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Register a new userDust in dwarf galaxies: The case of NGC 4214
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We have carried out a detailed modelling of the dust heating and emission in the nearby, starbursting dwarf galaxy NGC 4214. Due to its proximity and the great wealth of data from the UV to the millimeter range (from GALEX, HST, {it Spitzer}, Herschel, Planck and IRAM) it is possible to separately model the emission from HII regions and their associated photodissociation regions (PDRs) and the emission from diffuse dust. Furthermore, most model parameters can be directly determined from the data leaving very few free parameters. We can fit both the emission from HII+PDR regions and the diffuse emission in NGC 4214 with these models with normal dust properties and realistic parameters.
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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.
We present a compilation of galaxies in the NGC 1023 Group, an accumulation of late-type galaxies at a distance of 10 Mpc. Members at high and intermediate luminosities were identified from their spectroscopic velocities. Members at low luminosities were identified from their morphologies on wide-field CCD images. The faint-end slope is in the range -1.27 < alpha < -1.12. There is evidence for two dwarf galaxy populations: one in the halo of NGC 1023 that is dominated by dwarf elliptical galaxies, and one in the infall region surrounding NGC 1023 that contains mainly dwarf irregular galaxies. Similar distinctive populations are observed in the Local Group.
The [CII] 158 um fine structure line is one of the dominant cooling lines in the interstellar medium (ISM) and is an important tracer of star formation. Recent velocity-resolved studies with Herschel/HIFI and SOFIA/GREAT showed that the [CII] line can constrain the properties of the ISM phases in star-forming regions. The [CII] line as a tracer of star formation is particularly important in low-metallicity environments where CO emission is weak because of the presence of large amounts of CO-dark gas. The nearby irregular dwarf galaxy NGC 4214 offers an excellent opportunity to study an actively star-forming ISM at low metallicity. We analyzed the spectrally resolved [CII] line profiles in three distinct regions at different evolutionary stages of NGC 4214 with respect to ancillary HI and CO data in order to study the origin of the [CII] line. We used SOFIA/GREAT [CII] 158 um observations, HI data from THINGS, and CO(2-1) data from HERACLES to decompose the spectrally resolved [CII] line profiles into components associated with neutral atomic and molecular gas. We use this decomposition to infer gas masses traced by [CII] under different ISM conditions. Averaged over all regions, we associate about 46% of the [CII] emission with the HI emission. However, we can assign only around 9% of the total [CII] emission to the cold neutral medium (CNM). We found that about 79% of the total molecular hydrogen mass is not traced by CO emission. On average, the fraction of CO-dark gas dominates the molecular gas mass budget. The fraction seems to depend on the evolutionary stage of the regions: it is highest in the region covering a super star cluster in NGC 4214, while it is lower in a more compact, more metal-rich region.
It has been claimed in the recent literature that a non-trivial relation between the mass of the most-massive star, mmax, in a star cluster and its embedded star cluster mass (the mmax-Mecl relation) is falsified by observations of the most-massive stars and the Halpha luminosity of young star clusters in the starburst dwarf galaxy NGC 4214. Here it is shown by comparing the NGC 4214 results with observations from the Milky Way that NGC 4214 agrees very well with the predictions of the the mmax-Mecl relation and the integrated galactic stellar initial mass function (IGIMF) theory and that this difference in conclusions is based on a high degree of degeneracy between expectations from random sampling and those from the mmax-Mecl relation, but are also due to interpreting mmax as a truncation mass in a randomly sampled IMF. Additional analysis of galaxies with lower SFRs than those currently presented in the literature will be required to break this degeneracy.
We have performed an X-ray study of the nearby barred spiral galaxy NGC1672, primarily to ascertain the effect of the bar on its nuclear activity. We use both Chandra and XMM-Newton observations to investigate its X-ray properties, together with supporting high-resolution optical imaging data from the Hubble Space Telescope (HST), infrared imaging from the Spitzer Space Telescope, and ATCA ground-based radio data. We detect 28 X-ray sources within the D25 area of the galaxy, many of which correlate spatially with star-formation in the bar and spiral arms, while two are identified as background galaxies in the HST images. Nine of the X-ray sources are ULXs, with the three brightest (LX > 5E39 erg/s) located at the ends of the bar. With the spatial resolution of Chandra, we are able to show for the first time that NGC1672 possesses a hard (Gamma~1.5) nuclear X-ray source with a 2-10 keV luminosity of 4E38 erg/s. This is surrounded by an X-ray bright circumnuclear star-forming ring, comprised of point sources and hot gas, which dominates the 2-10 keV emission in the central region of the galaxy. The spatially resolved multiwavelength photometry indicates that the nuclear source is a low-luminosity AGN (LLAGN), but with star formation activity close to the central black hole. A high-resolution multiwavelength survey is required to fully assess the impact of both large-scale bars and smaller-scale phenomena such as nuclear bars, rings and nuclear spirals on the fueling of LLAGN.
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