A large Hα survey at z=2.23, 1.47, 0.84 & 0.40: the 11 Gyr evolution of star-forming galaxies from HiZELS (1202.3436v2)

Abstract: This paper presents new deep and wide narrow-band surveys undertaken with UKIRT, Subaru and the VLT; a combined effort to select large, robust samples of H-alpha (Ha) emitters at z=0.40, 0.84, 1.47 and 2.23 (corresponding to look-back times of 4.2, 7.0, 9.2 and 10.6 Gyrs) in a uniform manner over ~2 deg^2 in the COSMOS and UDS fields. The deep Ha surveys reach ~3M_sun/yr out to z=2.2 for the first time, while the wide area and the coverage over two independent fields allow to greatly overcome cosmic variance. A total of 1742, 637, 515 and 807 Ha emitters are homogeneously selected at z=0.40, 0.84, 1.47 and 2.23, respectively, and used to determine the Ha luminosity function and its evolution. The faint-end slope is found to be -1.60+-0.08 over z=0-2.23, showing no evolution. The characteristic luminosity of SF galaxies, L*, evolves significantly as log[L*(z)]=0.45z+log[L*(z=0)]. This is the first time Ha has been used to trace SF activity with a single homogeneous survey at z=0.4-2.23. Overall, the evolution seen in Ha is in good agreement with the evolution seen using inhomogeneous compilations of other tracers of star formation, such as FIR and UV, jointly pointing towards the bulk of the evolution in the last 11 Gyrs being driven by a strong luminosity increase from z~0 to z~2.2. Our uniform analysis reveals an Ha star formation history of the Universe which can be simply parameterised by log(SFRD)=-2.1/(1+z) for z<2.23. Both the shape and normalisation of the Ha star formation history are consistent with the measurements of the stellar mass density growth, confirming that our Ha analysis traces the bulk of the formation of stars in the Universe up to z~2.2. Star formation activity since z~2.2 is responsible for ~95% of the total stellar mass density observed locally today (with half being assembled between z=1.2-2.2, and the rest since z<1.2).

• The paper presents the largest homogeneous sample of Hα emitters, enabling a robust measurement of the Hα luminosity function over cosmic time.
• It reveals a nearly 10× increase in characteristic luminosity with a stable faint-end slope (α ≈ -1.60), challenging previous expectations.
• It reconstructs the evolution of the star formation rate density, linking Hα observations with stellar mass buildup and forecasting modest future growth.

Evolution of Star-forming Galaxies over 11 Billion Years as Traced by H$\alpha$ Emission Lines

The paper in focus presents an extensive paper on the evolution of star-forming galaxies over an 11-billion-year time span by utilizing a large H$\alpha$ survey. This investigation leverages multi-epoch, deep, and wide-area narrow-band imaging across the COSMOS and UDS fields, aiming to provide a comprehensive view of how H$\alpha$ emitters evolve from redshift $z=0.40$ to $z=2.23$. The employed datasets span observations from the UKIRT, Subaru, and VLT telescopes, offering a unique advantage by probing two independent fields to mitigate cosmic variance.

Key Contributions and Findings

A principal outcome of this paper is the establishment of the largest homogeneous sample of H$\alpha$ emitters at the aforementioned redshifts, numbering in the thousands across multiple epochs. This dataset enables robust determinations of the H$\alpha$ luminosity function (LF) and its evolution, revealing that the LF exhibits significant changes over time, predominantly characterized by an evolutionary increase in the characteristic luminosity $L^*_{\rm H\alpha}$ by about an order of magnitude from the local universe to $z\approx2.23$.

The paper posits that the faint-end slope ($\alpha$) remains relatively stable, with a value of $\alpha \approx -1.60$, challenging previously held views that it steepens significantly with redshift. This finding implies that the faint-end of the H$\alpha$ LF, consisting of low-luminosity star-forming galaxies, may be less sensitive to cosmic evolution than previously thought.

Furthermore, the paper contributes to a more consistent reconstruction of the star formation rate density (SFRD) evolution, corroborating the narrative of a rising SFRD towards $z \approx 2$, followed by a decline from that epoch onward. Impressively, this work aligns the evolution of SFRD traced by H$\alpha$ emitters with stellar mass buildup over cosmic time, concluding that 95% of the present-day stellar mass density has been accumulated since $z \approx 2.23$. If the historical decline in SFRD persists, this research forecasts only a mild additional growth of approximately 5% in stellar mass density.

Implications and Future Directions

The implications of these findings are manifold, particularly in advancing our understanding of galaxy evolution. The paper's depiction of a more stable faint-end slope contrasts with the expectation of universally steeper slopes at higher redshifts and suggests that cosmic mechanisms driving star formation across different epochs may homogenize the evolution of low-luminosity galaxies. This could influence theoretical models of galaxy evolution by indicating that processes affecting the faint-end, such as feedback and environmental effects, might be more uniform over cosmic time than anticipated.

Future research can leverage these findings to isolate the factors contributing to the observed stability of the faint-end slope, potentially involving simulations that incorporate various feedback mechanisms. Additionally, the forecast provided regarding the stellar mass density peak emphasizes the continued necessity of observing galaxies at even higher redshifts—possibly with the James Webb Space Telescope—to extend the observational frontier and build upon this characterization of cosmic star formation history.

In summary, this paper offers a crucial insight into the nature and progression of star-forming galaxies over a significant portion of the universe's history, solidifying the role of H$\alpha$ emission as a tracer for cosmic star formation and challenging preconceptions about the evolution of galactic populations.