre galaxies (see Section 1.6) and their possible association with high-redshift radio galaxies (HzRGs) (e.g. Stevens et al. [2003]; Chapman et al. [2005]) implies that there are regions of intense star-formation which also show strong radio emission at high redshift. Detection of emission lines from these galaxies in the infrared can be used to confirm this connection, and to provide an independent measure of star formation and AGN activity.

Near-infrared spectroscopy of high-redshift radio and submillimetre galaxies has previously been made in the K-band (Swinbank et al. [2004]). However, spectroscopy at longer near-infrared wavelengths between 2.5 µm and 5.0 µm is a region which has been relatively unexplored: it has poor sensitivity from the ground and was previously covered in space only with ISO-SWS (see Table 1.2), and was not, in particular, covered by Spitzer. This gave AKARI, which had a spectroscopic facility in this region, the unique possibility of detecting Hα emission from active galaxies at 3.0 < z < 6.5.

The AKARI FUHYU program was described in Chapter 6. The near-infrared spec- troscopic campaign which was part of this campaign was carried out during AKARI’s warm phase and details of the observations, data reduction and general results were presented in that chapter. The most interesting result, the detection of powerful Hα emission in four high redshift radio and submillimetre galaxies, is described in this chapter. The targets observed are described in Section 7.2. Results are presented in Section 7.3 and discussed in Section 7.4.