Age-dating Luminous Red Galaxies observed with the Southern African Large Telescope (1702.00418v2)

Abstract: We measure a value for the cosmic expansion of $H(z) = 89 \pm 23$(stat) $\pm$ 44(syst) km s$^{-1}$ Mpc$^{-1}$ at a redshift of $z \simeq 0.47$ based on the differential age technique. This technique, also known as cosmic chronometers, uses the age difference between two redshifts for a passively evolving population of galaxies to calculate the expansion rate of the Universe. Our measurement is based on analysis of high quality spectra of Luminous Red Galaxies (LRGs) obtained with the Southern African Large Telescope (SALT) in two narrow redshift ranges of $z \simeq 0.40$ and $z \simeq 0.55$ as part of an initial pilot study. Ages were estimated by fitting single stellar population models to the observed spectra. This measurement presents one of the best estimates of $H(z)$ via this method at $z\sim0.5$ to date.

• The paper presents a novel application of the differential age technique to estimate H(z) using SALT-observed LRG spectra.
• It employs high-quality spectral fitting with SSP and galaxev models over z ≃ 0.40 and 0.55 to derive precise galaxy ages.
• The findings align with ΛCDM predictions, emphasizing the effectiveness of cosmic chronometers in measuring cosmic expansion rates.

Age-Dating Luminous Red Galaxies with SALT

The paper, "Age--dating Luminous Red Galaxies observed with the Southern African Large Telescope" by Ratsimbazafy et al., presents an analysis of Luminous Red Galaxies (LRGs) to estimate cosmic expansion rates using the differential age technique. This method, also known as cosmic chronometers, relies on the age difference between two redshift epochs of a passively evolving galaxy population to infer the Universe's expansion rate, parameterized by the Hubble parameter, $H(z)$. The authors focus particularly on LRGs observed with the Southern African Large Telescope (SALT).

Methodology

The paper used high-quality spectra of LRGs, obtained from SALT, within two narrow redshift ranges centered around $z \simeq 0.40$ and $z \simeq 0.55$. The spectra were analyzed by fitting single stellar population (SSP) models to derive ages for these galaxies. A suite of stellar population synthesis models was employed, notably the galaxev models. The choice of models is a critical consideration as it impacts the derived ages, and thus the estimate of $H(z)$.

Results

From the analysis, the estimate for $H(z)$ at a redshift $z \simeq 0.47$ is found to be $$89 \pm 23 \text{(stat)} \pm 44 \text{(syst)} \text{ km s}^{-1} \text{ Mpc}^{-1}$$. This measurement aligns with predictions of the standard $\Lambda$CDM cosmological model and is one of the most precise estimates at this redshift made via the cosmic chronometer method.

Implications

The determination of $H(z)$ at this redshift, with improved precision due to the capabilities of SALT, supports the model-independent estimates of cosmic expansion. The CPP method's reliance on differential age measurements as opposed to absolute ages mitigates certain systematic errors, making it a valuable complementary technique to more traditional methods, such as observations of the cosmic microwave background or baryonic acoustic oscillations.

Future Directions

One limitation highlighted in this paper is the small sample size of galaxies, particularly at $z \simeq 0.55$, which suggests that more extensive observations could enhance the precision of $H(z)$ measurements. The research underscores the need for further studies with larger galaxy samples and push for the development of enhanced stellar population models that could adequately account for uncertainties in underlying assumptions such as metallicity and star formation history.

Overall, the paper presents a robust methodological approach to address cosmological parameter estimation. Further advancements in observational data and model sophistication will likely refine these estimates and could potentially resolve existing tensions in measures of the Hubble parameter. This research may also lead to new insights into the detailed evolutionary history of LRGs and other elliptical galaxy populations.