Hα Luminosity Research Articles

Hydrodynamic Model of Hα Emission from Accretion Shocks of a Proto-giant Planet and Circumplanetary Disk

Abstract Recent observations have detected excess Hα emission from young stellar systems with an age of several Myr such as PDS 70. One-dimensional radiation-hydrodynamic models of shock-heated flows that we developed previously demonstrate that planetary accretion flows of >a few ten km s−1 can produce Hα emission. It is, however, a challenge to understand the accretion process of proto-giant planets from observations of such shock-originated emission because of a huge gap in scale between the circumplanetary disk (CPD) and the microscopic accretion shock. To overcome the scale gap problem, we combine two-dimensional, high-spatial-resolution global hydrodynamic simulations and the one-dimensional local radiation-hydrodynamic model of the shock-heated flow. From such combined simulations for the protoplanet–CPD system, we find that the Hα emission is mainly produced in localized areas on the protoplanetary surface. The accretion shocks above the CPD produce much weaker Hα emission (approximately one to two orders of magnitude smaller in luminosity). Nevertheless, the accretion shocks above the CPD significantly affect the accretion process onto the protoplanet. The accretion occurs at a quasi-steady rate if averaged on a 10 day timescale, but its rate shows variability on shorter timescales. The disk surface accretion layers including the CPD shocks largely fluctuate, which results in the time-variable accretion rate and Hα luminosity of the protoplanet. We also model the spectral emission profile of the Hα line and find that the line profile is less time-variable despite the large variability in luminosity. High-spectral-resolution spectroscopic observation and monitoring will be key to revealing the property of the accretion process.

VALES

We characterize the ionized gas outflows in 15 low-redshift star-forming galaxies, a Valparaíso ALMA/APEX Line Emission Survey (VALES) subsample, using MUSE integral field spectroscopy and GAMA photometric broadband data. We measure the emission-line spectra by fitting a double-component profile, with the second and broader component related to the outflowing gas. This interpretation is in agreement with the correlation between the observed star-formation rate (SFR) surface density (ΣSFR) and the second-component velocity dispersion (σ2nd), expected when tracing the feedback component. By modeling the broadband spectra with spectral energy distribution fitting and obtaining the star-formation histories of the sample, we observe a small decrease in SFR between 100 and 10 Myr in galaxies when the outflow Hα luminosity contribution is increased, indicating that the feedback somewhat inhibits the star formation within these timescales. The observed emission-line ratios are best reproduced by photoionization models when compared to shock ionization, indicating that radiation from the young stellar population is dominant and seems to be a consequence of a continuous star-formation activity instead of a “bursty” event. The outflow properties, such as the mass outflow rate (∼0.1 M⊙ yr−1), the outflow kinetic power (∼5.2 × 10−4%Lbol), and the mass loading factor (∼0.12), point toward a scenario where the measured feedback is not strong and has a low impact on the evolution of galaxies in general.

Dark matter fraction in z ∼ 1 star-forming galaxies

Context. The study of dark matter (DM) across cosmic timescales is essential for understanding galaxy formation and evolution. Recent observational studies show that further back in time (z > 0.5), rotation-supported, star-forming galaxies (SFGs) begin to appear to be DM deficient compared to local SFGs. Aims. We present an observational study of the DM fraction in 225 rotation-supported, SFGs at z ≈ 0.9; these SFGs have stellar masses in the range 9.0 ≤ log(M* M⊙) ≤ 11.0 and star formation rates 0.49 ≤ log(SFR[M⊙ yr−1]) ≤ 1.77. Methods. We studied a subsample of the KMOS Redshift One Spectroscopic Survey (KROSS) studied by Sharma et al. (2021, MNRAS, 503, 1753). The stellar masses (M*) of these objects were previously estimated using mass-to-light ratios derived from fitting the spectral energy distribution of the galaxies. Star formation rates were derived from the Hα luminosities. In this paper, we determined the total gas masses (Mgas) by the scaling relations of molecular and atomic gas (Tacconi et al. 2018, ApJ, 853, 179; Lagos et al. 2011, MNRAS, 418, 1649, respectively). We derived the dynamical masses (Mdyn) from the rotation curves (RCs) at different scale lengths (effective radius: Re, ∼2 Re and ∼3 Re) and we then calculated the DM fractions (fDM = 1 − Mbar/Mdyn) at these radii. Results. We report that at z ∼ 1 only a small fraction (∼5%) of our sample has a low (<20%) DM fraction within ∼2 − 3Re. The majority (>72%) of SFGs in our sample have outer disks (∼5−10 kpc) dominated by DM, which agrees with local SFGs. Moreover, we find a large scatter in the fraction of DM at a given stellar mass (or circular velocity) with respect to local SFGs, suggesting that galaxies at z ∼ 1 span a wide range of stages in the formation of stellar disks and have diverse DM halo properties coupled with baryons.

