Context.
M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular ...clouds.
Aims.
We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum.
Methods.
We mapped the emission of gas and dust in M 33 using the far-infrared lines of C
II
and O
I
(63
μ
m) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the
Herschel
Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H
II
regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24
μ
m from
Spitzer
/MIPS, and at 70
μ
m, 100
μ
m, and 160
μ
m from
Herschel
/PACS were combined to obtain a map of the TIR.
Results.
TIR and C
II
intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of
G
0, obs
∼ 2 to 200 in units of the average Galactic radiation field. The binned C
II
/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the C
II
emission, as estimated from VLA H
I
data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the O
I
data before comparing it with models of photon dominated regions. Most of the ratios of C
II
/O
I
and (C
II
+O
I
)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at
n
∼ 2 × 10
2
cm
−3
,
G
0
∼ 60, and a second low-FUV solution at
n
∼ 10
4
cm
−3
,
G
0
∼ 1.5.
Conclusions.
The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.
Infrared emission is an invaluable tool for quantifying star formation in galaxies. Because the 8 mum polycyclic aromatic hydrocarbon (PAH) emission has been found to correlate with other well-known ...star formation tracers, it has widely been used as a star formation rate (SFR) tracer. There are, however, studies that challenge the accuracy and reliability of the 8 mum emission as a SFR tracer. Our study, part of the Herschel (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA) M33 Extended Survey (HERM33ES) open time key program, aims at addressing this issue by analyzing the infrared emission from the nearby spiral galaxy M33 at die high spatial scale of ~75 pc. We find that the majority of the 8 mum emission is fully consistent with heating by the diffuse interstellar medium, similar to what recently determined for the dust emission in M31 by Draine et al.
We have carried out a detailed modeling of the dust Spectral Energy Distribution (SED) of the nearby, starbursting dwarf galaxy NGC 4214. A key point of our modeling is that we distinguish the ...emission from (i) HII regions and their associated photodissociation regions (PDRs) and (ii) diffuse dust. For both components we apply templates from the literature calculated with a realistic geometry and including radiation transfer. The large amount of existing data from the ultraviolet (UV) to the radio allows the direct measurement of most of the input parameters of the models. We achieve a good fit for the total dust SED of NGC 4214. In the present contribution we describe the available data, the data reduction and the determination of the model parameters, whereas a description of the general outline of our work is presented in the contribution of Lisenfeld et al. in this volume.
Dust properties and distribution in dwarf galaxies Lisenfeld, Ute; Relaño, Monica; Vílchez, José ...
Proceedings of the International Astronomical Union,
06/2008, Letnik:
4, Številka:
S255
Journal Article
Recenzirano
Odprti dostop
We present a study of the extinction, traced by the Balmer decrement, in HII regions in the dwarf galaxies NGC 1569 and NGC 4214. We find that the large-scale extinction around the most prominent HII ...regions in both galaxies forms a shell in which locally the intrinsic extinction can adopt relatively high values (AV = 0.8 − 0.9 mag) despite the low metallicity and thus the low overall dust content. The small-scale extinction (spatial resolution ~0.3″) shows fluctuations that are most likely due to variations in the dust distribution. We compare the distribution of the extinction to that of the dust emission, traced by Spitzer emission at 8 and 24μm, and to the emission of cold dust at 850μm. We find in general a good agreement between all tracers, except for the 850μm emission in NGC 4214 which is more extended than the extinction and the other emissions. Whereas in NGC 1569 the dust emission at all wavelengths is very similar, NGC 4214 shows spatial variations in the 24-to-850μm ratio. We furthermore compared the 24μm and the extinction-corrected Hα emission from HII regions in a sample of galaxies with a wide range of metallicities and found a good correlation, independent of metallicity. We suggest that this lack of dependence on metallicity might be due to the formation of dust shells with a relatively constant opacity, like the ones observed here, around ionizing stars.
