Astronomy research

Here I summarize my research in astronomy.

Dark stars

Dark stars are hypothetical primordial stars powered by self-annihilating dark matter particles. They can be viewed as an evolutionary branch in the lives of Population III (metal-free stars, henceforth Pop III) stars if dark matter annihilation begins to power the core of the protostar prior to the onset of main-sequence hydrogen burning. Zackrisson et al. (2010a) we show that unlensed individual dark stars at z > 6 will not be observable within the foreseeable future but certain types of long-lived dark stars lensed by a foreground galaxy cluster could be observed by NIRCam on James Webb Telescope (JWST). The existence of supermassive dark stars (SMDS) with masses up to 10 million solar masses has been proposed (Freese et al. 2010) but in Zackrisson et al. (2010b) we set observational constraints on SMDS from already existing HST data.

Population III stars

Figure 1: The lowest necessary star formation rate (SFR) as a function of z is displayed with solid lines. The lines are color coded according to Lyα escape fraction. The dashed lines shows the SFR from several different simulations. None of them are above the minimum SFR necessary. Hence, no Pop III star is observed.

The first stars, metal-free Pop III stars, formed at redshift z ~ 25, or 200 Myr after the Big Bang. They began cosmological reionization, the process by which the universe was transformed from a cold, dark featureless void into the vast, hot cosmic web of galaxies and galaxy clusters observed today. They also began to pollute the universe with the first heavy elements (Whalen 2013), i.e. "metals", elements other than hydrogen and helium, which were later required for planets and life. As these stars congregated by mergers and accretion over cosmic time, the first galaxies were born (Bromm et al. 2009). In spite of the importance of the first stars and galaxies to the evolution of the primeval universe, little is known for certain of their properties. Numerical models of Pop III star formation are limited to just its initial stages (Bromm & Yoshida 2011).

In Rydberg et al. (2013), we examined detection thresholds for both single and small clusters of Pop III stars and their H II (ionized hydrogen) regions at z = 2 - 20. We used data from Schaerer (2002) and Tlusty (Hubeny & Lantz 1995) to model the stellar atmosphere, and then Cloudy (Ferland et al. 1998) to model the H II region. The spectra are used to assess the detectability of these stars and their nebulae with JWST.

Figure 2: Shows the same as Figure 1 with the difference that the typical mass of the Pop III stars are 300 Msol instead of 60 Msol. Here detections are possible for the most optimistic SFR models and highest Lyα emission.

We found that direct detection of Pop III stars would not be possible with JWST and then calculated threshold magnifications for their detection by strong gravitational lensing. With this we determined the minimum SFR required for a Pop III star to be observed behind the cluster lens MACS J0717.5+3745, which we derived by computing the volume that would be magnified above JWST thresholds as a function of background redshift. We performed the same calculation taking into account the possibility of strong Lyα emission. Lyα radiation originates from the transition of an electron between the second excited state and the first (ground) state in the hydrogen atom. It is intrinsically very strong but is resonant and easily scatters/gets absorbed when encountering neutral hydrogen. The fraction that is not scattered/absorbed is called the Lyα escape fraction, f(Lyα). We make the assumptions of escape fractions f(Lyα) = 0.2 and 0.5 along with the simple case of no Lyα emission. We compared our minimum SFRs to rates from simulations by Tornatore et al. (2007), Trenti & Stiavelli (2009), and Wise et al. (2012). If the characteristic masses of Pop III stars are a few 10s solar masses, even lensed detections would be implausible (Figure 1). In a scenario deemed very unlikely, in which Pop III stars have characteristic masses of 300 solar masses and f(Lyα) = 0.5, a minimum SFR consistent with detection in the most optimistic models is achieved (Figure 2).

Population III galaxies

In Zackrisson et al. (2011a, b) we used galaxy models provided by the Yggdrasil code to investigate the detectability and identification criteria for different types of galaxies at high redshift. A minimum stellar mass of M ~ 10,000 solar masses in a Pop III.1/III.2 galaxy surrounded by an H II region is necessary in order for detection by JWST to be possible.

We then investigated different broadband signatures (basically comparing radiation detected at different wavelengths) to single out Pop III galaxies from other galaxies with JWST and, assuming high Lyα emission, HST. Pop III identification criteria as color-color diagrams were derived, between m444- m560 and m560- m770 valid at z ~ 7 - 8 for JWST and between Y105-J125 vs. J125-H160 valid at z ~ 8 for HST.

Figure 3: A comparison of the quality of fit for the best Pop III galaxy candidate in Rydberg (2015). The model grid containing Pop III galaxies has a significantly better fit to the observations in z ~ [8.0, 9.0].