Exploring the evolution of giant molecular clouds in one of the nearest spiral galaxies M33

AbstractThe evolution of giant molecular clouds (GMCs), which are the main sites of star formation, is essential for unraveling how stars form and how galaxies evolve. We analyzed the M33 CO(J = 2–1) data with spatial resolution of 39 pc obtained by ALMA-ACA 7 m array combined with IRAM 30 m. We identified 736 GMCs and classified them into three types; Type I: associated with no Hii regions, Type II: associated with Hii regions with the Hα luminosity L(Hα) < 1037.5 erg s-1, Type III: associated with Hii regions with L(Hα) > 1037.5erg s-1. We found that mass, size, and velocity dispersion of GMCs slightly increase in the order of Type I, II, and III GMCs. Type III GMCs mainly exist in the spiral arm, while many of Type I and Type II GMCs are distributed in the inter-arm. Assuming that the star formation proceeds steadily, we roughly estimated the total GMC lifetime of 30 Myr.

Comparison of Planetary Hα-emission Models: A New Correlation with Accretion Luminosity

Abstract Accreting planets have been detected through their hydrogen-line emission, specifically Hα. To interpret this, stellar-regime empirical correlations between the Hα luminosity L Hα and the accretion luminosity L acc or accretion rate M ̇ have been extrapolated to planetary masses, however without validation. We present a theoretical L acc–L Hα relationship applicable to a shock at the surface of a planet. We consider wide ranges of accretion rates and masses and use detailed spectrally resolved, nonequilibrium models of the postshock cooling. The new relationship gives a markedly higher L acc for a given L Hα than fits to young stellar objects, because Lyα, which is not observable, carries a large fraction of L acc. Specifically, an L Hα measurement needs 10 to 100 times higher L acc and M ̇ than previously predicted, which may explain the rarity of planetary Hα detections. We also compare the M ̇ –L Hα relationships coming from the planet-surface shock or implied by accretion-funnel emission. Both can contribute simultaneously to an observed Hα signal, but at low (high) M ̇ the planetary-surface shock (heated funnel) dominates. Only the shock produces Gaussian line wings. Finally, we discuss accretion contexts in which different emission scenarios may apply, putting recent literature models in perspective, and also present L acc–L line relationships for several other hydrogen lines.

Characterizing star formation in the innermost kiloparsec of the galaxy NGC 1386

AbstractWe characterize the star formation going on in the inner kiloparsec region of the galaxy NGC 1386 as derived from the analysis of a multiwavelength dataset covering the optical, near-IR and mid-IR at subarsec resolution. We detect 61 point sources, distributed in a ring of 960 pc radius around the center of the galaxy. From SED fitting we conclude that these are low mass () young clusters, with age distributed from 1 to 10 Myr, with median at 3.6 Myr. Comparison of the Hα luminosity of the clusters derived from the Hα+[N ii] narrow band image with that expected from the fitted ionizing continuum shows that a large fraction of the ionizing photons escapes from the clusters. Moreover, a large fraction of these photons escapes from the regions around the star forming ring.

J-PLUS: The star formation main sequence and rate density at d ≲ 75 Mpc

Aims. Our goal is to estimate the star formation main sequence (SFMS) and the star formation rate density (SFRD) at z ≤ 0.017 (d ≲ 75 Mpc) using the Javalambre Photometric Local Universe Survey (J-PLUS) first data release, that probes 897.4 deg2 with twelve optical bands. Methods. We extract the Hα emission flux of 805 local galaxies from the J-PLUS filter J0660, being the continuum level estimated with the other eleven J-PLUS bands, and the dust attenuation and nitrogen contamination corrected with empirical relations. Stellar masses (M⋆), Hα luminosities (LHα), and star formation rates (SFRs) were estimated by accounting for parameters covariances. Our sample comprises 689 blue galaxies and 67 red galaxies, classified in the (u − g) vs. (g − z) color–color diagram, plus 49 AGN. Results. The SFMS is explored at log M⋆ ≳ 8 and it is clearly defined by the blue galaxies, with the red galaxies located below them. The SFMS is described as log SFR = 0.83log M⋆ − 8.44. We find a good agreement with previous estimations of the SFMS, especially those based on integral field spectroscopy. The Hα luminosity function of the AGN-free sample is well described by a Schechter function with log LHα∗ = 41.34, log ϕ* = −2.43, and α = −1.25. Our measurements provide a lower characteristic luminosity than several previous studies in the literature. Conclusions. The derived star formation rate density at d ≲ 75 Mpc is log ρSFR = −2.10 ± 0.11, with red galaxies accounting for 15% of the SFRD. Our value is lower than previous estimations at similar redshift, and provides a local reference for evolutionary studies regarding the star formation history of the Universe.

Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE

Context. Feedback from massive stars affects the interstellar medium (ISM) from the immediate surroundings of the stars (parsec scales) to galactic (kiloparsec) scales. High-spatial resolution studies of H II regions are critical to investigate how this mechanism operates. Aims. We study the ionised ISM in NGC 7793 with the MUSE instrument at ESO Very Large Telescope (VLT), over a field of view (FoV) of ∼2 kpc2 and at a spatial resolution of ∼10 pc. The aim is to link the physical conditions of the ionised gas (reddening, ionisation status, abundance measurements) within the spatially resolved H II regions to the properties of the stellar populations producing Lyman continuum photons. Methods. The analysis of the MUSE dataset, which provides a map of the ionised gas and a census of Wolf Rayet stars, is complemented with a sample of young star clusters (YSCs) and O star candidates observed with the Hubble Space Telescope (HST) and of giant molecular clouds traced in CO(2–1) emission with the Atacama Large Millimeter/submillimeter Array (ALMA). We estimated the oxygen abundance using a temperature-independent strong-line method. We determined the observed total amount of ionising photons (Q(H0)) from the extinction corrected Hα luminosity. This estimate was then compared to the expected Q(H0) obtained by summing the contributions of YSCs and massive stars. The ratio of the two values gives an estimate for the escape fraction (fesc) of photons in the region of interest. We used the [S II]/[O III] ratio as a proxy for the optical depth of the gas and classified H II regions into ionisation bounded, or as featuring channels of optically thin gas. We compared the resulting ionisation structure with the computed fesc. We also investigated the dependence of fesc on the age spanned by the stellar population in each region. Results. We find a median oxygen abundance of 12 + log(O/H) ∼ 8.37, with a scatter of 0.25 dex, which is in agreement with previous estimates for our target. We furthermore observe that the abundance map of H II regions is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are also observed. From the population synthesis analysis, we find that YSCs located in H II regions have a higher probability of being younger and less massive as well as of emitting a higher number of ionising photons than clusters in the rest of the field. Overall, we find fesc,HII = 0.67−0.12+0.08 for the population of H II regions. We also conclude that the sources of ionisation observed within the FoV are more than sufficient to explain the amount of diffuse ionised gas (DIG) observed in this region of the galaxy. We do not observe a systematic trend between the visual appearance of H II regions and fesc, pointing to the effect of 3D geometry in the small sample probed.

