Title: Feedback and dynamical masses in high-z galaxies: the advent of high-resolution NIRSpec spectroscopy
Authors: A. Saldana-Lopez, J. Chisholm, S. Gazagnes, R. Endsley, M. J. Hayes, D. A. Berg, S. L. Finkelstein, S. R. Flury, N. G. Guseva, A. Henry, Y. I. Izotov, E. Lambrides, R. Marques-Chaves, C. T. Richardson
First Author’s Institution: Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
Status: Submitted to MNRAS [open access]
In the aftermath of the Big Bang, around 13.8 billion years ago, the Universe was a pretty messy place. Small building blocks – primordial dark matter haloes – collapsed and merged together, gradually forming larger structures that would become the galaxies we see today. These early galaxies were likely compact, irregular, and turbulent due to rapid matter accretion and frequent collisions. Observing these young, distant galaxies has long been a challenging task, but the James Webb Space Telescope (JWST) has helped to revolutionise our ability to study them in detail. In today’s paper, the authors study a sample of 16 galaxies at redshifts of 4<z<7.6—when the Universe was only around 1 billion years old. They explore how ionised gas moves inside these young galaxies and how feedback impacts their overall mass budget.
The Stellar Feedback Loop: How stars shape—and are shaped by—their environments
Like many problems in science, stars and their surroundings (the interstellar medium, or ISM) are somewhat of a chicken-and-egg situation: stars (particularly massive ones) have a huge impact on their environment, but, in turn, the environment has a huge impact on the stars that form within it. These effects (and the mechanisms responsible for them) are referred to as stellar feedback. By adding energy and momentum into the ISM, stellar feedback drives winds and outflows that can regulate future star formation, and even enrich the circumgalactic medium (CGM) (e.g., see this bite). Similarly, inflowing gas from the surrounding medium can also impact star formation by increasing the amount of fuel available to form the next generation of stars. Out of the 16 galaxies studied in this sample, evidence of ionised gas flows (outflowing or inflowing) is found in five. This evidence is in the form of a two-component fit to the observed emission lines, i.e., one stationary component for the galaxy, and another component at a different velocity for the gas flowing out of or into the galaxy (see Figure 1 for an example).
Distinguishing between inflows and outflows is a tricky business. Researchers need to consider both geometrical and physical arguments, as well as the limitations of the observations. For now, the authors choose to consider both interpretations, but state that further observations would be needed for confirmation.
Case 1: If these are outflows…
For these five high redshift galaxies, the outflowing gas moves at velocities of 150-250 km/s, resulting in quite moderate mass outflow rates (the mass of ionised gas that is removed due to the outflow). These measurements (and others at similarly high redshift, see red circles and green diamonds in Figure 2) perfectly fill a gap in the current parameter space, fitting nicely between local dwarf galaxies with lower mass outflow and star formation rates (purple diamonds), and more highly star forming galaxies at the epoch where the cosmic star formation rate density peaks (orange stars), often referred to as Cosmic Noon (around z=2-3).
The ratio between how much mass is being ejected and the rate at which stars are being formed (called the mass loading factor, η) can help set the scene for future star formation. If η is greater than one the galaxy will eventually run out of fuel, and no new stars will be formed. For the comparison sample of local dwarf galaxies, η is typically lower than that of the higher redshift galaxies of similar mass or star formation rate studied here and in other works (right hand-side panel of Figure 2). This suggests that these galaxies in the early Universe remove ionised gas more efficiently.
In addition, these five galaxies with evidence for outflows tend to have more compact star formation (as traced by Σ SFR, which is the star formation rate surface density) in comparison to the other 11 galaxies in the sample, and other high redshift galaxies in the literature (see Figure 3). They also may contain more gas relative to their total mass, and may have had more recent bursts of star formation. Together, this paints a picture where these intense, compact bursts of star formation are what’s driving these outflowing winds in high redshift galaxies.
Case 2: But then if some of these are inflows…
For three out of the five galaxies with observed gas flows, the gas-flow component is redshifted. Normally, when we see redshifted emission in outflows, it is because the gas is moving away from us, on the far side of the galaxy (consider the outflowing arrows coming from the lower side of the galaxy, flowing down to the bottom of Figure 4). If the outflow is symmetric (like a bubble or cone), we should simultaneously see blueshifted emission from the near side, where gas is moving toward us. In dusty galaxies, we might miss the blueshifted part because the dust absorbs the associated emission. However, these galaxies have little to no dust, meaning there is no obvious reason why we don’t see both redshifted and blueshifted components. Instead of assuming the redshifted component comes from the far side of an outflow (which might halt future star formation by expelling gas), it could potentially be an inflow of gas falling into the galaxy on the near side (see the accreting gas arrow in Figure 4), which would instead provide more star forming fuel.
Observing inflows is quite rare in the local Universe. However, inflows are theoretically expected to be more significant in the early Universe due to the increase in accretion rates with redshift. If these inflow signatures are proven, this would be an impressive foray into testing these predictions at high redshift.
In summary, these findings highlight the delicate balance between star formation, feedback, and gas availability in early galaxies. Whether these galaxies are expelling gas through powerful outflows or gathering fuel through inflows, both processes play a crucial role in shaping their evolution. For now, the mystery remains: are these young galaxies in a state of loss, or are they just getting started?
Astrobite edited by Catherine Slaughter
Featured image credit: Tumlinson et al. (2017)