M33 is a gas rich spiral galaxy of the Local Group. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. We mapped the emission of gas ...and dust in M33 using the far-infrared lines of CII and OI(63um) and the TIR. The line maps were observed with Herschel/PACS. These maps have 50pc resolution and form a ~370pc wide stripe along its major axis covering the sites of bright HII regions, but also more quiescent arm and inter-arm regions from the southern arm at 2kpc galacto-centric distance to the south out to 5.7kpc distance to the north. Full-galaxy maps of the continuum emission at 24um from Spitzer/MIPS, and at 70um, 100um, and 160um from PACS were combined to obtain a map of the TIR. TIR and CII intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G0,obs~2 to 200 in units of the average Galactic radiation field. The binned CII/TIR ratio drops with rising TIR, with large, but decreasing scatter. Fits of modified black bodies (MBBs) to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the OI data before comparing it with models of photon dominated regions (PDRs). Most of the ratios of CII/OI and (CII+OI)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n~2x10^2 cm-3, G0~60, and and a second low-FUV solution at n~10^4 cm-3, G0~1.5. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ~1. A fraction of the gas may, however, be represented by the second solution.
The goal of the present study is to establish the physical origin of dust heating and emission based on radiation transfer models, which self-consistently connect the emission components from diffuse ...dust and the dust in massive star forming regions. NGC 4214 is a nearby dwarf galaxy with a large set of ancillary data, ranging from the ultraviolet (UV) to radio, including maps from SPITZER, HERSCHEL and detections from PLANCK. We mapped this galaxy with MAMBO at 1.2 mm at the IRAM 30 m telescope. We extract separate dust emission components for the HII regions (plus their associated PDRs on pc scales) and for the diffuse dust (on kpc scales). We analyse the full UV to FIR/submm SED of the galaxy using a radiation transfer model which self-consistently treats the dust emission from diffuse and SF complexes components, considering the illumination of diffuse dust both by the distributed stellar populations, and by escaping light from the HII regions. While maintaining consistency with the framework of this model we additionally use a model that provides a detailed description of the dust emission from the HII regions and their surrounding PDRs on pc scales. Due to the large amount of available data and previous studies for NGC 4214 very few free parameters remained in the model fitting process. We achieve a satisfactory fit for the emission from HII+PDR regions on pc scales, with the exception of the emission at 8\mi, which is underpredicted by the model. For the diffuse emission we achieve a good fit if we assume that about 30-70% of the emission escaping the HII+PDR regions is able to leave the galaxy without passing through a diffuse ISM, which is not an unlikely scenario for a dwarf galaxy which has recently undergone a nuclear starburst. We determine a dust-to-gas mass ratio of 350-390 which is close to the expected value based on the metallicity.
Because the 8 {\mu}m polycyclic aromatic hydrocarbon (PAH) emission has been found to correlate with other well-known star formation tracers, it has widely been used as a star formation rate (SFR) ...tracer. There are, however, studies that challenge the accuracy and reliability of the 8 {\mu}m emission as a SFR tracer. Our study, part of the Herschel M33 Extended Survey (HERM33ES) open time key program, aims at addressing this issue by analyzing the infrared emission from the nearby spiral galaxy M33 at the high spatial scale of 75 pc. Combining data from the Herschel Space Observatory and the Spitzer Space Telescope we find that the 8 {\mu}m emission is better correlated with the 250 {\mu}m emission, which traces cold interstellar gas, than with the 24 {\mu}m emission. The L(8)/L(24) ratio is highly depressed in 24 {\mu}m luminous regions, which correlate with known HII regions. We also compare our results with the dust emission models by Draine & Li (2007). We confirm that the depression of 8 {\mu}m PAH emission near star-forming regions is higher than what is predicted by models; this is possibly an effect of increased stellar radiation from young stars destroying the dust grains responsible for the 8 {\mu}m emission as already suggested by other authors. We find that the majority of the 8 {\mu}m emission is fully consistent with heating by the diffuse interstellar medium, similar to what recently determined for the dust emission in M31 by Draine at al. (2013). We also find that the fraction of 8 {\mu}m emission associated with the diffuse interstellar radiation field ranges between 60% and 80% and is 40% larger than the diffuse fraction at 24 {\mu}m.
PDF