Calibration of the Hα Age–Activity Relation for M Dwarfs

Abstract In this work, we calibrate the relationship between Hα emission and M dwarf ages. We compile a sample of 892 M dwarfs with Hα equivalent width (Hα EW) measurements from the literature that are either comoving with a white dwarf of known age (21 stars) or in a known young association (871 stars). In this sample we identify 7 M dwarfs that are new candidate members of known associations. By dividing the stars into active and inactive categories according to their Hα EW and spectral type (SpT), we find that the fraction of active dwarfs decreases with increasing age, and the form of the decline depends on SpT. Using the compiled sample of age calibrators, we find that Hα EW and fractional Hα luminosity (L Hα /L bol) decrease with increasing age. Hα EW for SpT ≤ M7 decreases gradually up until ∼1 Gyr. For older ages, we found only two early M dwarfs that are both inactive and seem to continue the gradual decrease. We also found 14 mid-type M dwarfs, out of which 11 are inactive and present a significant decrease in Hα EW, suggesting that the magnetic activity decreases rapidly after ∼1 Gyr. We fit L Hα /L bol versus age with a broken power law and find an index of for ages ≲776 Myr. The index becomes much steeper at older ages, but a lack of field age-calibrators (≫1 Gyr) leaves this part of the relation far less constrained. Finally, from repeated independent measurements for the same stars, we find that 94% of them have a level of Hα EW variability ≤5 Å at young ages (<1 Gyr).

The KMOS3D Survey: Investigating the Origin of the Elevated Electron Densities in Star-forming Galaxies at 1 ≲ z ≲ 3

Abstract We investigate what drives the redshift evolution of the typical electron density (n e ) in star-forming galaxies, using a sample of 140 galaxies drawn primarily from KMOS3D (0.6 < z < 2.6) and 471 galaxies from SAMI (z < 0.113). We select galaxies that do not show evidence of active galactic nucleus activity or outflows to constrain the average conditions within H ii regions. Measurements of the [S ii]λ6716/[S ii]λ6731 ratio in four redshift bins indicate that the local n e in the line-emitting material decreases from 187 cm−3 at z ∼ 2.2 to 32 cm−3 at z ∼ 0, consistent with previous results. We use the Hα luminosity to estimate the rms n e averaged over the volumes of star-forming disks at each redshift. The local and volume-averaged n e evolve at similar rates, hinting that the volume filling factor of the line-emitting gas may be approximately constant across 0 ≲ z ≲ 2.6. The KMOS3D and SAMI galaxies follow a roughly monotonic trend between n e and star formation rate, but the KMOS3D galaxies have systematically higher n e than the SAMI galaxies at a fixed offset from the star-forming main sequence, suggesting a link between the n e evolution and the evolving main sequence normalization. We quantitatively test potential drivers of the density evolution and find that n e (rms) , suggesting that the elevated n e in high-z H ii regions could plausibly be the direct result of higher densities in the parent molecular clouds. There is also tentative evidence that n e could be influenced by the balance between stellar feedback, which drives the expansion of H ii regions, and the ambient pressure, which resists their expansion.

The 60 pc Environment of FRB 20180916B

Abstract Fast radio burst FRB 20180916B in its host galaxy SDSS J015800.28+654253.0 at 149 Mpc is by far the closest-known FRB with a robust host galaxy association. The source also exhibits a 16.35 day period in its bursting. Here we present optical and infrared imaging as well as integral field spectroscopy observations of FRB 20180916B with the WFC3 camera on the Hubble Space Telescope and the MEGARA spectrograph on the 10.4 m Gran Telescopio Canarias. The 60–90 milliarcsecond (mas) resolution of the Hubble imaging, along with the previous 2.3 mas localization of FRB 20180916B, allows us to probe its environment with a 30–60 pc resolution. We constrain any point-like star formation or H ii region at the location of FRB 20180916B to have an Hα luminosity L Hα ≲ 1037 erg s−1, and we correspondingly constrain the local star formation rate to be ≲10−4 M⊙ yr−1. The constraint on Hα suggests that possible stellar companions to FRB 20180916B should be of a cooler, less massive spectral type than O6V. FRB 20180916B is 250 pc away (in projected distance) from the brightest pixel of the nearest young stellar clump, which is ∼380 pc in size (FWHM). With the typical projected velocities of pulsars, magnetars, or neutron stars in binaries (60–750 km s−1), FRB 20180916B would need 800 kyr to 7 Myr to traverse the observed distance from its presumed birth site. This timescale is inconsistent with the active ages of magnetars (≲10 kyr). Rather, the inferred age and observed separation are compatible with the ages of high-mass X-ray binaries and gamma-ray binaries, and their separations from the nearest OB associations.

