分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of Carbon-Oxygen WDs. For $\sim 10^4$ yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high stellar masses (high WD merger rate) and low star formation rates (low birth rate of $\sim 6-10 \,{\rm M_{\odot}}$ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines and strong carbon and oxygen recombination and fine-structure lines in the UV, optical and IR. With integral field spectrographs, we predict that hundreds of candidates are detectable in M87 and other nearby massive galaxies. Dust somewhat reduces the optical line emission for nebula radii $\lesssim 10^{17}$ cm, but the sources spend most of their time with larger radii when dust is less important. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne VI] and [Mg VII] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We introduce the neural network architecture SPENDER as a core differentiable building block for analyzing, representing, and creating galaxy spectra. It combines a convolutional encoder, which pays attention to up to 256 spectral features and compresses them into a low-dimensional latent space, with a decoder that generates a restframe representation, whose spectral range and resolution exceeds that of the observing instrument. The decoder is followed by explicit redshift, resampling, and convolution transformations to match the observations. The architecture takes galaxy spectra at arbitrary redshifts and is robust to glitches like residuals of the skyline subtraction, so that spectra from a large survey can be ingested directly without additional preprocessing. We demonstrate the performance of SPENDER by training on the entire spectroscopic galaxy sample of SDSS-II; show its ability to create highly accurate reconstructions with substantially reduced noise; perform deconvolution and oversampling for a super-resolution model that resolves the [OII] doublet; introduce a novel method to interpret attention weights as proxies for important spectral features; and infer the main degrees of freedom represented in the latent space. We conclude with a discussion of future improvements and applications.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Ultra-Diffuse Galaxies are both extreme products of galaxy evolution and extreme environments in which to test our understanding of star formation. In this work, we contrast the spatially resolved star formation activity of a sample of 22 HI-selected UDGs and 35 low-mass galaxies from the NASA Sloan Atlas (NSA) within 120 Mpc. We employ a new joint SED fitting method to compute star formation rate and stellar mass surface density maps that leverage the high spatial resolution optical imaging data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and the UV coverage of GALEX, along with HI radial profiles estimated from a subset of galaxies that have spatially resolved HI maps. We find that the UDGs have low star formation efficiencies as a function of their atomic gas down to scales of 500 pc. We additionally find that the stellar mass-weighted sizes of our UDG sample are unremarkable when considered as a function of their HI mass -- their stellar sizes are comparable to the NSA dwarfs at fixed HI mass. This is a natural result in the picture where UDGs are forming stars normally, but at low efficiencies. We compare our results to predictions from contemporary models of galaxy formation, and find in particular that our observations are difficult to reproduce in models where UDGs undergo stellar expansion due to vigorous star formation feedback should bursty star formation be required down to $z=0$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junyao Li
John D. Silverman
Xuheng Ding
Michael A. Strauss
Andy Goulding
Simon Birrer
Hassen M. Yesuf
Yongquan Xue
Lalitwadee Kawinwanichakij
Yoshiki Matsuoka
Yoshiki Toba
Tohru Nagao
Malte Schramm
Kohei Inayoshi
摘要： The relationship between quasars and their host galaxies provides clues on
how supermassive black holes (SMBHs) and massive galaxies are jointly
assembled. To elucidate this connection, we measure the structural and
photometric properties of the host galaxies of ~5000 SDSS quasars at 0.2
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junyao Li
John D. Silverman
Xuheng Ding
Michael A. Strauss
Andy Goulding
Malte Schramm
Hassen M. Yesuf
Mouyuan Sun
Yongquan Xue
Simon Birrer
Jingjing Shi
Yoshiki Toba
Tohru Nagao
Masatoshi Imanishi
摘要： We measure the evolution of the $M_{\rm BH}-M_*$ relation using 584
uniformly-selected SDSS quasars at $0.2
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jiaxuan Li
Jenny E. Greene
Johnny Greco
Rachael Beaton
Shany Danieli
Andy Goulding
Song Huang
Erin Kado-Fong
摘要： Recent observations have reignited interest in a population of dwarf galaxies that are large and diffuse for their mass, often called ultra-diffuse galaxies. However, the origin and evolution of these mass-size outliers and the role of environment are still unclear. Using the exquisitely deep and wide Hyper Suprime-Cam Strategic Survey images, we search for ultra-puffy galaxies, defined as being $1.5\sigma$ larger than the average size for their mass, around Milky-Way--like galaxies. We present the size and radial distributions of mass-size outliers, and derive their quenched fraction to explore the impact of environment. Surprisingly, despite being outliers in size, the ultra-puffy galaxies have a similar quenched fraction as normal-sized satellites of Milky Way analogs in both observations and simulations, suggesting that quenching is not tied to being a mass-size outlier. The quenched fraction is higher for the ultra-puffy galaxies associated with redder hosts as well as those that are closer to the host in projection. In contrast, the ultra-diffuse galaxies are overall redder and more quiescent compared with normal satellites. We show that the classic definition of ultra-diffuse galaxies is heavily weighted towards quenched galaxies and thus cannot be used for a study of quenching of mass-size outliers.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junyao Li
