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Register a new userBeyond the Solar Circle $-$ Trends in Massive Star Formation Between the Inner and Outer Galaxy
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We have compiled the most complete compact and ultracompact HII region catalogue to date via multi-wavelength inspection of survey data. We utilise data from the recently available SASSy 850$mu$m survey to identify massive star forming clumps in the outer Galaxy ($R_{rm{GC}}>8.5$ kpc) and cross-match with infrared and radio data of known UC HII regions from the RMS database. For the inner Galaxy sample ($R_{rm{GC}}<8.5$ kpc), we adopt the compact HII regions from previous works that used similar methods to cross match ATLASGAL with either CORNISH or RMS, depending on the location within the Galactic plane. We present a new UC HII region catalogue that more than doubles the original sample size of previous work, totaling 536 embedded HII regions and 445 host clumps. We examine the distance independent values of N$_{rm{Ly}}/$M and L$_{rm{bol}}/$M as proxies for massive star formation efficiency and overall star formation efficiency, respectively. We find a significant trend showing that L$_{rm{bol}}/$M decreases with increasing $R_{rm{GC}}$, suggesting that the overall star formation per unit mass is less in the outer Galaxy.
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We have conducted a study of star formation in the outer Galaxy from 65degr$< l <$265degr~in the region observed by the GLIMPSE360 program. This {it Spitzer} warm mission program mapped the plane of the outer Milky Way with IRAC at 3.6 and 4.5~$mu$m. We combine the IRAC, {it WISE}, and 2MASS catalogs and our previous results from another outer Galaxy survey and identify a total of 47,338 Young Stellar Objects (YSOs) across the field spanning $>$180degr in Galactic longitude. Using the $DBSCAN$ method on the combined catalog, we identify 618 clusters or aggregations of YSOs having 5 or more members. We identify 10,476 Class I, 29,604 Class II, and 7,325 anemic Class II/Class III YSOs. The ratio of YSOs identified as members of clusters was 25,528/47,338, or 54%. We found 100 of the clusters identified have previously measured distances in the {it WISE} ion{H}{2} survey. We used these distances in our spectral energy distribution (SED) fitting of the YSOs in these clusters, of which 96 had YSOs with $<3sigma$ fits. We used the derived masses from the SED model fits to estimate the initial mass function (IMF) in the inner and outer Galaxy clusters: dividing the clusters by Galactocentric distances, the slopes were $Gamma = 1.87 pm 0.31$ above 3~M$_{odot}$ for $R_{Gal} < 11.5$~kpc and $Gamma = 1.15 pm 0.24$ above 3~M$_{odot}$ for $R_{Gal} > 11.5$~kpc. The slope of the combined IMF was found to be $Gamma = 1.92 pm 0.42$ above 3~M$_{odot}$. These values are consistent with each other within the uncertainties, and with literature values in the inner Galaxy high-mass star formation regions. The slopes are likely also consistent with a universal Salpeter IMF.
White dwarfs are the remnants of low and intermediate mass stars. Because of electron degeneracy, their evolution is just a simple gravothermal process of cooling. Recently, thanks to Gaia data, it has been possible to construct the luminosity function of massive (0.9 < M/Msun < 1.1) white dwarfs in the solar neighborhood (d < 100 pc). Since the lifetime of their progenitors is very short, the birth times of both, parents and daughters, are very close and allow to reconstruct the (effective) star formation rate. This rate started growing from zero during the early Galaxy and reached a maximum 6-7 Gyr ago. It declined and ~5 Gyr ago started to climb once more reaching a maximum 2 - 3 Gyr in the past and decreased since then. There are some traces of a recent star formation burst, but the method used here is not appropriate for recently born white dwarfs.