MAMMOTH: confirmation of two massive galaxy overdensities at z = 2.24 with Hα emitters

ABSTRACT Massive galaxy overdensities at the peak epoch of cosmic star formation provide ideal testbeds for the formation theories of galaxies and large-scale structure. We report the confirmation of two massive galaxy overdensities at z = 2.24, BOSS1244 and BOSS1542, selected from the Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH) project using Lyα absorption from the intergalactic medium over the scales of 15−30 h−1 Mpc imprinted on the quasar spectra. We use Hα emitters (HAEs) as the density tracer and identify them using deep narrow-band H2S(1) and broad-band Ks imaging data obtained with the wide-field infrared camera (WIRCam) at the Canada–France–Hawaii Telescope. In total, 244 and 223 line emitters are detected in these two fields, and 196 ± 2 and 175 ± 2 are expected to be HAEs with an Hα flux of >2.5 × 10−17 erg s−1 cm−2 (corresponding to a star formation rate of >5 M⊙ yr−1). The detection rate of HAE candidates suggests an overdensity factor of δgal = 5.6 ± 0.3 and 4.9 ± 0.3 over the volume of 54 × 32 × 32 co-moving Mpc3. The overdensity factor increases two to three times when focusing on the high-density regions of scales 10–15 co-moving Mpc. Interestingly, the HAE density maps reveal that BOSS1244 contains a dominant structure, while BOSS1542 manifests as a giant filamentary structure. We measure the Hα luminosity functions (HLFs), finding that BOSS1244’s HLF is nearly identical to that of the general field at the same epoch, while BOSS1542 shows an excess of HAEs with high Hα luminosity, indicating the presence of enhanced star formation or active galactic nuclei activity. We conclude that the two massive MAMMOTH overdensities are undergoing a rapid galaxy mass assembly.

NGC 2770: High supernova rate due to interaction

Context. Galaxies that hosted many core-collapse supernova (SN) explosions can be used to study the conditions necessary for the formation of massive stars. NGC 2770 was dubbed an SN factory because it hosted four core-collapse SNe in 20 years (three type Ib and one type IIn). Its star formation rate (SFR) was reported to not be enhanced and, therefore, not compatible with such a high SN rate. Aims. We aim to explain the high SN rate of NGC 2770. Methods. We used archival H I line data for NGC 2770 and reinterpreted the Hα and optical continuum data. Results. Even though the continuum-based SFR indicators do not yield high values, the dust-corrected Hα luminosity implies a high SFR, consistent with the high SN rate. Such a disparity between the SFR estimators is an indication of recently enhanced star formation activity because the continuum indicators trace long timescales of the order of 100 Myr, unlike the line indicators, which trace timescales of the order of 10 Myr. Hence, the unique feature of NGC 2770 compared to other galaxies is the fact that it was observed very shortly after the enhancement of the SFR. It also has high dust extinction, E(B − V) above 1 mag. We provide support for the hypothesis that the increased SFR in NGC 2770 is due to the interaction with its companion galaxies. We report an H I bridge between NGC 2770 and its closest companion and the existence of a total of four companions within 100 kpc (one identified for the first time). There are no clear H I concentrations close to the positions of SNe in NGC 2770 such as those detected for hosts of gamma-ray bursts (GRBs) and broad-lined SNe type Ic (IcBL). This suggests that the progenitors of type Ib SNe are not born out of recently accreted atomic gas, as was suggested for GRB and IcBL SN progenitors.