We took a deeper look at gravitational lensing of Pop III galaxies in Zackrisson et al. (2012) and found that ~ one Pop III galaxy could be found in CLASH (Postman et al. 2012) if ~ 1% of the baryons (ordinary matter, as opposed to dark matter, DM) in each halo is converted into stars.

In Rydberg et al. (2015) we reported the search for and discovery of a few Pop III galaxy candidates in CLASH. We used the color-color criterion from Zackrisson et al. (2011b) and SED fitting (galaxy models fitted to observations) to identify viable Pop III galaxy candidates. We identified five possible candidates, Abell209-994, MACS0416-1828, MACS0647-610, MACS1931-777, and RXJ1347-1951. Their SED fits indicate young, low-mass, Lyα emitting objects with stellar population masses of the order of one million solar masses. The two objects MACS1931-777 and RXJ1347-1951 are singled out as Pop III galaxy candidates. MACS1931-777, at z ≈ 8.2, is the best candidate and its quality of fit for a variety of model grids is shown in Figure 3.

Figure 4: Left: 100’’ times 100’’ overview of the Abell 2261 cluster with critical line and the regions predicted to contain counter-images. Right: magnification map of the cluster, i.e. the magnification in each point is color-coded. The names of the three counter-images we have identified are printed beside corresponding region.

In Rydberg et al. (2016) we present the discovery of a triply lensed galaxy behind Abell 2261 at z ≈ 6.3. The first image, Abell 2261-9000, was identified by the same method used in Rydberg et al. (2015). Using a lens model (Zitrin et al. 2015) we identified three areas where counter-images were predicted. In two of them counter-images, named Abell 2261-14000 and Abell 2261-1366, with morphologies corresponding to Abell 2261-9000, were found. See Figure 4 for the positions of the predicted counter-images in the Abell 2261 cluster and Figure 5 for close multi-color images of each identified counter-image. By stacking the three images and using SED fitting, high Lyα emission was indicated and a photometric redshift of 6.3 was derived. We have secured time on Large Binocular Telescope (LBT) in 2017 for spectroscopic follow-up on each of the three images. The spectroscopic observations have the potential to confirm the redshift, its high Lyα emission and its multiply lensed nature.

Figure 5: Thumbnail images of the multiply-lensed galaxy candidate behind the Abel 2261 cluster lens at z ≈ 6.3 that may be a galaxy merger (blue: F850LP; green: F125W; red : F160W; Rydberg et al. 2016).

Population III supernovae

A direct way to probe the properties of the first stars is to look for the explosions resulting from the death of the more massive stars. These so called supernovae (SN) emit time dependent radiation, light curves, that depend on the progenitor stars mass. Recent simulations have shown that Pop III pair-instability (PI) SNe (explosion type for stars in the mass interval [140, 260] solar masses) will be visible to JWST at z > 30 (Whalen et al. 2013a, 2014) while much less energetic core-collapse (CC) SNe (explosion type for stars in the mass interval [8, 40] solar masses) will be visible out to z ~ 10 – 15 (Whalen et al. 2013b,c; Smidt et al. 2014). They will also be visible at redshift up to 15 to Euclid and WFIRST. But both types of explosions must be found at z ~ 20 to constrain Pop III stellar masses and formation rates and ensure that metal-enriched Pop II stars do not contaminate the sample. JWST will be extremely sensitive but will not detect CC SNe at z > 15. Its narrow field of view may also prevent it from encountering many PI SNe at z ~ 20 over its lifetime. On the other hand, LSST, Euclid and WFIRST will have much larger survey areas that could discover far greater numbers of transients. But they are less sensitive than JWST and will not find SNe at z > 15. This picture could change if strong gravitational lensing by galaxy clusters and massive elliptical galaxies amplifies light from background SNe above the detection limits of these missions.

In our current project, we will study how strong lensing in much larger (> 5 square degrees) survey fields may reveal ancient SNe and galaxies that would otherwise be too faint to be detected. We have developed magnification functions for large areas of the sky due to elliptical galaxies and galaxy clusters. We will convolve these functions, which predict what fraction of the sky is magnified by a given factor at a given redshift, with SN light curves and later protogalactic luminosity functions to determine detection rates for transients and galaxies in wide-field surveys by LSST, Euclid and WFIRST. These functions, which are statistical in nature, will allow us to compute approximate detection rates which could be used when setting the survey strategies for LSST, Euclid and WFIRST. We have started this project by estimating the number of SNe from Pop III stars that will be observed by LSST, Euclid and WFIRST. Both directly observed SNe as well as SNe lensed by galaxies/galaxy clusters distributed throughout the universe is counted.


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