John D. Silverman
Andrea Merloni
Mara Salvato
Johannes Buchner
Andy Goulding
Teng Liu
Riccardo Arcodia
Johan Comparat
Xuheng Ding
Kohei Ichikawa
Masatoshi Imanishi
Toshihiro Kawaguchi
Lalitwadee Kawinwanichakij
Yoshiki Toba
摘要： We investigate the physical properties, such as star-forming activity, disk
vs. bulge nature, galaxy size, and obscuration of 3796 X-ray selected AGNs at
$0.2
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present an unsupervised outlier detection method for galaxy spectra based on the spectrum autoencoder architecture spender, which reliably captures spectral features and provides highly realistic reconstructions for SDSS galaxy spectra. We interpret the sample density in the autoencoder latent space as a probability distribution, and identify outliers as low-probability objects with a normalizing flow. However, we found that the latent-space position is not, as expected from the architecture, redshift invariant, which introduces stochasticity into the latent space and the outlier detection method. We solve this problem by adding two novel loss terms during training, which explicitly link latent-space distances to data-space distances, preserving locality in the autoencoding process. Minimizing the additional losses leads to a redshift-invariant, non-degenerate latent space distribution with clear separations between common and anomalous data. We inspect the spectra with the lowest probability and find them to include blends with foreground stars, extremely reddened galaxies, galaxy pairs and triples, and stars that are misclassified as galaxies. We release the newly trained spender model and the latent-space probability for the entire SDSS-I galaxy sample to aid further investigations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jiaxuan Li
Jenny E. Greene
Johnny P. Greco
Song Huang
Peter Melchior
Rachael Beaton
Kirsten Casey
Shany Danieli
Andy Goulding
Remy Joseph
Erin Kado-Fong
Ji Hoon Kim
Lauren A. MacArthur
摘要： Large diffuse galaxies are hard to find, but understanding the environments where they live, their numbers, and ultimately their origins, is of intense interest and importance for galaxy formation and evolution. Using Subaru's Hyper Suprime-Cam Strategic Survey Program, we perform a systematic search for low surface brightness galaxies and present novel and effective methods for detecting and modeling them. As a case study, we surveyed 922 Milky Way analogs in the nearby Universe ($0.01 < z < 0.04$) and build a large sample of satellite galaxies that are outliers in the mass-size relation. These ``ultra-puffy'' galaxies (UPGs), defined to be $1.5\sigma$ above the average mass-size relation, represent the tail of the satellite size distribution. We find that each MW analog hosts $N_{\rm UPG} = 0.31\pm 0.05$ ultra-puffy galaxies on average, which is consistent with but slightly lower than the observed abundance at this halo mass in the Local Volume. We also construct a sample of ultra-diffuse galaxies (UDGs) in MW analogs and find an abundance of $N_{\rm UDG} = 0.44\pm0.05$ per host. With literature results, we confirm that the UDG abundance scales with the host halo mass following a sublinear power law. We argue that our definition for ultra-puffy galaxies, which is based on the mass-size relation, is more physically-motivated than the common definition of ultra-diffuse galaxies, which depends on surface brightness and size cuts and thus yields different surface mass density cuts for quenched and star-forming galaxies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shenli Tang
John D. Silverman
Xuheng Ding
Junyao Li
Khee-Gan Lee
Michael A. Strauss
Andy Goulding
Malte Schramm
Lalitwadee Kawinwanichakij
J. Xavier Prochaska
Joseph F. Hennawi
Masatoshi Imanishi
Kazushi Iwasawa
Yoshiki Toba
Issha Kayo
Masamune Oguri
Yoshiki Matsuoka
Kohei Ichikawa
Tilman Hartwig
Nobunari Kashikawa
摘要： We report on a spectroscopic program to search for dual quasars using Subaru Hyper Suprime-Cam (HSC) images of SDSS quasars which represent an important stage during galaxy mergers. Using Subaru/FOCAS and Gemini-N/GMOS, we identify three new physically associated quasar pairs having projected separations less than 20 kpc, out of 26 observed candidates. These include the discovery of the highest redshift ($z=3.1$) quasar pair with a separation $<$ 10 kpc. Based on the sample acquired to date, the success rate of identifying physically associated dual quasars is $19\%$ when excluding stars based on their HSC colors. Using the full sample of six spectroscopically confirmed dual quasars, we find that the black holes in these systems have black hole masses ($M_{BH} \sim 10^{8-9}M_{\odot}$) similar to single SDSS quasars as well as their bolometric luminosities and Eddington ratios. We measure the stellar mass of their host galaxies based on 2D image decomposition of the five-band ($grizy$) optical emission and assess the mass relation between supermassive black holes (SMBHs) and their hosts. Dual SMBHs appear to have elevated masses relative to their host galaxies. Thus mergers may not necessarily align such systems onto the local mass relation, as suggested by the Horizon-AGN simulation. This study suggests that dual luminous quasars are triggered prior to the final coalescence of the two SMBHs, resulting in early mass growth of the black holes relative to their host galaxies.
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