Extended He ii 4686 emission in Markarian 387

ABSTRACT In order to explore the effect of an active galactic nucleus (AGN) on the interstellar medium of its host galaxy, we selected a promising case for study, Markarian 387 (Mrk 387), based on the strength of its extended He ii 4686 emission, a high-ionization line that can be excited by a hard source of radiation. We use area-resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey and the availability of additional multiwavelength data. Besides harbouring an obscured AGN and the extended He ii 4686 emission, Mrk 387 has a number of other unusual properties, including a high far-infrared luminosity, a low neutral hydrogen mass compared to the stellar mass, a high Hα luminosity and high Hα equivalent width throughout the disc, and strong He i 5876 in the exterior regions. He ii 4686 and [O iii] 5008 extend with a bilateral morphology beyond 6 kpc from the nucleus. We interpret this emission as due to photoionization from the central source, where the interstellar medium must be sufficiently porous to allow the ionizing flux to reach these relatively distant regions.

Thermally Unstable Cooling Stimulated by Uplift: The Spoiler Clusters

Abstract Chandra X-ray observations are analyzed for five galaxy clusters whose atmospheric cooling times, entropy parameters, and ratios of cooling time to freefall time within the central galaxies lie below 1 Gyr, below 30 keV cm2, and between 20 ≲ min(t cool/t ff) ≲ 50, respectively. These thermodynamic properties are commonly associated with molecular clouds, bright Hα emission, and star formation in central galaxies. However, all have Hα luminosities below 1040 erg s−1 in the ACCEPT database. Star formation and molecular gas are absent at the levels seen in other central galaxies with similar atmospheric properties. Only RBS 0533 may host a radio/X-ray bubble, which are commonly observed in cooling atmospheres. Signatures of uplifted, high-metallicity atmospheric gas are absent. Their atmospheres are apparently thermodynamically stable despite the absence of strong nuclear feedback. We suggest that extended filaments of nebular emission and associate molecular clouds are absent at appreciable levels because their central radio sources have failed to lift low-entropy atmospheric gas to an altitude where the ratio of the cooling time to the freefall time falls below unity and the gas becomes thermally unstable.

Spectral signatures of H-rich material stripped from a non-degenerate companion by a Type Ia supernova

The single-degenerate scenario for Type Ia supernovae should yield metal-rich ejecta that enclose some stripped material from the non-degenerate H-rich companion star. We present a large grid of non-local thermodynamic equilibrium steady-state radiative transfer calculations for such hybrid ejecta and provide analytical fits for the Hα luminosity and equivalent width. Our set of models covers a range of masses for 56Ni and the ejecta, for the stripped material (Mst), and post-explosion epochs from 100 to 300 d. The brightness contrast between stripped material and metal-rich ejecta challenges the detection of H I and He I lines prior to ~100 d. Intrinsic and extrinsic optical depth effects also influence the radiation emanating from the stripped material. This inner denser region is marginally thick in the continuum and optically thick in all Balmer lines. The overlying metal-rich ejecta blanket the inner regions, completely below about 5000 Å, and more sparsely at longer wavelengths. As a consequence, Hβ should not be observed for all values of Mst up to at least 300 days, while Hα should be observed after ~100 d for all Mst ≥ 0.01 M⊙. Observational non-detections capable of limiting the Hα equivalent width to <1 Å set a formal upper limit of Mst < 0.001M⊙. This contrasts with the case of circumstellar-material (CSM) interaction, not subject to external blanketing, which should produce Hα and Hβ lines with a strength dependent primarily on CSM density. We confirm previous analyses that suggest low values of order 0.001 M⊙ for Mst to explain the observations of the two Type Ia supernovae with nebular-phase Hα detection, in conflict with the much greater stripped mass predicted by hydrodynamical simulations for the single-degenerate scenario. A more likely solution is the double-degenerate scenario, together with CSM interaction, or enclosed material from a tertiary star in a triple system or from a giant planet.

