DOI: 10.65398/VHPX3538
Abstract. A brief overview of early results of JWST-MIRI deep imaging and spectroscopy observations of very high redshift galaxies taken as part of the Guaranteed Time program is presented.
1. Introduction
Understanding how galaxies form and evolve is one of the key, open questions in the field of modern-day astrophysics and has been a constant theme in NASA decadal surveys (e.g. National Academies of Sciences and Medicine 2021). In particular, identifying the epoch in which the first galaxies formed, deciphering how they formed and what their fundamental properties are is a major goal of today’s observational surveys and tomorrow’s state-of-the-art instrumentation projects (e.g. Block et al. 2003; Gilmozzi and Spyromilio 2007; Laureijs et al. 2011).
With the recent developments in instrumentation, in the form of the James Webb Space Telescope (JWST; Gardner et al. 2023) it is now possible to observe the rest-frame infrared Universe beyond our local neighbourhood at unprecedented resolution. In particular, the Near Infrared Camera (NIRCam; Beichman et al. 2012) and the Mid-Infrared Instrument (MIRI; Rieke et al. 2015) instruments on JWST now enable observations from 0.9 to 25μm. Consequently in its first few years of operation, JWST has started to shed light on the rest-frame near-infrared Universe beyond cosmic noon (z > 2, ≥10 Gyr ago), probing into the earliest origins of our Universe, as depicted in Figure 1.
These technological advances have been exploited in the first few years of operation of JWST by the astronomy community, with deep multi-band, multi-instrument and wide-area surveys of the cosmos (e.g. Bagley, Finkelstein, et al. 2023; Bagley, Pirzkal, et al. 2023; Casey et al. 2023; Eisenstein et al. 2023). Combining these surveys with targeted near-infrared spectroscopic observations has been an effective tool in unlocking the secrets of the early Universe (e.g. Naidu et al. 2022). This is the approach adopted by the the European MIRI Consortium’s (GTO) High-Redshift Working Group, with the goal of studying the distant Universe with state-of-the-art MIRI observations. In particular, we utilise both the imaging component of MIRI (MIRIM; Bouchet et al. 2015) to produce one of the deepest images of the Universe in the near-infrared at 5.6μm as part of the MIRI Deep Imaging Survey (MIDIS; Östlin et al. in prep.) as well as spectroscopy. In addition to targeted observations with the medium resolution spectrograph (MRS; Wells et al. 2015) which constrains the spectral properties of a multitude of galaxies across cosmic time from high-redshift line emitters, quasars and sub-millimetre galaxies (SMGs) are observed. In total the MIRI GTO High-Redshift observing time equates to 125 hours, with 60 hours on the MIRI Deep Imaging Survey and 65 hours of MRS spectroscopy, in addition to 125 hours of coordinated parallels and simultaneous observations with NIRCam and MIRI.
2. MIRI Deep Imaging Survey (MIDIS)
The MIRI Deep Imaging survey consists of 60 hours of MIRI imaging centred on the HUDF, with 50 hours dedicated to the 5.6μm filter and the remaining 10 hours in the 10μm filter. Alongside these primary observations, deep (5σ point-source depth ∼29mag) multi-band parallel observations in the HUDF-P2 region with NIRCam (e.g. Pérez-González et al. 2023; Caputi et al. 2023) and HUDF-P3 with NIRISS (Melinder in prep.) were obtained.
Amongst the ongoing projects being undertaken with one of the deepest images ever taken in the near-infrared (see Figure 2), including the analysis of the cosmic evolution of stellar morphology (Gillman et al. in prep.), 5.6μm-only (NIRCam-dark) sources (Jermann et al. in prep.), ALMA sources in the HUDF (e.g. Boogaard et al. 2023), and the stellar populations of Lyman-Break galaxies (e.g. Iani et al. 2023) (see Karina Caputi’s talk), we are also analysing the F560W emission of one of the highest redshift, spectroscopically confirmed, galaxies JADES-GS-z11-0, at a redshift of z ∼ 11.58 as first identified by the JADES team (e.g. Robertson et al. 2023).
