We present deep HST near-IR and Spitzer mid-IR observations of a large sample of spectroscopically-confirmed galaxies at z \geq 6 .
The sample consists of 51 Ly \alpha emitters ( LAEs ) at z \simeq 5.7 , 6.5 , and 7.0 , and 16 Lyman-break galaxies ( LBGs ) at 5.9 \leq z \leq 6.5 .
The near-IR images were mostly obtained with WFC3 in the F125W and F160W bands , and the mid-IR images were obtained with IRAC in the 3.6 \mu m and 4.5 \mu m bands .
Our galaxies also have deep optical imaging data from Subaru Suprime-Cam .
We utilize the multi-band data and secure redshifts to derive their rest-frame UV properties .
These galaxies have steep UV continuum slopes roughly between \beta \simeq - 1.5 and –3.5 , with an average value of \beta \simeq - 2.3 , slightly steeper than the slopes of LBGs in previous studies .
The slope shows little dependence on UV continuum luminosity except for a few of the brightest galaxies .
We find a statistically significant excess of galaxies with slopes around \beta \simeq - 3 , suggesting the existence of very young stellar populations with extremely low metallicity and dust content .
Our galaxies have moderately strong rest-frame Ly \alpha equivalent width ( EW ) in a range of \sim 10 to \sim 200 Å .
The star-formation rates are also moderate , from a few to a few tens solar masses per year .
The LAEs and LBGs in this sample share many common properties , implying that LAEs represent a subset of LBGs with strong Ly \alpha emission .
Finally , the comparison of the UV luminosity functions between LAEs and LBGs suggests that there exists a substantial population of faint galaxies with weak Ly \alpha emission ( EW < 20 Å ) that could be the dominant contribution to the total ionizing flux at z \geq 6 .