Context : The afterglows and host galaxies of long gamma-ray bursts ( GRBs ) offer unique opportunities to study star-forming galaxies in the high- z Universe .
Until recently , however , the information inferred from GRB follow-up observations was mostly limited to optically bright afterglows , biasing all demographic studies against sight-lines that contain large amounts of dust .

Aims : Here we present afterglow and host observations for a sample of bursts that are exemplary of previously missed ones because of high visual extinction ( A _ { V } ^ { GRB } \gtrsim 1 mag ) along the sight-line .
This facilitates an investigation of the properties , geometry and location of the absorbing dust of these poorly-explored host galaxies , and a comparison to hosts from optically-selected samples .

Methods : This work is based on GROND optical/NIR and Swift /XRT X-ray observations of the afterglows , and multi-color imaging for eight GRB hosts .
The afterglow and galaxy spectral energy distributions yield detailed insight into physical properties such as the dust and metal content along the GRB sight-line as well as galaxy-integrated characteristics like the host ’ s stellar mass , luminosity , color-excess and star-formation rate .

Results : For the eight afterglows considered in this study we report for the first time the redshift of GRB 081109 ( z = 0.9787 \pm 0.0005 ) , and the visual extinction towards GRBs 081109 ( A _ { V } ^ { GRB } = 3.4 _ { -0.3 } ^ { +0.4 } mag ) and 100621A ( A _ { V } ^ { GRB } = 3.8 \pm 0.2 mag ) , which are among the largest ever derived for GRB afterglows .
Combined with non-extinguished GRBs , there is a strong anti-correlation between the afterglow ’ s metals-to-dust ratio and visual extinction .

The hosts of the dustiest afterglows are diverse in their properties , but on average redder ( \langle ( R - K ) _ { AB } \rangle \sim 1.6 mag ) , more luminous ( \langle L \rangle \sim 0.9 L ^ { \ast } ) and massive ( \langle \log M _ { \ast } [ M _ { \odot } ] \rangle \sim 9.8 ) than the hosts of optically-bright events .
We hence probe a different galaxy population , suggesting that previous host samples miss most of the massive , chemically-evolved and metal-rich members .
This also indicates that the dust along the sight-line is often related to host properties , and thus probably located in the diffuse ISM or interstellar clouds and not in the immediate GRB environment .
Some of the hosts in our sample , are blue , young or of small stellar mass illustrating that even apparently non-extinguished galaxies possess very dusty sight-lines due to a patchy dust distribution .

Conclusions : The afterglows and host galaxies of the dustiest GRBs provide evidence for a complex dust geometry in star-forming galaxies .
In addition , they establish a population of luminous , massive and correspondingly chemically-evolved GRB hosts .
This suggests that GRBs trace the global star-formation rate better than studies based on optically-selected host samples indicate , and the previously-claimed deficiency of high-mass host galaxies was at least partially a selection effect .