The UV spectral slope beta and stellar population of most active star-forming galaxies at z~4

Abstract

We investigate the stellar population of star-forming galaxies at $z\sim 4$ by focusing on their slope of rest-frame ultraviolet (UV) continuum, $\beta$ where $f_{\lambda }\propto {\lambda }^{\beta }$. We investigate the sample of bright Lyman Break Galaxies (LBGs) with $i^{\prime }\le 26.0$ in the Subaru/XMM-Newton Deep Survey field by using the SED fitting analysis. We find that the apparently redder (${\beta }_{\mathrm{obs}}>-1.73$) LBGs tend to be dusty ($\mathrm{Av}>1.0$), have the young stellar population (${\beta }_{\mathrm{int}}<-2.42$), and the intrinsically active star-forming galaxies (SFR $\gtrsim$ a few $\times 10^2$ ${\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$). It means that a significant fraction of the UV-selected LBGs at $z\sim 4$ is on-going active and dust obscured star-forming galaxies. We compare the IR to UV luminosity ratio assuming the dust attenuation laws with the sub-millimeter observations from previous works. The result suggests that the Calzetti-like dust attenuation law is preferable for the active and dusty star-forming LBGs at $z=4$. We also find that an extrapolation of the ${\beta }_{\mathrm{int}}$-$M_{\mathrm{UV},\mathrm{int}}$ relation toward the fainter magnitude range below our sample magnitude limit intersects the ${\beta }_{\mathrm{obs}}$-$M_{\mathrm{UV},\mathrm{obs}}$ relation previously obtained in the deeper narrow-area observations at $M_{\mathrm{UV}}=-18.9$ and $\beta =-1.94$, which coincides with the break point of ${\beta }_{\mathrm{obs}}$-$M_{\mathrm{UV},\mathrm{obs}}$ relation observed so far. The coincidence suggest that we see the almost dust-free population at $M_{\mathrm{UV},\mathrm{obs}}\gtrsim -18.9$.

20 pages, 20 figures

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