Transcript The IRX-* relation of star formation regions in NGC628 based on

The IRX-β relation of star formation regions in
NGC628 based on integral field spectroscopy
Cheng-Yun Ye
Department of Astronomy at USTC
2015.9.18
Supervised by Prof. Xu Kong
IRX-β relation
• A major obstacle to understanding the intrinsic SED of a galaxy is
correcting for the ultraviolet (UV) and optical flux lost to dust
attenuation, which is re-emitted in the infrared(IR).
• IRX ≡ log(LTIR/LFUV), where LTIR is the luminosity of the infrared thermal
radiation of dust and LFUV is the far-ultraviolet monochromatic
luminosity
• ultraviolet spectral slope (β,defined as fλ∝λβ, where 1250Å ≤ λ ≤
2600Å), UV color or UV luminosity ratio as a surrogate for the
traditional UV spectral slope
For 57 nearby starburst galaxies, there is
a remarkably tight correlation between
ratio of total far-infrared to ultraviolet
luminosity and ultraviolet spectral slope β.
Meurer et al. 1999
Kong et al. 2004
Star formation history is relevant to
the second parameter affecting the
IRX-UV relation.
Mao et al. 2012
The age signature is indicated by systematic offsets of
the local background regions toward redder color ranges
from the UV clusters in the IRX–UV diagrams due to the
intrinsic redder UV colors for evolved stellar populations.
My work
overview of the data
• NGC 628 is a nearby (D≈7.3Mpc;z = 0.00219) nearly face-on spiral
galaxy
• make use of IFS observations of NGC 628 taken as part of the PINGS
survey
• The data covers the spectral range between 3700Å -7100Å, with a
spectral resolution of FWHM ≈ 8Å, comprising 11094 individual
spectra, covering a nearly circular field-of-view of ~6 arcmin in
diameter
• FUV,NUV from GALEX, 3.6um,8um,24um from Spitzer
• Star formation region:
SFH,dust,geometry more simple than
galaxy
• The segregation of HII regions and the
extraction of the corresponding
spectra is performed using HIIexplorer
Stellar populations age
Dn(4000) index
Dn(4000) is an indicator of the stellar population
mean age and small in young, massive stars.
SFR surface density
SFR(M⊙. yr-1) = 5.3 × 10-42 [L(Hα)obs + 0.031L(24μm)]
Hα equivalent width EW(Hα)
EW(Hα) can be used as a proxy of birthrate parameter b,
the ratio of the current star formation rate to the average
over the lifetime of the galaxy.
Oxygen abundance
Metallicity correlated well with ultraviolet spectral slope
(Heckman et al. 1998), and it may have a significant
additional effect on the IRX-β relation offset.
Oxygen abundance
Color excess E(B-V)
Intrinsic ultraviolet spectral slope β0
ANUV = -0.0495x3 + 0.4718x2 +
0.8998x + 0.2269
AFUV = -0.0333y3 + 0.3522y2 +
1.1960y + 0.4967
x = log(LTIR/LNUV), y = LTIR/LFUV
Buat et al. 2005
Conclusions
• The EW(Hα) of HII regions correlates clearly with perpendicular distance
dp and ultraviolet spectral slope βGLX. This clearly hints that birthrate
parameter affects the distribution of star formation regions on the IRX-β
diagram. There a considerable correlation between stellar population
ages, Dn(4000), SFR and dp, βGLX, with a non negligible dispersion.
• For oxygen abundance from O3N2 and N2 index, we do not observe it
correlates with dP or βGLX, and also for Av.
• Intrinsic ultraviolet spectral slope β0 does not show clearly trend with
ages, Dn(4000), but a significant correlation with EW(Hα).
Thank you