In Figure 3, we show the multi-wavelength observations of JADES-GS-z11-0, including the MIRI/F560W emission, with indications of an extended morphology at rest-frame UV-optical (0.43μm) wavelengths (Östlin et al. in prep.). As identified by the JADES team (e.g. Robertson et al. 2023), through spectral energy distribution fitting, this galaxy has a stellar mass of log10(M∗[M⊙]) = 8.9 with a star-formation rate of log10(SFR[M⊙ yr-1]) = 0.3. The moderate star-formation rates and compact sizes indicate elevated star-formation efficiencies, providing insights into the formation of one the earliest known galaxies.
3. MACS1149-JD1
One of the targets of the MRS observing program was MACS1149-JD1, a lensed galaxy at redshift z = 9.1 when the Universe was 530 Myr years old. In these observations we spatially resolve, for the first time the Hα emission, identifying two clumps, one in the north (N) and one in the south (S) of the galaxy, as shown in Figure 4 from Álvarez-Márquez et al. (2023). In total the galaxy has a star-formation rate of ∼5 M⊙ yr-1 with a velocity map that reflects a rotating disk.
The extended overall structure of Hα follows the [OIII] 88μm emission of the galaxy imaged with ALMA, whilst the clumps show very different kinematics, with a factor ∼2 difference between the velocity dispersions, potentially associated with the UV clump identified in the north and a potential outflowing component.
4. J1120+0641
In addition to lensed galaxies, our GTO MRS observations also comprise of the first infrared spectrum of a z > 7 quasar, J1120+0641 at z=7.0848, during the epoch of re-ionisation. For the first time, we detect a hot dust torus at this epoch, defined by a upturn in continuum emission at λrest ≃ 1.3μm, as shown in Figure 5 from Bosman et al. (2023), leading to a black-body temperature of Tdust = 1413.5−7.4+5.7 K. Compared to similarly-luminous quasars at 0 < z < 6, the hot dust in J1120+0641 is somewhat elevated in temperature (top 1%). The temperature is more typical among 6 < z < 6.5 quasars (top 25%), leading us to postulate a weak evolution in the hot dust temperature at z > 6 (2σ significance).
We measure the black hole mass of J1120+0641 based on the Hα Balmer line, MBH = 1.52 ± 0.17 · 109M⊙, which is consistent with previous measurements using the rest-UV Mg II line and measurements utilising the Paschen-series lines indicating that no significant extinction is biasing the MBH measurement obtained from the rest-frame UV. By comparing the ratios of the Hα, Pa-α and Pa-b emission lines to predictions from models, we find that the hydrogen broad lines are consistent with originating in a common broad-line region. Overall, we find that both J1120+0641’s hot dust torus and hydrogen broad-line region properties show no significant peculiarity when compared to luminous quasars down to z = 0. The quasar accretion structures must have therefore assembled very quickly, as they appear fully ‘mature’ less than 760 million years after the Big Bang.
5. GNz11
Finally, we obtained targeted MIRI Imaging observations of the highest redshift galaxy pre-JWST at z ∼ 11 (Oesch et al. 2016). The galaxy has clear detections in the NIRCam observations from the JADES survey with a spectroscopic redshift of z = 10.60, stellar mass of log10(M∗[M⊙]) = 9 and star-formation rate of log10(SFR[M⊙ yr-1]) = 21, with a physical extent of 64 pc (Tacchella et al. 2023; Bunker et al. 2023). We have two high signal to noise detections in the MIRI filters F770W and F560W, as shown in Figure 6, with very red colours, with a flux ratio of F560W/F444W = 2.7 and F770W/F444W = 1.6, indicating either a extreme line emitter ([OIII], Hα) or active galactic nuclei (AGN) origin of this intriguing source, however the MRS spectroscopy upcoming in March will reveal the true nature of GNz11.
6. Summary
MIRI is the only instrument capable of exploring the (rest-frame) near-infrared spectral range in high redshift galaxies with both deep spectroscopy and imaging. The unique instrument allows the first characterization of the old stellar populations, in addition to mapping the obscured star-formation in galaxies and its connection with molecular interstellar medium. Furthermore, the state-of-the-art spectral capabilities of the MIRI enable the derivation of the physical properties of the black hole, torus and broad line regions in high-redshift quasars, in addition to quantifying the ionized interstellar medium properties and stellar populations in epoch of re-ionisation sources at z > 9. No other instrument or observatory will be active in this spectral range with similar capabilities as planned for the foreseeable future (the next 20 years). We have a unique opportunity to exploit the legacy value of JWST/MIRI now.
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