Hα Luminosity of ATLAS18qtd Does Not Plateau in the Nebular Phase

Abstract We present new spectroscopic and photometric observations of ATLAS18qtd/SN 2018cqj, a fast-declining SN Ia with variable Hα emission in previously published nebular phase spectra. ATLAS18qtd is undetected in both spectroscopic and photometric observations which occurred at ∼540 days after maximum light and ∼230 days after the last Hα detection. With these new non-detections, we place an upper limit on the Hα luminosity of ≲ 1.1 × 10 36 erg s − 1 indicating the Hα flux decreased by a factor of ≳ 4 since the previous detection. This upper limit excludes Hα emission that plateaus or increases since the previous detection but cannot confirm that the Hα emission decay rate is equivalent to the supernova decay rate.

Quantifying the Effect of Field Variance on the Hα Luminosity Function with the New Numerical Galaxy Catalog (ν2GC)

Abstract We construct a model of Hα emitters (HAEs) based on a semianalytic galaxy formation model, the New Numerical Galaxy Catalog (ν 2GC). In this paper, we report our estimate for the field variance of the HAE distribution. By calculating the Hα luminosity from the star formation rate of galaxies, our model well reproduces the observed Hα luminosity function (LF) at z = 0.4. The large volume of the ν 2GC makes it possible to examine the spatial distribution of HAEs over a region of (411.8 Mpc)3 in the comoving scale. The surface number density of z = 0.4 HAEs with erg s−1 is 308.9 deg−2. We have confirmed that the HAE is a useful tracer for the large-scale structure of the universe because of their significant overdensity (>5σ) at clusters and the filamentary structures. The Hα LFs within a survey area of ∼2 deg2 (typical for previous observational studies) show a significant field variance up to ∼1 dex. Based on our model, one can estimate the variance on the Hα LFs within given survey areas.

Revealing the effects of galaxy interaction in the main galaxies of the southern group Arp 314

ABSTRACT We present new Gemini imaging and spectroscopic data of the system Arp 314, which consists of a triplet of interacting galaxies. This new imagery exhibits tidal tails and stellar bridges between the galaxies' members and confirms the past interactions. Using this data set, we have analysed the physical properties of 22 star-forming regions located in the main disc of these galaxies, as well as in the intergalactic medium. All these regions have emission lines typical of young ages and a couple of them display very high Hα luminosities (LHα ∼ 1040 erg s−1). Using the star-forming regions located in Arp 314-1, we derive its gas-phase oxygen-abundance distribution, which suggests a flatter behaviour than the distribution shown by non-interacting systems. This is in agreement with results obtained for other interacting systems and simulations. The presence of gas flows, as indicated by its complex kinematics, could explain this finding. Most of the star formation in Arp 314-2 is located in a central starburst, where double Hα profiles can be identified, as shown by archival Fabry–Perot data. Additionally, we found that the irregular galaxy Arp 314-3 has a low oxygen abundance. Considering its luminosity, this object has a primordial origin, and it was not formed during the interaction event that this system has experienced.

Paβ, Hα, and Attenuation in NGC 5194 and NGC 6946

Abstract We combine Hubble Space Telescope Paschen β (Paβ) imaging with ground-based, previously published Hα maps to estimate the attenuation affecting Hα, A(Hα), across the nearby, face-on galaxies NGC 5194 and NGC 6946. We estimate A(Hα) in ∼2000 independent 2″ ∼ 75 pc diameter apertures in each galaxy, spanning out to a galactocentric radius of almost 10 kpc. In both galaxies, A(Hα) drops with radius, with a bright, high-attenuation inner region, though in detail the profiles differ between the two galaxies. Regions with the highest attenuation-corrected Hα luminosity show the highest attenuation, but the observed Hα luminosity of a region is not a good predictor of attenuation in our data. Consistent with much previous work, the IR-to-Hα color does a good job of predicting A(Hα). We calculate the best-fit empirical coefficients for use combining Hα with 8, 12, 24, 70, or 100 μm to correct for attenuation. These agree well with previous work, but we also measure significant scatter around each of these linear relations. The local atomic plus molecular gas column density, N(H), also predicts A(Hα) well. We show that a screen with magnitude ∼0.2 times that expected for a Milky Way gas-to-dust value does a reasonable job of explaining A(Hα) as a function of N(H). This could be expected if only ∼40% of gas and dust directly overlap regions of Hα emission.