STAR FORMATION ALONG A CLUSTER-FEEDING FILAMENT DARIO FADDA IPAC / Caltech
Outline * Discovery of the filament * The obscured star formation in different environments * Radio observation and the density of the intra-filament medium * GALEX observations * Estimation of stellar masses * Star formation map
Outline * Discovery of the filament * The obscured star formation in different environments * Radio observation and the density of the intra-filament medium * GALEX observations * Estimation of stellar masses * Star formation map
We discovered a filament by observing the cluster A1763 with Spitzer (Fadda et al. 2008) and doing a spectroscopic follow-up survey with WIYN/Hydra. Galaxies with intense star formation detected in the IR (blue) live preferentially along the filament. Starbursts galaxies in the filament are twice as common as in the core or the external part of the cluster.
The filament is evident in a density map of the 24um sources belonging to the system at a redshift of 0.23. We can divide the system in 3 parts: a) core (red circle), b) filament (blue lines), and c) outskirts (green). In the figure, starbusts galaxies are pink points. Symbols are proportional to the ssfr.
By comparing the total SFR normalized by the richness of the three different environments, the filament has the highest total star formation. However, when considering the specific star formation, there is virtually no difference between the population of the three different environments.
The distribution of SED classes in the three environments is also very similar with the exception of early-type galaxies marginally detected in the IR. This shows that similar modes of star formation take place in galaxies in different environments (Biviano et al. 2011).
We observed the A1763 supercluster at 20cm with the VLA. Although the observations were not as deep as the Spitzer images (0.03mJy), we were able to find a similar overdensity of star-forming galaxies along the filament as well as a negligible AGN contamination (Edwards et al. 2010).
However, one of these AGN happened to be traveling along the filament and allowed us to estimate the density of the intra-filament medium (Edwards, Fadda & Frayer 2010) for the first time with this technique. The density is (1-20) E-29 g/cm3 consistent with another measurement using a X-ray detection in another cluster and theoretical simulations.
After having explored the dark side of the star formation, we obtained time on GALEX to complete our panchromatic vision of the supercluster. We were awarded 21 ks for a pointing on a filament. A separate program had already pointed the A1763 cluster in G4.
Blue: GI5 data on the filament Red: GI4 data on the A1763 cluster The NUV data on the filament are roughly half mag deeper than those on the cluster. FUV data are comparable because we didn't obtain all the FUV data due to the failure of the FUV detector.
Li et al. (2007) Eskew et al. (2012) To estimate stellar masses, we used the Maraston's (2005) models with a Kroupa IMF, solar metallicity, and a grid of E(B-V) between 0 and 1. It is interesting to note that popular methods to use WISE fluxes to estimate the masses seem to fail with high extinctions ( E(B-V) > 0.6). In the figures, we compare our values with the Li et al. (2007) and the Eskew et al. (2012) methods. Empty dots mark galaxies with extinction greater than 0.6.
We have a total of 350 members spectroscopically Of these: 95 UV + FIR 185 FIR only 12 UV only 58 no UV, no FIR confirmed. (green) (red) (black) (empty)
Sources with FUV detections tend to have low extinctions. Moreover, the ones without FIR counterparts (red dots), have typically very low extinctions and low FUV fluxes. Therefore, extinction corrections are generally not very important for these sources.
To compute the UV star formation, we adopted the Bouat (2005) correction for the extinction which gives an excellent agreement with the SFR computed using the FIR. For the points without FIR detection, we used the Meurer (1999) correction. For the points without UV detection, we used the Rieke (2011) relationship to derive the SFR from the FIR.
To improve the statistics, we used all the z-phot identified members. Density plot with stellar masses greater than 10.5 in log(m/msun). Red dots are the spectroscopic members and the two crosses indicate the centers of A1763 and A1770. A1770 shows up in the z-phot selection although we do not have enough spectroscopic members to confirm its existence.
SFR normalized to the density for galaxies more massive than 10.5 (in log(m/msun)) and SFR greater than 2 Msun/yr. A clear overdensity shows up along the filament between 3 and 7 Mpc from the center of the A1763 cluster. A similar plot for the ssfr appears flatter confirming the previous study based only on FIR detected sources.
Summary * Most of the sources detected in the UV have an infrared counterpart and the SFR based on the UV agrees well with the SFR based on the IR. * Stellar masses from an SED fitting using points from the UV to the IR allow us to identify a little population of low mass galaxies with high absorption which will be overlooked using only near-ir magnitudes as proxies. * Using UV and IR observations, we still identify an overdensity of SF between 3 and 7 Mpc from the center of A1763. The ssfr density map is more uniform.