Title: High-velocity outflows persist up to 1 Gyr after starburst in recently-quenched galaxies at z>1
Authors: Elizabeth Taylor, David Maltby, Omar Almaini, Michael Merrifield, Vivienne Wild, Kate Rowlands, Jimi Harrold
First Author’s Institution: University of Nottingham, School of Physics and Astronomy, Nottingham
Status: Accepted to MNRAS [open access]
Case Background
Cosmic noon refers to the era in the Universe’s history when star formation was at its highest. But as time passed, this rapid star formation slowed, and many galaxies transitioned from being lively, star-forming systems to more dormant, quenched states. What causes this transformation? And how do galaxies evolve from young, blue, gas-filled, disc-shaped structures to old, compact, red, and passive ones?
Prime Suspects
The key lies in a process called feedback. Feedback occurs when energy released by stars or active galactic nuclei (AGN) influences the star formation within a galaxy. This can happen in different ways. For instance, stars can emit energy through radiation or explosive supernova events. Meanwhile, AGNs—supermassive black holes at the centers of galaxies—can release energy when they consume gas. This energy can either heat the surrounding gas or push it out of the galaxy as outflows, halting star formation. AGNs produce powerful jets in another type of feedback that keeps the gas hot and prevents it from condensing to form new stars.
In their study, the authors focused on understanding how much stars contribute to these outflows. Research has indicated that the speed of these outflows is strongly linked to certain properties of stars in the galaxy, such as their star formation rate (SFR), the star formation rate density, and, to a small degree, the total stellar mass.
To explore this, the authors examined post-starburst galaxies—galaxies that recently experienced intense star formation followed by a rapid halt—around the time of cosmic noon. This helps shed light on how outflows have evolved since their starburst phase.
Evidence Collection
To distinguish between star-forming galaxies, passive galaxies, and post-starburst galaxies (PSBs), the authors use a method called principal component analysis (PCA). We need many filters to describe the spectral energy distribution (SED) of a galaxy. Using PCA, you make a few combinations of these filters that can explain the spectral energy distribution the same way as the filters. Each combination is called a “supercolour” (SC1, SC2, and SC3). The authors note that you need only three supercolours to describe the SED.
For this work, the authors focus on the first two supercolours. SC1 is linked to the average age of the stars in the galaxy, while SC2 relates to how much stellar mass has formed in the past billion years. By plotting galaxies on an SC1-SC2 diagram, they can classify them with previously set boundaries by comparing the data to models.
The authors divided their sample into four categories:
- Progenitors: Galaxies that are likely to stop forming stars soon.
- Quenched-young: Galaxies that had a burst of star formation less than 0.6 billion years ago.
- Quenched-mid: Galaxies with a starburst between 0.6 and 1 billion years ago.
- Quenched-old: Galaxies that had a starburst more than 1 billion years ago.
The authors focused on detecting the MgII line, which helps trace the ionized gas in the interstellar medium (ISM)—the gas and dust filling the space between stars. This MgII line shifts to bluer wavelengths if outflows are present due to the Doppler effect, showing that gas is moving outward from the galaxy. To get an accurate read on the galaxy’s redshift, they also checked for other spectral lines, like [OII] and the CaII H and K lines.
Crime Scene Analysis
To improve the signal-to-noise ratio, the rest frame spectra from each group are combined into one by picking the median value of the flux at each wavelength (median stacking) in Figure 2. The continuum is then measured, and the spectrum is normalized based on the continuum near the MgII line.
If outflows are present, the MgII emission will show two components: one from the ISM inside the galaxy, which appears at the original wavelength, and another blue-shifted component from the outflow. The most straightforward approach is to fit two Gaussian curves to the spectra. Using this method, they detect a blue-shifted component in all groups except the quenched galaxies. This blueshifted wavelength is then converted into a velocity of the outflow.
- Progenitors: 1510 km/s
- Quenched-young: ~1350 km/s
- Quenched-mid: ~1450 km/s
- Quenched-old: No outflows!
These velocities are high enough for the gas to escape the galaxies, leaving them depleted of the material needed to form new stars.
Unsolved Mysteries
Outflow velocities generally drop off over time following a starburst in nearby galaxies. However, in this sample, the outflow speeds hold steady, suggesting that something besides star formation could drive these outflows. Could it be AGN activity? Yet, there’s no sign of AGN in the spectrum—no highly ionized lines that typically indicate an active AGN. Still, it’s possible the AGN was active in the past and is now dormant, meaning these outflows could be a “fossil” from an earlier phase of AGN activity.
This lingering mystery keeps us searching for a more complete picture of the processes that transform galaxies over cosmic time. As we gather more data, we may get closer to uncovering the whole story behind how galaxies evolve.
Astrobite edited by Sonja Panjkov
Featured image credit: NGC 3351 (stellar feedback), NCG 1232 (young galaxy), NGC 5643 (AGN feedback), and NGC 936 (old galaxy) from ESO collaged into a casefile using Canva
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