[Galaxies sprinting] through the valley

Title: Across the Green Valley with HST grisms: colour evolution, crossing time-scales and the growth of the red sequence at z=1.0−1.8

Authors: Gaël Noirot, Marcin Sawicki, Roberto Abraham, Maruša Bradač, Kartheik Iyer, Thibaud Moutard, Camilla Pacifici, Swara Ravindranath, Chris J. Willott

First Author’s Institution: Institute for Computational Astrophysics and Department of Astronomy & Physics, Saint Mary’s University, Halifax, Canada

Status: Accepted to MNRAS

Not unlike Ellie in the Last of Us, many galaxies too walk through the valley of the shadow of death. While roughly half of the galaxies, including our own, are blue and actively star-forming, the other half are less lively. At some point in their lifetime, they stopped forming stars, or quenched, earning a grim label of “red-and-dead” galaxies. But how and when galaxies quench is still a million dollar question!

The authors of today’s paper study a relatively rare population of green valley galaxies in the early Universe, at redshift z=1-2, or 4-8 billion years ago. Green valley galaxies are dying, losing their star-formation-driven blue colors and becoming redder. The dearth of these intermediate-color galaxies (Fig. 1) tells us that the transition is rapid, and so this population of galaxies was romantically named the green valley

The authors chose a particularly rich sample of galaxies from the GOODS-S and the UDS fields, that have been observed in a number of bands from the IR to the UV using Hubble Space Telescope, Spitzer, and many ground-based observatories. This rich dataset provided the authors with over 10,000 galaxies to study.

Figure 1. Left:  the galaxy color-mass diagram (adapted from Fig. 2, Schawinski et al., 2014). Star-forming galaxies are blue and primarily reside in the left lower corner, while quiescent galaxies are red and reside in the upper half of the diagram. Green valley, shown in green, is the transition region with relatively few galaxies. Right:  examples of some green valley galaxies, listed in Salim et al. (2014). Image credit: Sloan Digital Sky Survey.

The bottom line of the green valley

The authors of the paper were most interested in the speed of galaxies moving through the green valley from the blue cloud to the red sequence. To measure the transition along this space more easily, they defined the bottom of green valley (dashed line in Fig. 2, left) and transformed the diagram relatively to that line. This gave a new parameter: the distance to the bottom of the green valley, ΔGV, that all galaxies fall towards. The new parameter has several advantages: it is easier to calculate, to interpret, and it accounts for the loss of dust in spiral galaxies as they transition across the valley.

Figure 2. Left: color-color diagram showing the red sequence of quiescent galaxies, the blue cloud of star-forming galaxies, and the green valley. The dashed line (highlighted in green) shows the bottom of the green valley used to define ΔGV.  Color of the points represents SFR (bluer = higher). Right: ΔGV versus color for a sample of 265 objects with spectroscopic observations. The bottom of the green valley is a straight line in this representation. Adapted from Fig. 1, Noirot et al., 2021.

Winners and losers of the valley race

The authors then measured the star formation history of each galaxy. Most of their conclusions are made using a subsample of 265 objects that have grism (think grating+prism) spectroscopy, making the analysis of their star formation history more robust. They used a Flexible Stellar Population Synthesis method to estimate the star formation rate (SFR) of each galaxy as a function of time. This method lets scientists recover the time the galaxy first formed, and the SFR since then.

Fig. 3 shows examples of some star formation histories. Each line shows the SFR at different ages of a galaxy, from the time it was formed to the time it is observed. Gray shaded region shows the predicted future SFR. The authors use these curves to define a new parameter, τ, the time between galaxy formation and the peak of its star formation. Fast galaxies reach the peak and quench quickly after they form, intermediate ones peak after ~1 Gyr, and slow galaxies form stars for several Gyr before they peak. Keep in mind that galaxies in the sample may be very young, and their peak might be in the future! Fig. 3 (right) shows the distribution of the galaxies in their sample, and the clear division between fast, intermediate, and slow populations.

Figure 3. Left: example star formation history models for a fast (yellow), intermediate (purple) and slow (grey) case, observed 1.25 Gyr after birth. The fast, intermediate, and slow models reach SFR peak after τ=0.1, 0.9, and 2.1 Gyr respectively. The SFR of the slow model is still increasing at time of observation. Adapted from Fig. 4, Noirot et al., 2021. Right: the distribution of τ in the spectroscopic sample clearly shows 3 distinct populations: fast, intermediate, and slow. Adapted from Fig. 6, Noirot et al., 2021.

The race across the green valley

Of course, we can’t watch a single galaxy evolve across the green valley – it takes upwards of a billion years. But we can assume that galaxies from one group evolve similarly. Then fast galaxies that formed earlier are just a look in the future of the younger fast galaxies – the sample becomes a sort of a time machine!

The authors plotted the ΔGV and measurements for fast, intermediate, and slow galaxies and looked at how long it takes a galaxy from each population to cross the green valley (Fig. 4). They found that while intermediate and slow galaxies basically never pass through the valley, fast galaxies quench extremely quickly – and reach the bottom of the green valley in ~1 Gyr!

Figure 4.  Distance to the bottom of green valley, ΔGV, as a function of galaxy age for fast (yellow), intermediate (purple), and slow (gray) populations. Only the fast galaxies cross the green valley, with an approximate timescale of 1 Gyr. Fig. 11, Noirot et al., 2021.

Using their results and the galaxy counts between redshifts 0.9 to 1.8, the authors took the time machine for a spin. They estimated how many of z=1.8 galaxies will become quenched by z=1.3, and how many of z=1.3 galaxies will quench by z=1 assuming the timescales they worked out. They then compared the actual number of “red-and-dead” galaxies at these times (Fig. 5). There is a remarkable agreement, and they successfully predicted the future!

Figure 5. Luminosity function of quiescent galaxies at different redshifts. Colored points show measurements from literature, while the black arrows show the prediction from the author’s approach. Assuming fast galaxies cross the green valley in ~1 Gyr, they reproduce precisely the expected build-up of quiescent galaxies with time. Fig. 14, Noirot et al., 2021.

The authors of today’s paper used a new method – looking at the time to reach the bottom of the green valley, and the delay between start and peak of star formation – to figure out how different galaxies cross the green valley. Turns out, only fast galaxies cross – or rather run – through. They start forming stars violently soon after birth, peak quickly, and then quench in less than 1 Gyr. Looking over a larger time frame, it’s the death of these galaxies that builds up the “red-and-dead” population over cosmic time. So if you are a galaxy and want to stay alive, slow and steady wins the race!

Edited by: Roan Haggar

Featured image credit: Green Valley in Krkonose by Roman Boed, CC 2.0 + Sloan Digital Sky Survey.

About Liza Sazonova

I study how galaxies quench and die, together with the amazing SPOGs team. I look at the structure of galaxies as they are quenching, and see what processes transform them from a disk to a blob. When not coding or looking at galaxies, I like playing board games, climbing rocks, and writing a short novel about Phil the Photon!

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