Candidates of z ~ 5.5--7 Galaxies in the HST Ultra Deep Field

We report results from our $z\simeq 5.5$--7 galaxy search in the HST Ultra Deep Field (UDF). Using the 400-orbit of ACS data, we found 108 plausible $5.5\leq z\leq 6.5$ (or $z\simeq 6$ for short) candidates to $m_{AB}(z_{850})=30.0$ mag. The contamination to the sample, either due to image artifacts or known types of astronomical objects, is likely negligible. The inferred surface densities of $z\simeq 6$ galaxies are consistent with our earlier predictions from $m_{AB}(z_{850})=26.5$ to 28.5 mag. After correcting for detection incompleteness, the counts of $z\simeq 6$ candidates to $m_{AB}(z_{850})=29.2$ mag suggests that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between $\alpha=-1.8$ and -1.9, which is sufficient to account for the entire Lyman photon budget necessary to complete the reionization of the universe at $z\simeq 6$. We also searched for $z\simeq 6.5$--7 candidates using the UDF NICMOS data, and have found four candidates to $J_{110}=27.2$ mag. However, the infrared colors of three candidates cannot be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at $z\simeq 7$ of 0.36 per arcmin$^2$ to $J_{110}=26.6$ mag, which suggest a noticeable drop in the LF amplitude from $z\simeq 6$ to $z\simeq 7$.Comment: accepted for publication in ApJL (submitted March 17 2004, accepted July 20 2004

The Major Sources of the Cosmic Reionizing Background at z ~ 6

In this paper, we address which sources contributed most of the reionizing photons. Our argument assumes that the reionization ended around z ~ 6 and that it was a relatively quick process, i.e., that there was a non-negligible fraction of neutral hydrogen in the Universe at somewhat earlier epochs. Starting from our earlier estimate of the luminosity function (LF) of galaxies at z ~ 6, we quantitatively show that the major sources of reionization are most likely galaxies with L < L*. Our approach allows us to put stronger constraints to the LF of galaxies at z ~ 6. To have the Universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than $\alpha=-1.6$, which is the value measured at lower redshifts (z ~ 3), unless either the normalization (Phi*) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Phi* is actually lower than what we assumed by a factor of two, a steep slope close to $\alpha=-2.0$ is required. Our LF predicts a total of 50 -- 80 z ~ 6 galaxies in the HST Ultra Deep Field (UDF) to a depth of AB=28.4 mag, which can be used to constraint both Phi* and $\alpha$. We conclude that the least luminous galaxies existing at this redshift should reach as low as some critical luminosity in order to accumulate the entire reionizing photon budget. On the other hand, the existence of significant amounts of neutral hydrogen at slightly earlier epochs, e.g. z ~ 7, requires that the least luminous galaxies should not be fainter than another critical value (i.e., the LF should cut-off at this point).Comment: ApJL in press (Jan 1, 2004 issue

Galaxy Formation In The Reionization Epoch As Hinted By Wide Field Camera 3 Observations Of The Hubble Ultra Deep Field

We present a large sample of candidate galaxies at z~7--10, selected in the HUDF using the new observations made by the HST/WFC3. Our sample is composed of 20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of the z-dropouts are close to what predicted by earlier studies, however, those of the Y- and J-dropouts are quite unexpected. While no Y- or J-dropouts have been found at AB < 28.0 mag, their surface densities seem to increase sharply at fainter levels. While some of these candidates seem to be close to foreground galaxies and thus could possibly be gravitationally lensed, the overall surface densities after excluding such cases are still much higher than what would be expected if the luminosity function does not evolve from z~7 to 10. Motivated by such steep increases, we tentatively propose a set of Schechter function parameters to describe the LFs at z~8 and 10. As compared to their counterpart at z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. Although such parameters are not yet demanded by the existing observations, they are allowed and seem to agree with the data better than other alternatives. If these LFs are still valid beyond our current detection limit, this would imply a sudden emergence of a large number of low-luminosity galaxies when looking back in time to z~10, which, while seemingly exotic, would naturally fit in the picture of the cosmic hydrogen reionization. These early galaxies could easily account for the ionizing photon budget required by the reionization, and they would imply that the global star formation rate density might start from a very high value at z~10, rapidly reach the minimum at z~7, and start to rise again towards z~6. In this scenario, the majority of the stellar mass that the universe assembled through the reionization epoch seems still undetected by current observations at z~6. [Abridged]Comment: accepted for publication in Research in Astronomy and Astrophysic

Searching for z ~ 6 Objects with the HST Advanced Camera for Surveys: Preliminary Analysis of a Deep Parallel Field

Recent results suggest that z ~ 6 marks the end of the reionization era. A large sample of objects at z ~ 6, therefore, will be of enormous importance, as it will enable us to observationally determine the exact epoch of the reionization and the sources that are responsible for it. With the HST Advanced Camera for Surveys (ACS) coming on line, we now have an unique opportunity to discover a significant number of objects at z ~ 6. The pure parallel mode implemented for the Wide Field Camera (WFC) has greatly enhanced this ability. We present our preliminary analysis of a deep ACS/WFC parallel field at |b|=74.4^o. We find 30 plausible z ~ 6 candidates, all of which have S/N > 7 in the F850LP-band. The major source of contamination could be faint Galactic cool dwarfs, and we estimated that they would contribute at most 4 objects to our candidate list. We derived the cumulative number density of galaxies at 6.0 <= z <= 6.5 as 2.3 arcmin^{-2} to a limit of 28.0 mag in the F850LP-band, which is slightly higher than our prediction. If this is not due to an underestimated contamination rate, it could possibly imply that the faint-end slope of the z ~ 6 luminosity function is steeper than alpha=-1.6. At the very least, our result suggests that galaxies with L<L^* do exist in significant number at z ~ 6 and they could be the major sources that contributed to the reionizing photons.Comment: accepted for publication in the ApJL; replaced with slightly modified versio

The Stellar Masses and Star Formation Histories of Galaxies at z ≈ 6: Constraints from Spitzer Observations in the Great Observatories Origins Deep Survey

Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z approx 6 based on the i_(775)-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i_(775)-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~10^(10) M_⊙ and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ≈ 6 galaxies. The existence of such massive galaxies at z ≈ 6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i_(775)-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≾ 40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ≈ 6, which is 1.1-6.7 × 10^6 M_⊙ Mpc^(-3). The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability