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Register a new userA Cautionary Tale of Attenuation in Star Forming Regions
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M. Molina
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The attenuation of light in star forming galaxies is correlated with a multitude of physical parameters including star formation rate, metallicity and total dust content. This variation in attenuation is even more prevalent on the kiloparsec scale, which is relevant to many current spectroscopic integral field unit surveys. To understand the cause of this variation, we present and analyse textit{Swift}/UVOT near-UV (NUV) images and SDSS/MaNGA emission-line maps of 29 nearby ($z<0.084$) star forming galaxies. We resolve kiloparsec-sized star forming regions within the galaxies and compare their optical nebular attenuation (i.e., the Balmer emission line optical depth, $tau^l_Bequivtau_{textrm{H}beta}-tau_{textrm{H}alpha}$) and NUV stellar continuum attenuation (via the NUV power-law index, $beta$) to the attenuation law described by Battisti et al. The data agree with that model, albeit with significant scatter. We explore the dependence of the scatter of the $beta$-$tau^l_B$ measurements from the star forming regions on different physical parameters, including distance from the nucleus, star formation rate and total dust content. Finally, we compare the measured $tau^l_B$ and $beta$ between the individual star forming regions and the integrated galaxy light. We find a strong variation in $beta$ between the kiloparsec scale and the larger galaxy scale not seen in $tau^l_B$. We conclude that the sight-line dependence of UV attenuation and the reddening of $beta$ due to the light from older stellar populations could contribute to the $beta$-$tau^l_B$ discrepancy.
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Resolution studies of test problems set baselines and help define minimum resolution requirements, however, resolution studies must also be performed on scientific simulations to determine the effect of resolution on the specific scientific results. We perform a resolution study on the formation of a protostar by modelling the collapse of gas through 14 orders of magnitude in density. This is done using compressible radiative non-ideal magnetohydrodynamics. Our suite consists of an ideal magnetohydrodynamics (MHD) model and two non-ideal MHD models, and we test three resolutions for each model. The resulting structure of the ideal MHD model is approximately independent of resolution, although higher magnetic field strengths are realised in higher resolution models. The non-ideal MHD models are more dependent on resolution, specifically the magnetic field strength and structure. Stronger magnetic fields are realised in higher resolution models, and the evolution of detailed structures such as magnetic walls are only resolved in our highest resolution simulation. In several of the non-ideal MHD models, there is an off-set between the location of the maximum magnetic field strength and the maximum density, which is often obscured or lost at lower resolutions. Thus, understanding the effects of resolution on numerical star formation is imperative for understanding the formation of a star.
We compile a sample of about 157,000 spaxels from the Mapping Nearby Galaxies at the Apache Point Observatory survey to derive the average dust attenuation curve for subgalactic star-forming regions of local star-forming galaxies (SFGs) in the optical wavelength, following the method of cite{Calzetti1994}. We obtain a $D_n(4000)$-independent average attenuation curve for spaxels with $1.1leq D_n(4000)<1.3$, which is similar to the one derived from either local starbursts or normal SFGs. We examine whether and how the shape of the average attenuation curve changes with several local and global physical properties. For spaxels with $1.2leq D_n(4000)<1.3$, we find no dependence on either local or global physical properties for the shape of the average attenuation curve. However, for spaxels with younger stellar population ($1.1leq D_n(4000)<1.2$), shallower average attenuation curves are found for star-forming regions with smaller stellar mass surface density, smaller star formation rate surface density, or those residing in the outer region of galaxies. These results emphasize the risk of using one single attenuation curve to correct the dust reddening for all types of star-forming regions, especially for those with fairly young stellar population.
Maser emission plays an important role as a tool in star formation studies. It is widely used for deriving kinematics, as well as the physical conditions of different structures, hidden in the dense environment very close to the young stars, for example associated with the onset of jets and outflows. We will summarize the recent observational and theoretical progress on this topic since the last maser symposium: the IAU Symposium 242 in Alice Springs.
We investigate the dust attenuation in both stellar populations and ionized gas in kpc-scale regions in nearby galaxies, using integral field spectroscopy data from MaNGA MPL-9. We identify star-forming (HII) and diffuse ionized gas (DIG) regions from MaNGA datacubes. From the stacked spectrum of each region, we measure the stellar attenuation, $E(B-V)_{rm star}$, using the technique developed by Li et al.(2020), as well as the gas attenuation, $E(B-V)_{rm gas}$, from the Balmer decrement. We then examine the correlation of $E(B-V)_{rm star}$, $E(B-V)_{rm gas}$, $E(B-V)_{rm gas}-E(B-V)_{rm star}$ and $E(B-V)_{rm star}/E(B-V)_{rm gas}$ with 16 regional/global properties, and for regions with different $rm H{alpha}$ surface brightnesses ($Sigma_{rm Halpha}$). We find a stronger correlation between $E(B-V)_{rm star}$ and $E(B-V)_{rm gas}$ in regions of higher $Sigma_{rm Halpha}$. Luminosity-weighted age ($t_L$) is found to be the property that is the most strongly correlated with $E(B-V)_{rm star}$, and consequently with $E(B-V)_{rm gas}-E(B-V)_{rm star}$ and $E(B-V)_{rm star}/E(B-V)_{rm gas}$. At fixed $Sigma_{rm Halpha}$, $log_{10}t_L$ is linearly and negatively correlated with $E(B-V)_{rm star}/E(B-V)_{rm gas}$ at all ages. Gas-phase metallicity and ionization level are important for the attenuation in the gas. Our results indicate that the ionizing source for DIG regions is likely distributed in the outer-skirt of galaxies, while for HII regions our results can be well explained by the two-component dust model of Charlot & Fall (2000).
We investigate the specific angular momentum (sAM) $ j(<r)$ profiles of intermediate redshift ($0.4<z<1.4$) star-forming galaxies (SFGs) in the relatively unexplored regime of low masses (down to $M_starsim 10^8$M$_{odot}$) and small sizes (down to $R_{rm e}sim 1.5$ kpc) and characterize the sAM scaling relation and its redshift evolution. We have developed a 3D methodology to constrain sAM profiles of the star-forming gas using a forward modeling approach with galpak{} that incorporates the effects of beam smearing, yielding the intrinsic morpho-kinematic properties even with limited spatial resolution data. Using mock observations from the TNG50 simulation, we find that our 3D methodology robustly recovers the SFR-weighted $j(<r)$ profiles down to low effective signal-to-noise ratio (SNR) of $gtrapprox3$. We apply our methodology blindly to a sample of 494 OII{}-selected SFGs in the MUSE Ultra Deep Field (UDF) 9~arcmin$^2$ mosaic data, covering the unexplored $8<log M_*/$M$_{odot}<9$ mass range. We find that the (SFR-weighted) sAM relation follows $jpropto M_star^{alpha}$ with an index $alpha$ varying from $alpha=0.3$ to $alpha=0.5$, from $log M_star/$M$_{odot}=8$ to $log M_*/$M$_{odot}=10.5$. The UDF sample supports a redshift evolution consistent with the $(1+z)^{-0.5}$ expectation from a Universe in expansion. The scatter of the sAM sequence is a strong function of the dynamical state with $log j|_{M_*}propto 0.65 times log(V_{rm max}/sigma)$ where $sigma$ is the velocity dispersion at $2 R_{rm e}$. In TNG50, SFGs also form a $j-M_{star}-(V/sigma)$ plane but correlates more with galaxy size than with morphological parameters. Our results suggest that SFGs might experience a dynamical transformation before their morphological transformation to becoming passive via either merging or secular evolution.
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