More Like this

1. Now You See It, Now You Don’t: Star Formation Truncation Precedes the Loss of Molecular Gas by ∼100 Myr in Massive Poststarburst Galaxies at z ∼ 0.6

   https://doi.org/10.3847/1538-4357/ac3dfa  

   Bezanson, Rachel; Spilker, Justin S.; Suess, Katherine A.; Setton, David J.; Feldmann, Robert; Greene, Jenny E.; Kriek, Mariska; Narayanan, Desika; Verrico, Margaret (February 2022, The Astrophysical Journal)

   Abstract We use ALMA observations of CO(2–1) in 13 massive (M_*≳ 10¹¹M_⊙) poststarburst galaxies atz∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, angu $\overrightarrow{L}\mathrm{ar}$momentum, and Evolution ( $\mathit{SQuIGG}\vec{L}E$) program. Early results showed that two poststarburst galaxies host large H₂reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with $M_{{\mathrm{H}}_2}\gtrsim {10}^9$M_⊙. Given their high stellar masses, this mass limit corresponds to an average gas fraction of $〈f_{{\mathrm{H}}_2}\equiv M_{{\mathrm{H}}_2}/M_{*}〉\sim 7\%$or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with theD_n4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support this empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H₂reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, the $\mathit{SQuIGG}\vec{L}E$galaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H₂scaling relations. 

   more » « less
2. An Estimate of the Binary Star Fraction among Young Stars at the Galactic Center: Possible Evidence of a Radial Dependence

   https://doi.org/10.3847/1538-4357/ad26e6  

   Gautam, Abhimat K.; Do, Tuan; Ghez, Andrea M.; Chu, Devin S.; Hosek, Jr., Matthew W.; Sakai, Shoko; Naoz, Smadar; Morris, Mark R.; Ciurlo, Anna; Haggard, Zoë; et al (March 2024, The Astrophysical Journal)

   Abstract We present the first estimate of the intrinsic binary fraction of young stars across the central ≈0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 spectroscopically confirmed young stars, using 119 nights of 10″ wide adaptive optics imaging observations taken at W. M. Keck Observatory over 16 yr in the $K\prime$-[2.1μm] andH-[1.6μm] bands. We photometrically detected three binary stars, all of which are situated more than 1″ (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. All are contact binaries or have photometric variability originating from stellar irradiation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined the experimental sensitivity through detailed light-curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population, and underlying binary system parameters (periods, mass ratios, and eccentricities) similar to those of local massive stars. As might be expected, our experimental sensitivity decreases for eclipses narrower in phase. The detections and simulations imply that the young, massive stars in the GC have a stellar binary fraction ≥71% (68% confidence), or ≥42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. based on radial velocity measurements for stars ≲1″ of the SMBH. Constrained with these two studies, the probability that the same underlying young star binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction, and therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH. 

   more » « less
3. Fundamental Parameters for Central Stars of 103 Infrared Bow Shock Nebulae

   https://doi.org/10.3847/1538-4357/ade22f  

   Patten, Nikhil; Kobulnicky, Henry A; Povich, Matthew S; Whisnant, Angelica S; Andrews, Sydney; Boone, Alexandra; Dandu, Srujan; Jones, Naomi; Justice, S Nick; Hope, Dylan; et al (July 2025, The Astrophysical Journal)

   Abstract Stellar bow shock nebulae are arcuate shock fronts formed by the interaction of radiation-driven stellar winds and the relative motion of the ambient interstellar material. Stellar bow shock nebulae provide a promising means to measure wind-driven mass loss, independent of other established methods. In this work, we characterize the stellar sources at the center of bow shock nebulae drawn from all-sky catalogs of 24μm–selected nebulae. We obtain new, low-resolution blue optical spectra for 104 stars and measure stellar parameters temperatureT_eff, surface gravity $\log g$, and projected rotational broadening $v\sin i$. We perform additional photometric analysis to measure stellar radiusR_*, luminosityL_*, and visual-band extinctionA_V. All but one of our targets are O and early B stars, with temperatures ranging fromT= 16.5 to 46.8 kK, gravities from $\log g=$2.57 to 4.60, and $v\sin i$from <100 to 400 km s⁻¹. With the exception of rapid rotatorζOph, bow shock stars do not rotate at or near critical velocities. At least 60 of 103 (60%) OB bow shock stars are binaries, consistent with the multiplicity fraction of other OB samples. The sample shows a runaway fraction of 23%, with 19 stars havingv_2D≥ 25 km s⁻¹. Of the 19 runaways, at least 15 (≥79%) are binaries, favoring dynamical ejection over the binary supernova channel for producing runaways. We provide a comprehensive census of stellar parameters for bow shock stars, useful as a foundation for determining the mass-loss rates for OB-type stars—one of the single most critical factors in stellar evolution governing the production of neutron stars and black holes. 

   more » « less
4. Lyα Escape from Low-mass, Compact, High-redshift Galaxies

   https://doi.org/10.3847/1538-3881/ac83a9  

   Pucha, Ragadeepika; Reddy, Naveen A.; Dey, Arjun; Juneau, Stéphanie; Lee, Kyoung-Soo; Prescott, Moire K.; Shivaei, Irene; Hong, Sungryong (September 2022, The Astronomical Journal)

   Abstract We investigate the effects of stellar populations and sizes on Lyαescape in 27 spectroscopically confirmed and 35 photometric Lyαemitters (LAEs) atz≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive (M_⋆≈ 10⁷–10⁹M_⊙), younger (ages ≲1 Gyr), smaller (r_e< 1 kpc), and less dust-attenuated (E(B−V) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1–100M_⊙yr⁻¹). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (Z_neb≲ 0.2Z_⊙) and/or high ionization parameters ( $\log \left(\mathrm{U}\right)\gtrsim -2.4$). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Lyαescape by computing SFR surface density, Σ_SFR, and specific SFR surface density, Σ_sSFR. For a given Σ_SFR, the Lyαescape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher Σ_sSFR, on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Lyαphotons. These results have important implications for the physics of Lyαradiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization. 

   more » « less
5. Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

   https://doi.org/10.3847/1538-4357/ac91d1  

   Nugent, Anya E.; Fong, Wen-Fai; Dong 董, Yuxin 雨欣; Leja, Joel; Berger, Edo; Zevin, Michael; Chornock, Ryan; Cobb, Bethany E.; Kelley, Luke Zoltan; Kilpatrick, Charles D.; et al (November 2022, The Astrophysical Journal)

   Abstract We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of $z={0.64}_{-0.32}^{+0.83}$(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oft_m= ${0.8}_{-0.53}^{+2.71}$Gyr, stellar mass of log(M_*/M_⊙) = ${9.69}_{-0.65}^{+0.75}$, star formation rate of SFR = ${1.44}_{-1.35}^{+9.37}$M_⊙yr⁻¹, stellar metallicity of log(Z_*/Z_⊙) = $-{0.38}_{-0.42}^{+0.44}$, and dust attenuation of $A_V={0.43}_{-0.36}^{+0.85}$mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution, with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website. 

   more » « less