Research On IRX-β Relation With Integral Field Spectroscopy

It has been found that the infrared-to-ultraviolet luminosity ratio (IRX= LIR/LUV) and ultraviolet spectral slope (β) have a tight correlation in starburst galaxies, while in the IRX-3 diagram the distribution of normal galaxies is deviated from the IRX-β relation of starburst galaxies and has a much larger scatter. The main cause which drives normal star forming galaxies to disperse is still controversial.Star forming regions are much simpler in both morphology and physical properties than galaxies, so their photometric and spectroscopic properties are more easily and ac-curately determined. In this work the star forming regions of nearby galaxy NGC 628 are selected as our samples, and we have used the integral field spectroscopy and multi-band photometric images to study the IRX-β relation of H ii regions in NGC 628. The spectrum data came from PPAK IFS Galaxies Survey(PINGS), ultraviolet and in-frared images were taken by GALEX and Spitzer respectively. The Integral field spec-troscopy should provider a great opportunity to extend these studies to a much higher accuracy. What’s more, the degeneration effect in stellar age, metallicity and dust ex-tinction can be effectively avoided when photometrc images were used. Finally we adopted photometry and spectral fitting to derive some physical properties for each H ii region, including Dn(4000) index, stellar ages, Ho emission equivalent width EW(Ho), star formation rate (SFR), oxygen abundance and Hα/Hβ line ratio.Both the Dn(4000), stellar population age, star formation rate and EW(Ha) corre-lates with parameter dp, which represents deviation distance from the starburst IRX-β relation, and the correlation is more obvious for EW(Ha). These results indicate that the stellar age and star forming activity are responsible for the devation and scatter of H ii regions in the IRX-β diagram. The ultraviolet color of stellar population with younger stars and stronger star forming activity tends to be much redder at given IRX. In addition, we find no correlations between Hα/Hβ line ratio, metallicity and dp in our samples, indicating the differences both in metallicity and dust extinction are not the main cause which drives normal star forming galaxies to disperse.From the above analysis, our main conclusion is that the difference in star forma-tion history (SFH) of galaxies (star forming regions) is the primary cause. Meanwhile, we find that EW(Hq) charactering star birthrate parameter correlates with parameter dp. However there is still a considerable dispersion in the relation, this could be caused by kinds of reasons. For example, although the dust type and geometry of H ii regions are simple, the contribution of underlying stellar populations still can not be ignored. Both older stars and larger dust attenuation can make UV color get redder, especially for the regions near the galaxy center. In order to evaluate the impact of dust factors on IRX-β relation more accurately, we need a more reliable method to reduce the effect of old stellar population.