Our understanding of galaxy formation was fostered by highly multiplexed spectroscopic surveys like SDSS at low redshift and with a variety of facilities at higher redshift. The statistical power of the SDSS galaxy sample has revealed how the SFRs, chemical en-
Figure 1.16: Comparison between different observed CO SLEDs, as indicated in the legend. All SLEDs are normalized to the CO(1–0) of the average BzK (star forming galaxies selected with B, z and K filters) SLED, except the Milky Way, which is nor- malized using CO(2–1). The dotted blue, dashed-dotted green and dashed cyan curves are models taken from Papadopoulos et al. (2012),Narayanan & Krumholz (2014) and
Bournaud et al.(2015). This figure is taken from Daddi et al. (2015).
richment, stellar populations, morphologies, and BH growth rates of galaxies vary with stellar mass, environment, and cosmic time (e.g. Kauffmann et al. 2003a,b; Tremonti et al. 2004;Brinchmann et al. 2004;Kewley et al. 2006;Mannucci et al. 2010;Johansson et al. 2012). However, SDSS like all the spectroscopic surveys larger than a few hundred galaxies suffers from a serious drawback. Indeed, these surveys sample just the circum- nuclear regions of each targeted galaxy (with a 3”-diameter fibre in the case of SDSS), with the aim of deriving integrated spectroscopic properties. Consequently, the com- plexity of galaxy internal structure (e.g. spiral arms, bulge, H ii regions) is completely lost. Furthermore, the inferred measurements may not be representative of the entire galaxy, but biased by the fact that they target only its center along a particular axis. Therefore, large aperture corrections (e.g. Brinchmann et al. 2004; Salim et al. 2007) must be taken into account to derive galaxy properties, even though it is not trivial to calibrate them accurately.
internal structures and kinematics. Specifically, an integral field spectrograph collects spectra over a two-dimensional FOV. Basically, it allows to observe an astronomical object in three dimensions in one go: each pixel of the image is associated with a full spectrum (i.e. spectral pixel, or “spaxel”), measuring the intensity of the light at each wavelength (that can be converted into a velocity towards or away from Earth). These instruments consist of both a spectrograph and an IFU, that divides the FOV into a continuous array. The main techniques to do this step are through a lenslet array (e.g. SAURON@CFHT, OSIRIS@Keck), fibres (e.g. PMAS@CAHA, MaNGA@APO), or an image-slicer (e.g. SINFONI and MUSE@VLT), illustrated in Fig. 1.17. In a lenslet array, first the source image is split up by a microlens array, concentrated in small dots, and then dispersed by the spectrograph. The microlens is tilted in order to prevent the spectra from overlapping, but this limits the length of the spectrum and the packing efficiency of the CCD. The technique that uses a 2D bundle of optical fibres that transfer the input light to the spectrograph (with or without lenslets) is the most common. Finally, the image-slicer technique uses mirrors to segment the source image in thin horizontal sections and to order the slices end to end to form the slit of the spectrograph.
Figure 1.17: Main techniques for achieving integral field spectroscopy. Credit: M. Westmoquette, taken from INTEGRAL FIELD SPECTROSCOPY WIKI.
In the last twenty years, local galaxies have been explored through IFU spectro- graphs in a variety of surveys, pioneered by SAURON (72 E/S0/Sa galaxies,de Zeeuw et al. 2002) and ATLAS (260 E/S0 galaxies,Cappellari et al. 2011), that took a step for- ward in the classification of early-type galaxies, exploiting their IFU kinematics instead of their morphology. Subsequent surveys such as VENGA (30 spiral galaxies, Blanc et al. 2013) prioritised high spatial resolution to investigate SF and the ISM, while oth- ers such as discMass (146 face-on disc galaxies Bershady et al. 2010) optimised their sample selection and instrument setup, providing insight on the internal mass profiles and mass-to-light ratios of disc populations. Then, the development of IFS instruments in the NIR such as SINFONI (Eisenhauer et al.,2003) at the VLT has allowed to inves- tigate spatially resolved properties of galaxies at z ∼ 2. For instance, these observations revealed that clumpy and highly star forming high-z galaxies generally display a regular rotating discs (F¨orster Schreiber et al., 2009; Law et al., 2009) and gave new insights on large-scale winds and outflows (Cresci et al., 2015a; Carniani et al., 2016).
State-of-the-art instruments, such as the ALMA telescope (Wootten & Thompson,
2009) in the submillimeter wavelength range and the MUSE spectrograph (Bacon et al.,
2010) in the optical at the VLT, are allowing astronomers to obtain spatially resolved observations with an unprecedented spatial sampling (e.g. of 0.2” for MUSE) on entire galaxies both in the local and high-z Universe. ALMA is an interferometer, and thus exploits a completely different technology, but still delivers data cubes, in which the third axis is frequency, and so the final data products are comparable to an IFU with up to a million spaxels. For instance, the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) Collaboration is aiming at inspecting GMC scales (∼ 50 pc) with ALMA and MUSE large programs, analysing CO emission at cloud scales across the discs of 74 nearby galaxies and MUSE data across a subsample of 19 galaxies (at a matched resolution) to map the ionised gas and stellar populations (e.g. Kreckel et al. 2018). This is giving new insights on the time evolution of star forming regions, thanks to the discovery of a systematic spatial offset between molecular clouds and H ii regions, explained in terms of rapid evolutionary cycling between GMCs, SF, and feedback effects (Kreckel et al., 2018; Kruijssen et al., 2019). Another novel MUSE large program is GAs Stripping Phenomena in galaxies (GASP,Poggianti et al. 2017), that is aimed at studying gas removal processes in galaxies, investigating the ionised gas phase and the stellar component both in the discs and in the extraplanar tails of local jellyfish galaxies5 in different environments (galaxy clusters and groups), as well as a control sample of disc galaxies with no morphological anomalies. Concerning high redshift galaxies instead, an 5Objects with optical signatures of unilateral debris or tails reminiscent of gas-stripping processes
ESO Large Program among others aimed at spatially investigating their dynamics, gas excitation properties and chemical abundances is given by the KLEVER survey (Curti et al., 2019a), that exploits KMOS, first multi-IFUs spectrograph in the NIR at VLT (Sharples et al., 2006), to investigate a sample of 120 galaxies (1.2 < z < 2.5).
All these surveys are focused on small samples, and thus they cannot provide an unbiased view of the galaxy population in the nearby and high-z Universe. In the local Universe, this challenge has been recently addressed by the CALIFA (S´anchez et al.,
2012), Sydney-AAO Multi-object Integral field (SAMI, Croom et al. 2012) and MaNGA (Bundy et al., 2015) surveys on 600, 3400 and ∼ 10000 galaxies, respectively. MaNGA represents by far the largest IFS survey and is one of the three major programs of the fourth generation SDSS (SDSS-IV), with the aim of observing 10000 local (z ∼ 0.03) galaxies within 2020. These data have been using to carry out quantitatively studies to understand the life history of present-day galaxies, from their initial birth and assembly, through their ongoing growth via SF and mergers, to their death from quenching of SF at late times (e.g. Belfiore et al. 2016, 2018).