he SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatiallyresolved local star-forming galaxies 

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The GOODS-ALMA survey

The SMGs studied by ALMA have been the follow-ups of previously detected sources by single dish submillimeter telescopes and color- and mass-selected galaxies from deep surveys at optical/near-infrared wavelength. This will lead to a bias towards some specific population. In addition, the enormous dust content indicated by the bright submillimeter emission hinders some SMGs, perhaps the most massive ones, from emitting in the optical/NIR, hence missed from pre-selections. A homogeneous blind survey is needed to obtain a more general population.
The GOODS-ALMA survey (PI: D.Elbaz Franco et al., 2018; Franco et al., 2020b), covers the largest contiguous area of 69 arcmin2 in the GOODS-South field by ALMA at 1.1 mm. The original image has a 0.24ÕÕ beam and mean depth of 110 µJy beam≠1. It was then tapered to 0.60ÕÕ to reduce the number of independent beams, hence the number of statistical spurious detections. The rms sensitivity is then 182 µJy beam≠1. A total of 35 galaxies have been detected above 3.5‡ in GOODS-ALMA (Franco et al., 2020b). These include 19 galaxies detected above the 4.8‡ limit from a blind detection approach (Franco et al., 2018) and 16 galaxies within 3.5 and 4.8‡ detected using ancillary information, mainly Spitzer -IRAC prior positions (Franco et al., 2020b). The median redshift and stellar mass of the S/N Ø 4.8 sources are z = 2.73 and Mı = 1.0 ◊ 1011 M§, whereas the sources with a 4.8 > S/N Ø 3.5, are both slightly closer (z = 2.40) and less massive (Mı = 7.2 ◊ 1010 M§).
Compared with other blind surveys of dust continuum (Walter et al., 2016; Dunlop et al., 2017; Hatsukade et al., 2018; Decarli et al., 2019), GOODS-ALMA is the most efficient to probe distant galaxies, not only because it covers the largest area, but also because of the strategy to observe a
shallower but wider field within a limited amount of time. This is also confirmed by the flattening at ≥ 0.1 mJy in the observed 1.1 mm number counts (Muñoz Arancibia et al., 2018; González-López et al., 2020; Popping et al., 2020). The significant decrease of galaxies fainter than 0.1 mJy suggests that deep pencil-beam surveys would be less efficient in searching for the sources contributing to the cosmic infrared background. Despite being counter-intuitive, this can be understood by the fact that submillimeter detected galaxies tend to be the most massive ones, which are rare and bright.

Star formation fueled by gas

As the key ingredient of star formation, the gas component is vital to our understanding of galaxy evolution. The physical processes involving gas span a vast range of physical scales: from the accretion of intergalactic gas (≥ Mpc), to the settling down and the cooling in galaxies (≥ kpc), to the collapse into molecular clouds (≥ 10-100 pc), then to the fragmentation into denser cores (≥ 0.1-1 pc), finally to the formation of stars (see review by Kennicutt and Evans, 2012).
In extragalactic astrophysics, we are limited by observations to the processes down to sub-kpc scale, which leads to mostly statistical studies on the gas fraction, star formation efficiency/gas depletion time, etc.

Gas velocity dispersion and the energy sources

Galaxies at different cosmic epochs show quite distinct properties. Compared to their high-redshift counterparts at similar stellar masses, local star-forming galaxies are larger, and have relatively lower gas fractions and lower SFRs (Leroy et al., 2005; Daddi et al., 2010a; Tacconi et al., 2010; Madau and Dickinson, 2014). They are also less likely to experience violent events such as major mergers and gas accretion (Baugh et al., 1996; Genzel et al., 2008; Robotham et al., 2014). Despite of all the various properties, galaxy discs at all epochs tend to be in a state of marginal gravitational stability, which can be characterized by the close to unity Toomre (1964) Q parameter, as Q = Ÿ‡/fiGΣ, where Ÿ is the epicyclic frequency of the galaxy’s rotation, ‡ the velocity dispersion, represents the effect of pressure, and Σ the mass surface density, represents the effect of gravity. However, many theoretical and observational studies suggest that gas in higher-z galaxies has larger random motions compared to gas in nearby galaxies. Galaxies at z > 1 have velocity dispersions in the range of 50–100 km s≠1 (Nesvadba et al., 2006; Lehnert et al., 2009; Lehnert et al., 2013; Förster Schreiber et al., 2009; Wisnioski et al., 2015) and show an almost linear correlation with the SFR, while local galaxies show typical velocity dispersions of < 50 km s≠1 (Varidel et al., 2016; Yu et al., 2019). On the other hand, both local and high-z galaxies show velocity dispersions higher than expected from only the thermal contribution of gas. The characteristic temperature of 104 K corresponds to a typical velocity dispersion of ≥9 km s≠1 for the ionized gas emitting at H– (Glazebrook, 2013).
The dominant energy source of the non-thermal turbulent motions is unclear. Numerous drivers have been proposed, including star formation feedback (Mac Low and Klessen, 2004; Krumholz and Matzner, 2009; Murray et al., 2010), radial transport of gas in discs due to gravitation (Krumholz and Burkhart, 2016; Krumholz et al., 2018), gas accretion from the intergalactic medium and minor mergers (Dekel et al., 2009; Glazebrook, 2013), galactic shear from the differential rotation in disc galaxies (Krumholz and Burkhart, 2015; Federrath et al., 2016; Federrath et al., 2017), etc. In Chapter 5, I studied the driver of turbulent motion of eight spatially-resolved nearby star-forming galaxies. The results show that star formation feedback is not the main energy source of the turbulent motions in galaxies with low SFR surface density. However, recent studies by Krumholz et al. (2018) and Varidel et al. (2020) on the global properties of galaxies found that the models taking into consideration of both the star formation feedback and the gravitational energy release from radial transport of gas can yield excellent agreement with the observations of galaxies with SFR ranging from 10≠4 M§ yr≠1 to 103 M§ yr≠1. The model predicts a transition from gravity-dominantly-driven turbulence in high-z galaxies to star-formation-driven turbulence in low-z galaxies, where SFR is lower. The distinct conclusions come from the different treatments of beam smearing effects, as will be explained below. This model also explains why galaxy bulges tend to form at high redshift and discs at lower redshift, and why galaxies tend to quench inside-out, because the gas accretion rate increases much faster with velocity dispersion, than SFR with velocity dispersion (M˙acc à ‡gas3 vs. SFR à ‡gas) and then masses are transported more inward to a bulge in high-z galaxies and remain in the outskirts to form a disk in low-z galaxies (Krumholz et al., 2018).

