¶ Science Background
Galaxies are composed of baryonic matter (planets, stars and gas), radiation and dark matter. Integral Field Spectroscopy (IFS, also known as 2D spectroscopy or 3D optical data) is used in large galaxy surveys including CALIFA , MaNGA and the SAMI Galaxy Survey to give insight into galaxy formation and evolution. Such surveys mainly target the ‘normal’ galaxy population and as such dwarf galaxies (with stellar masses of 106 – 10 9 M⊙) tend to be underrepresented.
(Centre) Same image as observed using Hi-KIDS, with a spatial resolution of 20 pc/pixel.
(Right) Galaxy surveys like SAMI and HECTOR observe more distant galaxies than those in the Local Volume, the highest spatial resolution they achieve is 200 pc/pixel, which is insufficient to resolve the galaxy.
- They do not observe very nearby galaxies due to limited spatial resolution and their targets are too far for exploring processes on intermediate and small scales (see Figure 1).
- They use, if any, unresolved galaxy gas data. Atomic hydrogen is the foundation of star formation emitting a 21cm radio line. Radio interferometric observations are pivotal to understand gas and star formation rates.
- Dwarf galaxies tend to be underrepresented as they can be harder to detect. Local dwarf galaxies are analogous to primitive galaxies formed during the early Universe, therefore a comprehensive study is essential for an understanding of galaxy evolution (e.g. White & Frenk 1991; Kauffmann & White 1993; Springel+ 2005).
Combining H I radio data (neutral gas, dark matter) and the IFS optical data (stars, ionised gas) provide constraints on physical processes that regulate the star formation rate and the build-up of stellar mass over cosmic time.
¶ Innovative Survey
The aim of HI-KIDS is to obtain high quality 2D optical galaxy spectra of nearby unexplored dwarf galaxies using the KOALA Integral Field Unit in conjunction with the AAOmega spectrograph and the available 21cm H I interferometric radio data. The samples have mainly been chosen from the Local Volume (D < 10 Mpc) H I Survey (LVHIS, Koribalski et al. 2018), which analyses the H I distribution of ∼80 gas-rich galaxies using data from the Australian Telescope Compact Array.
Additional dwarf galaxies that are visible from the Anglo-Australian Telescope (AAT) from the Faint Irregular Galaxies GMRT Survey have also been observed. Many of these galaxies don’t have reliable estimations of stellar populations and their gas-phase metallicities. Blue Compact Dwarf Galaxies (BCDGs) with H I interferometric data have also been observed. BCDGs represent the subset of low-luminosity, metal-poor galaxies undergoing a strong and short-lived episode of star formation, in many cases triggered by interactions. This data can be used to trace gas accretion, massive stellar feedback and their influence on other properties (e.g., chemical composition, kinematics).
¶ Parameter space:
The parameter space of the Hi-KIDS galaxies is:
- Stellar masses: 6 < log Mstar / M⊙ < 10
- HI mass: 5 < log MHI / M⊙ < 10
- Metallicity (Te-scale): 7.8 < 12+log(O/H) < 8.3
- SFR: -3 < SFR/(M⊙/yr) < 1
¶ Objectives
- To understand the role of metals in the evolution of dwarf galaxies and how galaxy properties depend on gas and stellar metallicity.
- To develop chemical evolution models considering not only the O/H ratio but N/O, S/O, Ar/O, Ne/O and Fe/O, the gas-to-star mass ratio, the star-formation history or the star formation efficiency.
- To consider the ionisation mechanism of the gas in dwarf galaxies and the mechanical and chemical feedback between massive stars and the ISM.
- To study the dust content in dwarf galaxies, the effect of the dust reddening, and its dependence on other galaxy properties.
- To study the efficiency of the conversion of gas into stars as a function of local (surface densities, metallicity) and global (total, stellar, and gas masses, galaxy morphology, escape velocity) properties.
- To globally study the star-formation processes and star-formation histories in dwarf and irregular galaxies to be compared with theoretical models
- To investigate cold gas accretion in galaxies, metal-loss by outflows, and star-formation feedback.
- To understand galaxy scaling relationships in dwarf galaxies between stellar mass, gas mass, metallicity, star formation, and others, and compare them with those obtained in large galaxy samples.
- To study dwarf galaxy dynamics using gas and stars, quantify the importance of non-rotating gas movements (interactions, mergers, inflows and outflows).
- To compare, for the first time, the universal scaling relation S0.5 - M using 3 tracers: star, ionised gas and neutral gas
¶ Specific Aims
The specific scientific objectives of our IFS observations using KOALA+AAOmega at the AAT are:
- To derive precise chemical abundances (O and N, but also Ne, Ar, S, Ne, and Fe) using the direct Te-method (i.e., considering the electron temperatures derived from the faint auroral lines of [O III], [S III], [Ar III], [O II], or [S II]), or applying to empirical calibrations which include an ionization parameter which considers the hardness of the ionizing radiation including the [S III]/[S II] ratio. We will consider both calibrations based on photo-ionization models and those based on direct estimations of the electron temperature
- To analyse the physical conditions of the gas: electron density and temperature, extinction (derived from H I Balmer ratios and from stellar population fitting) and derive their local and global star formation rate (we will combine this with GALEX FUV and MIPS 24µm data when available).
- To analyse the ionization structure of the gas and constrain its excitation mechanism (massive stars or shocks) using diagnostic diagrams (e.g., [O III]/Hβ vs [N II]/Hα [O III]/Hβ vs [S II]/Hα). An analysis of the emission lines profiles will be performed.
- To estimate the properties of the stellar populations underlying the star-forming regions. Using the absorption lines and the stellar continuum the age and metallicity of the stellar populations may be inferred via stellar population synthesis models. TH Is analysis will strongly constrain the star-formation history of each galaxy.
- To analyse the kinematics of the stars and the ionized gas via the profiles of the absorption/emission lines to understand the feedback between the ISM and the massive stars and to get complement of the galaxy dynamics to that derived using the H I kinematics. This analysis may also reveal non-rotating gas movements due to recent interactions, mergers, and inflows or outflows of gas.
- To computing modelling the chemical evolution of galaxies. We will develop detailed chemical evolution models that will consider the effect of the star-formation history in the evolution of the O/H and N/O ratios and the gas-star fraction.