This work aims to the study of the Sh 2-307 HII region and related stellar population. Near-infrared imaging and spectroscopic observations in the direction of Sh 2-307 were performed using OSIRIS at SOAR Telescope. From J-, H- and K-band spectra of the brightest source in the cluster, we conclude that it has a near-infrared spectra compatible with that taken for O9v-O9.5v stars. Using the derived spectral type and the respective J, H and K-band photometry, we compute a heliocentric distance of 3.2(0.5) kpc, which for R0 = 8 kpc, puts this cluster at more than 10 kpc from the Galactic centre. From the Brg, H2, and continuum narrow-band images we were able to detect both the NIR counterpart of the associated HII region, as well as, the interface between the ionised and the cool molecular gas. Using the 5 Ghz continuum flux density taken from the PMN catalogue and the Brg narrow band image we estimate that the HII region has a mean diameter of 0.94(0.15) pc, mean electron density of 550 cm-3 and an estimated dynamical age of 1.6 Myears years. The large fraction of sources presenting excess emission at 2micron suggests that the stellar population is very young, with many sources still in the pre-main sequence accreting phase. By the use of theoretical pre-main sequence tracks we derived a cluster mean age of about 2.5 Myears, and from the analyses of the fraction of excess emission sources as a function of their spatial distribution we found evidence for an age spread for the embedded pre-main sequence stellar population. Finally, from the study of the spatial distribution of the low-mass sources relative to the main-cluster source and associated photo-dissociation zones, we conclude that the O-type star probably has been triggering the star formation process in the region.
Deuteration has been used as a tracer of the evolutionary phases of low- and high-mass star formation. The APEX Telescope Large Area Survey (ATLASGAL) provides an important repository for a detailed statistical study of massive star-forming clumps in the inner Galactic disc at different evolutionary phases. We study the amount of deuteration using NH2D in a representative sample of high-mass clumps discovered by the ATLASGAL survey covering various evolutionary phases of massive star formation. Unbiased spectral line surveys at 3 mm were thus conducted towards ATLASGAL clumps between 85 and 93 GHz with the Mopra telescope and from 84 to 115 GHz using the IRAM 30m telescope. A subsample was followed up in the NH2D transition at 74 GHz with the IRAM 30m telescope. We determined the deuterium fractionation from the column density ratio of NH2D and NH3 and measured the NH2D excitation temperature for the first time from the simultaneous modelling of the 74 and 110 GHz line using MCWeeds. We find a large range of the NH2D to NH3 column density ratio up to 1.6+/-0.7 indicating a high degree of NH3 deuteration in a subsample of the clumps. Our analysis yields a clear difference between NH3 and NH2D rotational temperatures for a fraction of the sources. We therefore advocate observation of the NH2D transitions at 74 and 110 GHz simultaneously to determine the NH2D temperature directly. We determine a median ortho-to-para column density ratio of 3.7+/-1.2. The high detection rate of NH2D confirms a high deuteration previously found in massive star-forming clumps. Using the excitation temperature of NH2D instead of NH3 is needed to avoid an overestimation of deuteration. We measure a higher detection rate of NH2D in sources at early evolutionary stages. The deuterium fractionation shows no correlation with evolutionary tracers such as the NH3 (1,1) line width, or rotational temperature.
There is now a large consensus that the current epoch of the Cosmic Star Formation History (CSFH) is dominated by low mass galaxies while the most active phase at 1<z<2 is dominated by more massive galaxies, which undergo a faster evolution. Massive galaxies tend to inhabit very massive halos such as galaxy groups and clusters. We aim to understand whether the observed galaxy downsizing could be interpreted as a halo downsizing, whereas the most massive halos, and their galaxy populations, evolve more rapidly than the halos of lower mass. Thus, we study the contribution to the CSFH of galaxies inhabiting group-sized halos. This is done through the study of the evolution of the Infra-Red (IR) luminosity function of group galaxies from redshift 0 to ~1.6. We use a sample of 39 X-ray selected groups in the Extended Chandra Deep Field South (ECDFS), the Chandra Deep Field North (CDFN), and the COSMOS field, where the deepest available mid- and far-IR surveys have been conducted with Spitzer MIPS and Hersche PACS. Groups at low redshift lack the brightest, rarest, and most star forming IR-emitting galaxies observed in the field. Their IR-emitting galaxies contribute <10% of the comoving volume density of the whole IR galaxy population in the local Universe. At redshift >~1, the most IR-luminous galaxies (LIRGs and ULIRGs) are preferentially located in groups, and this is consistent with a reversal of the star-formation rate vs .density anti-correlation observed in the nearby Universe. At these redshifts, group galaxies contribute 60-80% of the CSFH, i.e. much more than at lower redshifts. Below z~1, the comoving number and SFR densities of IR-emitting galaxies in groups decline significantly faster than those of all IR-emitting galaxies. Our results are consistent with a halo downsizing scenario and highlight the significant role of environment quenching in shaping the CSFH.
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