Integral field unit (IFU), datacube and beam smearing effect

The integral field unit technique has been widely used in optical and near-IR to study galaxy kinematics. It allows us to obtain spatially resolved spectral information in the galaxies. In Chapter 5, I will present my work on the energy sources of the turbulent motions in local star-forming galaxies making use of the IFU survey, the SAMI Galaxy Survey (Croom et al., 2012; Bryant et al., 2015). The three dimensional information from such IFU observations are saved in the so-called datacubes (Figure 1.14). This is similar to the data product obtained from the interferometers at submillimeter to radio wavelengths.
Measurements of velocity dispersions from these datacubes are limited by the spectral and spatial resolution of the instruments. The observed emission line is broadened by the spectral resolution, but this can be addressed by convolving the line spread function into the emission line fitting. The limited spatial resolution blurs the spatial distribution of the intrinsic flux, the line of sight velocity profile, and the line of sight velocity dispersion within the smallest resolved area. As a result, the observed velocity dispersion is elevated with the unresolved velocity gradient. This is the so-called beam smearing effect. Several tools have been developed to account for beam-smearing effect, e.g., 3DBBAROLO12 (DiTeodoro and Fraternali, 2015),GBKFIT13 (Bekiaris et al., 2016), GALPAK3D 14(Bouché et al., 2015), and BLOBBY3D (Varidel et al., 2019). They construct a 3D modelled cube for the galaxy and then spatially convolve the cube per spectral slice to simulate the effect of beam smearing. The convolved cube is finally compared to the observed data.

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Optically dark galaxies and their association with a proto-cluster in formation at z ≥ 3.5

I studied thoroughly the properties of the six optically dark galaxies detected in the GOODS-ALMA 1.1 mm continuum survey. None of them is listed in the deepest H-band based CANDELS catalog in the GOODS-South field. Five of them suffer from the confusion with bright neighboring galaxies even in the highly resolved optical to near-IR images. To retrieve information at these wavelengths, I performed a de-blending procedure with the method developed by one of the co-authors, C. Schreiber (Schreiber et al., 2018a, code). It turns out that after this deblending analysis two out of the six galaxies end up having H-band counterparts with magnitudes brighter than the detection limit determined by in the CANDELS team on their catalog. They were missed because they were considered as part of the neighboring galaxy because of the confusion limit as a drawback of the source extraction procedure. I fitted the optical-to-MIR SEDs of the optically dark galaxies and their neighbors respectively. The derived redshifts confirm the confusion due to a projection effect, meaning that the optically dark galaxies and their neighbors are at different redshifts.
As co-I, I contributed to a spectroscopic follow-up using ALMA (2018.1.01079.S, PI: M. Franco) to identify the exact redshifts of these optically dark galaxies. I analyzed the data with CASA and GILDAS and found that one emission line was detected in two of the galaxies.
To study the environment where these galaxies reside in, I constructed the surface number density map of galaxies in the GOODS-South field based on the ZFOURGE catalog. We choose ZFOURGE rather than CANDELS because the ZFOURGE catalog is based on detections extracted from the near-IR Ks band images, where galaxies at higher redshift appear brighter than they are in the optical images. We present evidence that nearly 70 % of the optically dark galaxies belong to the same over-density of galaxies at z ≥ 3.5. We also found that the most massive one of the optically dark galaxies is also the most massive galaxy at z > 3 in the GOODS-ALMA field after excluding galaxies hosting a luminous AGN potentially responsible for an overestimation of their stellar mass. This galaxy, AGS24, falls at the very center of the peak of the galaxy surface density. This suggests that the surrounding over-density is a proto-cluster in the process of virialization and this massive galaxy is the candidate progenitor of the future Brightest Cluster Galaxy (BCG).
These optically dark galaxies unveiled by ALMA are good tracers of such large-scale structures in the early Universe and they can serve to test current theories on the formation of the most massive galaxies during the first billin years of the Universe.

