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Register a new userThe distribution of star formation in galactic bars as seen with H$alpha$ and stacked GALEX UV imaging
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Sim\\'on D\\'iaz-Garc\\'ia
Publication date
2020
fields
Physics
and research's language is
English
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We investigate the spatial distribution of star formation (SF) within bars of nearby disk galaxies (inclination $< 65^{circ}$) from the S$^4$G survey. We use archival GALEX far- and near-UV imaging for 772 barred galaxies. We also assemble a compilation of continuum-subtracted H$alpha$ images for 433 barred galaxies, of which 70 are produced by ourselves from ancillary photometry and MUSE/CALIFA IFU data cubes. We employ two complementary approaches: i) the analysis of bar/disk stacks built from co-added UV images of hundreds of galaxies; and ii) the classification of the morphology of ionised regions in galaxies into three main SF classes: A) only circumnuclear SF, B) SF at the bar ends, but not along the bar, and C) SF along the bar. Lenticular galaxies typically belong to SF class A: this is probably related to bar-induced SF quenching. The distribution of SF class B peaks for early- and intermediate-type spirals: this most likely results from the interplay of gas flow, shocks, and enhanced shear in centrally concentrated galaxies with large bar amplitudes. Late-type galaxies are mainly assigned to SF class C: we argue that this is a consequence of low shear. In bar stacks of spirals, the UV emission traces the stellar bars and dominates on their leading side, as witnessed in simulations. For early-types, the central UV emission is $sim$0.5 mag brighter in strongly barred galaxies, relative to their weakly barred counterparts: this is related to the efficiency of strong bars sweeping the disk gas and triggering central starbursts. We also show that the distributions of SF in inner ringed galaxies are broadly the same in barred and non-barred galaxies, including a UV/H$alpha$ deficit in the middle part of the bar: this hints at the effect of resonance rings trapping gas. Distinct distributions of SF within bars are reported in galaxies of different morphological types (Abridged).
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We present deep H{alpha} imaging of seven Hickson Compact Groups (HCGs) using the 4.1m Southern Astrophysics Research (SOAR) Telescope. The high spatial resolution of the observations allow us to study both the integrated star-formation properties of the main galaxies as well as the 2D distribution of star-forming knots in the faint tidal arms that form during interactions between the individual galaxies. We derive star-formation rates and stellar masses for group members and discuss their position relative to the main sequence of star-forming galaxies. Despite the existence of tidal features within the galaxy groups, we do not find any indication for enhanced star-formation in the selected sample of HCGs. We study azimuthally averaged H{alpha} profiles of the galaxy disks and compare them with the g and r surface-brightness profiles. We do not find any truncated galaxy disks but reveal that more massive galaxies show a higher light concentration in H{alpha} than less massive ones. We also see that galaxies that show a high light concentration in r, show a systematic higher light concentration in H{alpha}. TDG candidates have been previously detected in R-band images for 2 groups in our sample but we find that most of them are likely background objects as they do not show any emission in H{alpha}. We present a new tidal dwarf galaxy (TDG) candidate at the tip of the tidal tail in HCG 91.
Ionization feedback should impact the probability distribution function (PDF) of the column density around the ionized gas. We aim to quantify this effect and discuss its potential link to the Core and Initial Mass Function (CMF/IMF). We used in a systematic way Herschel column density maps of several regions observed within the HOBYS key program: M16, the Rosette and Vela C molecular cloud, and the RCW 120 H ii region. We fitted the column density PDFs of all clouds with two lognormal distributions, since they present a double-peak or enlarged shape in the PDF. Our interpretation is that the lowest part of the column density distribution describes the turbulent molecular gas while the second peak corresponds to a compression zone induced by the expansion of the ionized gas into the turbulent molecular cloud. The condensations at the edge of the ionized gas have a steep compressed radial profile, sometimes recognizable in the flattening of the power-law tail. This could lead to an unambiguous criterion able to disentangle triggered from pre-existing star formation. In the context of the gravo-turbulent scenario for the origin of the CMF/IMF, the double peaked/enlarged shape of the PDF may impact the formation of objects at both the low-mass and the high-mass end of the CMF/IMF. In particular a broader PDF is required by the gravo-turbulent scenario to fit properly the IMF with a reasonable initial Mach number for the molecular cloud. Since other physical processes (e.g. the equation of state and the variations among the core properties) have already been suggested to broaden the PDF, the relative importance of the different effects remains an open question.
Because the timescale of H$alpha$ emission (several tens of Myr) following star formation is significantly shorter than that of UV radiation (a few hundred Myr), the H$alpha$/UV flux ratio provides insight on the star formation histories (SFHs) of galaxies on timescales shorter than $sim100$ Myr. We present H$alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive dataset. The data provide us with robust measurements of dust-corrected SFRs in both H$alpha$ and UV for 1,050 galaxies. The results show a correlation between the H$alpha$/UV ratio and the deviation from the main sequence in the sense that galaxies above/below the main sequence tend to have higher/lower H$alpha$/UV ratios. This trend increases the dispersion of the main sequence by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting that diversity of recent SFHs of galaxies has a direct impact on the observed main sequence scatter. We caution that the results suffer from incompleteness and a selection bias which may lead us to miss many sources with high H$alpha$/UV ratios, this could further increase the scatter from SFHs in the star-forming main sequence.
Radio continuum and polarization observations of several nearby galaxies allowed to determine their vertical scaleheights, magnetic field strengths and large-scale magnetic field patterns. They all show a similar large-scale magnetic field pattern, which is parallel to the galactic disk along the midplane and X-shaped further away from the disk plane, indepenent of their Hubble type or star formation in the disk or nuclear region. We conclude that - though a high star formation rate (SFR) in the disk increases the total magnetic field strength in the disk and the halo - the SFR does not significantly change the global field configuration nor influence the global scale heights of the radio emission. The observed similar scale heights indicate that star formation regulates the galactic wind velocities. The galactic wind itself may be essential for an effective dynamo action.
While some galactic bars show recent massive star formation (SF) along them, some others present a lack of it. Whether bars with low level of SF are a consequence of low star formation efficiency (SFE), low gas inflow rate, or dynamical effects, remains a matter of debate. We perform a multi-wavelength analysis of 12 strongly barred massive galaxies, chosen to host different degrees of SF along the bar major axis without any prior condition on gas content. We observe the CO(1-0) and CO(2-1) emission within bars with the IRAM-30m telescope, which we use to estimate molecular gas masses. SF rates (SFR) are calculated from GALEX near- and far- ultraviolet (UV) and WISE 12 and 22 micron images within the beam pointings, covering the full bar extent. We detect molecular gas along the bars of all probed galaxies. The SFE in bars varies between galaxies by up to an order of magnitude. On average, SFEs are roughly constant along bars. SFEs are not significantly different from the mean value in spiral galaxies reported in the literature. Interestingly, the higher the total stellar mass of the host galaxy, the lower the SFE within their bars. In particular, the two galaxies in our sample with lowest SFEs and SFR surface densities (NGC 4548 and NGC 5850) are also the ones hosting massive bulges and signs of past interactions with nearby companions. The SFE in strong bars is not systematically inhibited (either in the central, mid- or end-parts of the bar). Both environmental and internal quenching are likely responsible for the lowest SFEs reported in this work (Abridged).
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