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Dust Heating in Nearby Galaxies

Kong, Lingjie

[Thesis]. Manchester, UK: The University of Manchester; 2013.

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Abstract

Dust is an important part of the interstellar medium (ISM). Dust absorbs ultravioletand optical star light and re-emits in the mid- and far-infrared light. More than onethird of the total star light in the universe is absorbed and re-emitted by dust.Observations show that dust emission is strongly correlated with star formationrates. Recent research with Herschel Space Observatory data has shown that dustemission at ≥ 250 µm appears to be heated by the total stellar population ratherthan just star forming regions, questioning the use of dust to measure star formation.To further study dust heating sources, we carried out an analysis on NGC4254,NGC4303, NGC4321 and NGC4579 using 100, 160, 250, 350 and 500 µm imagesfrom the Herschel Reference Survey and Herschel Virgo Cluster Survey. We usedHα images to trace star forming regions and 1.6 µm image to trace the total stellarpopulations. Colour temperature maps (based on 100/160, 160/250, 250/350 and350/500 µm ratio maps) were compared to the Hα and 1.6 µm images. We maderatio plots against Hα and 1.6 µm pixel by pixel to ﬁnd out whether there is a linearrelation.All the infrared ratios in four galaxies show strong correlation both with Hα and1.6 µm data. NGC4303, NGC4321 and NGC4579 shows no signiﬁcant diﬀerencebetween Hα and 1.6 µm data which indicate the dust in these three galaxies maybeheated by both star forming regions and the evolved star regions. NGC4254 shows abetter correlation between the colour temperature maps with the Hα image than the1.6 µm image, which means more dust is heated by star forming regions than by theevolved stars. There is no clear evidence for a dust component which is heated onlyby the evolved stars. From the colour temperature maps, I ﬁnd dust temperaturesin the range of 12-30 K assuming that β = 2. There is no evidence for dust at lowertemperature than 12 K.All the four galaxies show hotter dust at longer wavelength which is unexpected.Changing the value of β to β = 2.2 ﬁxes the problem. Further study is needed todetermine if β is indeed higher than 2 in these and possibly other galaxies.

Layman's Abstract

Dust is widely distributed in galaxies. It absorbs star light and re-emit in Infrared and Far-Infrared. One third of star light is absorbed and re-emitted by dust in galaxies. Star light represent what type of star and what life stage the star is. So if we know which dust emission present which star light, we can use dust emission as a tracer for the star formation. This thesis studied dust emissions in four nearby galaxies at Far-Infrared and tried to find out if dust is a good tracer for star forming regions. The conclusion is not clear because we found dust emission also trace evolved stars equally well. We need further study to find out the truth.