The SAMI Integral field Units (IFU) Survey

Similar to ALMA, IFU observations also produce 3D datacubes. Then we can associate the physical properties extracted from spectra, with their spatial information to understand the underlying physical processes. I collaborated with F. Bian, T. Yuan, C. Federrath, A. Melding and other members in the SAMI survey team to study spatially resolved nearby star-forming galaxies.

Energy source of turbulence in star-forming galaxies

This work has been published in Monthly Notices of the Royal Astronomical Society (Zhou et al., 2017) and is presented in Chapter 5.
I studied the energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies based on the SAMI survey. We found that on sub-kpc scales, our galaxies display a flat distribution of ionized gas velocity dispersions as a function of SFR surface density. A major fraction of our galaxies shows higher velocity dispersions than the predictions by feedback-driven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium in star-forming galaxies.

Table of contents :

1 Introduction 
1.1 Star formation: the driver of galaxy evolution
1.2 Star formation obscured by interstellar dust
1.2.1 Multi-wavelength study of galaxies
1.2.2 The main sequence of star-forming galaxies
1.2.3 Cosmic star formation history
1.2.4 Observing star-forming galaxies in the (sub)millimeter
1.2.5 Optically-dark galaxies
1.3 Star formation fueled by gas
1.3.1 Gas tracers
1.3.2 Star formation law
1.3.3 Gas content
1.4 Star formation as one of the energy sources of gas
1.4.1 Gas velocity dispersion and the energy sources
1.4.2 Integral field unit (IFU), datacube and beam smearing effect
2 Summary of the work done in this thesis 
2.1.1 Optically dark galaxies and their association with a proto-cluster in formation at z ≥3.5
2.1.2 Data compilation
2.1.3 Nascent AGNs in GOODS-ALMA
2.1.4 Contribution as third author
2.2 Extremely metal-poor galaxies
2.2.1 Spatially resolved dust emission
2.2.2 Gas content in IZw18
2.3 The SAMI Integral field Units (IFU) Survey
2.3.1 Energy source of turbulence in star-forming galaxies
2.4 Observations
3 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5 
3.1 Introduction
3.2 Data and observations
3.2.1 ALMA data and observations
3.2.2 Ancillary data
3.2.3 Origin of the redshifts and stellar masses
3.2.4 Derived parameters of the optically dark galaxies
3.3 Results of the ALMA spectroscopic follow-up
3.3.1 AGS4
3.3.2 AGS17
3.3.3 Upper limits of AGS11, AGS15 and AGS24
3.4 GOODS-ALMA optically-dark galaxies
3.4.1 AGS4, an extremely massive galaxy at z=3.556 and a case of blending in the Hubble H-band image
3.4.2 AGS25, the most distant optically-dark galaxy in GOODS-ALMA
3.5 An over-density at z ≥3.5 in GOODS-ALMA
3.5.1 Clustering properties of optically dark galaxies
3.5.2 A clear peak at z ≥3.5 in the redshift distribution
3.5.3 Optically-dark galaxies at z ≥3.5
3.5.4 Spatial distribution of galaxies at z ≥3.5 in the GOODS-ALMA field
3.5.5 Dynamical state of the proto-cluster at z ≥3.5
3.6 Conclusions
4 Extremely weak CO emission in IZw 18 
4.1 Introduction
4.2 Observations
4.3 Results
4.3.1 CO J=2-1
4.3.2 1.3mm continuum
4.4 Discussion
4.4.1 SED and Submilimetre excess
4.4.2 Infrared luminosity and SFR versus LÕCO
4.4.3 The structure of the interstellar medium
4.5 Conclusion
5 The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatiallyresolved local star-forming galaxies 
5.1 Introduction
5.2 Sample and data analysis
5.2.1 Sample selection
5.2.2 Gas kinematic information
5.2.3 Spatial resolution
5.3 Results
5.3.1 The spatial distribution of !SFR, vgas, and ‡gas
5.3.2 The ‡gas – !SFR relation in local and high redshift star-forming galaxies
5.4 Discussion
5.4.1 Main driver(s) of velocity dispersion
5.4.2 Caveats
5.5 conclusion
6 Spatially resolved dust emission of extremely metal poor galaxies1 
6.1 Introduction
6.2 Sample, observations and data analysis
6.2.1 The sample
6.2.2 Observations
6.2.3 Photometric measurements
6.3 The far-IR SEDs
6.3.1 The color-color diagrams
6.3.2 Modified black-body fitting
6.3.3 Spatial variations of SEDs and dust heating Mechanism
6.4 Dust-to-stellar mass ratio
6.5 Conclusions
7 Conclusion and perspectives 
7.1 Conclusion
7.2 Perspective
7.2.1 More on the optically dark galaxies
7.2.2 AGN feedback on high-z star-forming galaxies


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