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Register a new userUnbiased large spectroscopic surveys of galaxies selected by SPICA using dust bands
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The mid-infrared (IR) range contains many spectral features associated with large molecules and dust grains such as polycyclic aromatic hydrocarbons (PAHs) and silicates. These are usually very strong compared to fine-structure gas lines, and thus valuable in studying the spectral properties of faint distant galaxies. In this paper, we evaluate the capability of low-resolution mid-IR spectroscopic surveys of galaxies that could be performed by SPICA. The surveys are designed to address the question how star formation and black hole accretion activities evolved over cosmic time through spectral diagnostics of the physical conditions of the interstellar/circumnuclear media in galaxies. On the basis of results obtained with Herschel far-IR photometric surveys of distant galaxies and Spitzer and AKARI near- to mid-IR spectroscopic observations of nearby galaxies, we estimate the numbers of the galaxies at redshift z > 0.5, which are expected to be detected in the PAH features or dust continuum by a wide (10 deg^2) or deep (1 deg^2) blind survey, both for a given observation time of 600 hours. As by-products of the wide blind survey, we also expect to detect debris disks, through the mid-IR excess above the photospheric emission of nearby main-sequence stars, and we estimate their number. We demonstrate that the SPICA mid-IR surveys will efficiently provide us with unprecedentedly large spectral samples, which can be studied further in the far-IR with SPICA.
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The main energy-generating mechanisms in galaxies are black hole (BH) accretion and star formation (SF) and the interplay of these processes is driving the evolution of galaxies. MIR/FIR spectroscopy are able to distinguish between BH accretion and SF, as it was shown in the past by infrared spectroscopy from the space by the Infrared Space Observatory and Spitzer. Spitzer and Herschel spectroscopy together can trace the AGN and the SF components in galaxies, with extinction free lines, almost only in the local Universe, except for a few distant objects. One of the major goals of the study of galaxy evolution is to understand the history of the luminosity source of galaxies along cosmic time. This goal can be achieved with far-IR spectroscopic cosmological surveys. SPICA in combination with ground based large single dish submillimeter telescopes, such as CCAT, will offer a unique opportunity to do this. We use galaxy evolution models linked to the observed MIR-FIR counts (including Herschel) to predict the number of sources and their IR lines fluxes, as derived from observations of local galaxies. A shallow survey in an area of 0.5 square degrees, with a typical integration time of 1 hour per pointing, will be able to detect thousands of galaxies in at least three emission lines, using SAFARI, the far-IR spectrometer onboard of SPICA.
The physical processes driving the chemical evolution of galaxies in the last $sim 11, rm{Gyr}$ cannot be understood without directly probing the dust-obscured phase of star-forming galaxies and active galactic nuclei. This phase, hidden to optical tracers, represents the bulk of star formation and black hole accretion activity in galaxies at $1 < z < 3$. Spectroscopic observations with a cryogenic infrared (IR) observatory like SPICA will be sensitive enough to peer through the dust-obscured regions of galaxies and access the rest-frame mid- to far-IR range in galaxies at high-$z$. This wavelength range contains a unique suite of spectral lines and dust features that serve as proxies for the abundances of heavy elements and the dust composition, providing tracers with a feeble response to both extinction and temperature. In this work, we investigate how SPICA observations could be exploited to understand key aspects in the chemical evolution of galaxies: the assembly of nearby galaxies based on the spatial distribution of heavy element abundances, the global content of metals in galaxies reaching the knee of the luminosity function up to $z sim 3$, and the dust composition of galaxies at high-$z$. Possible synergies with facilities available in the late 2020s are also discussed.
Mass outflow rates and loading factors are typically used to infer the quenching potential of galactic-scale outflows. However, these generally rely on observations of a single gas phase which can severely underestimate the total ejected gas mass. To address this, we use observations of high mass ($geqslant$10$^{10}$ M$_{odot}$), normal star-forming galaxies at $zsim$0 from the MaNGA, xCOLD GASS, xGASS and ALFALFA surveys and a stacking of NaD, H$alpha$, CO(1-0) and HI 21cm tracers with the aim of placing constraints on an average, total mass outflow rate and loading factor. We find detections of outflows in both neutral and ionised gas tracers, with no detections in stacks of molecular or atomic gas emission. Modelling of the outflow components reveals velocities of $|$v$_{text{NaD}}|$=131 km s$^{-1}$ and $|$v$_{text{H}alpha}|$=439 km s$^{-1}$ and outflow rates of $dot{M}_{text{NaD}}$=7.55 M$_{odot}$yr$^{-1}$ and $dot{M}_{text{H}alpha}$=0.10 M$_{odot}$yr$^{-1}$ for neutral and ionised gas, respectively. Assuming a molecular/atomic outflow velocity of 200 km s$^{-1}$, we derive upper limits of $dot{M}_{text{CO}}<$19.43 M$_{odot}$yr$^{-1}$ and $dot{M}_{text{HI}}<$26.72 M$_{odot}$yr$^{-1}$ for the molecular and atomic gas, respectively. Combining the detections and upper limits, we find average total outflow rates of $dot{M}_{text{tot}}lesssim$27 M$_{odot}$yr$^{-1}$ and a loading factor of $eta_{text{tot}}lesssim$6.39, with molecular gas likely contributing $lesssim$72% of the total mass outflow rate, and neutral and ionised gas contributing $sim$28% and $<$1%, respectively. Our results suggest that, to first order, a degree of quenching via ejective feedback could occur in normal galaxies when considering all gas phases, even in the absence of an AGN.
We have investigated the possibilities to quantify how much stars move in the Milky Way stellar disk due to diffuse processes (i.e. so called blurring) and due to influences from spiral arms and the bar (i.e. so called churning). To this end we assume that it is possible to infer the formation radius of a star if we know their elemental abundances and age as well as the metallicity profile of the interstellar medium at the time of the formation of the star. Using this information, coupled with orbital information derived from Gaia DR2 data and radial velocities from large spectroscopic surveys, we show that it is possible to isolate stellar samples such that we can start to quantify how important the role of churning is. We use data from APOGEE DR14, parallaxes from Gaia and stellar ages based on C and N elemental abundances in the stars. In our sample, we find that about half of the stars have experienced some sort of radial migration (based solely on their orbital properties), 10 % have likely have suffered only from churning, whilst a modest 5-7 % of stars have never experienced either churning or blurring making them ideal tracers of the original properties of the cool stellar disk. Our investigation shows that it is possible to put up a framework where we can begin to quantify churning and blurring an important. Important aspects for future work would include to investigate how selection effects should be accounted for.
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