Astrophysics of Galaxies (astro-ph.GA)

Abstract: Compact elliptical (cE) galaxies remain an elusively difficult galaxy class to study. Recent observations have suggested that isolated and host-associated cEs have different formation pathways, while simulation studies have also shown different pathways can lead to a cE galaxy. However a solid link has not been established, and the relative contributions of each pathway in a cosmological context remains unknown. Here we combine a spatially-resolved observational sample of cEs taken from the SAMI galaxy survey with a matched sample of galaxies within the IllustrisTNG cosmological simulation to establish an overall picture of how these galaxies form. The observed cEs located near a host galaxy appear redder, smaller and older than isolated cEs, supporting previous evidence for multiple formation pathways. Tracing the simulated cEs back through time, we find two main formation pathways; 32 $\pm$ 5 percent formed via the stripping of a spiral galaxy by a larger host galaxy, while 68 $\pm$ 4 percent formed through a gradual build-up of stellar mass in isolated environments. We confirm that cEs in different environments do indeed form via different pathways, with all isolated cEs in our sample having formed via in-situ formation (i.e. none were ejected from a previous host), and 77 $\pm$ 6 percent of host-associated cEs having formed via tidal stripping. Separating them by their formation pathway, we are able to reproduce the observed differences between isolated and host-associated cEs, showing that these differences can be fully explained by the different formation pathways dominating in each environment.

Abstract: When subhalos infall into galaxy clusters, their gas content is ram pressure stripped by the intracluster medium (ICM) and may turn into cometary tails. We report the discovery of two spectacular X-ray double tails in a single galaxy cluster, Z8338, revealed by 70 ks Chandra observations. The brighter one, with an X-ray bolometric luminosity of $3.9 \times 10^{42}{\rm\ erg\ s}^{-1}$, is a detached tail stripped from the host halo and extended at least 250 kpc in projection. The head of the detached tail is a cool core with the front tip of the cold front $\sim$ 30 kpc away from the nucleus of its former host galaxy. The cooling time of the detached cool core is $\sim 0.3$ Gyr. For the detached gas, the gravity of the once-associated dark matter halo further enhances the Rayleigh-Taylor (RT) instability. From its survival, we find that a magnetic field of a few $\mu$G is required to suppress the hydrodynamic instability. The X-ray temperature in the tail increases from 0.9 keV at the front tip to 1.6 keV in the wake region, which suggests the turbulent mixing with the hotter ICM. The fainter double X-ray tail, with a total X-ray luminosity of $2.7 \times 10^{42}{\rm\ erg\ s}^{-1}$, appears to stem from the cool core of a subcluster in Z8338, and likely was formed during the ongoing merger. This example suggests that X-ray cool cores can be displaced and eventually destroyed by mergers, while the displaced cool cores can survive for some extended period of time.

Abstract: J1044+0353 is considered a local analog of the young galaxies that ionized the intergalactic medium at high-redshift due to its low mass, low metallicity, high specific star formation rate, and strong high-ionization emission lines. We use integral field spectroscopy to trace the propagation of the starburst across this small galaxy using Balmer emission- and absorption-line equivalent widths and find a post-starburst population (~ 15 - 20 Myr) roughly one kpc east of the much younger, compact starburst (~ 3 - 4 Myr). Using the direct electron temperature method to map the O/H abundance ratio, we find similar metallicity (1 to 3 sigma) between the starburst and post-starburst regions but with a significant dispersion of about 0.3 dex within the latter. We also map the Doppler shift and width of the strong emission lines. Over scales several times the size of the galaxy, we discover a velocity gradient parallel to the galaxy's minor axis. The steepest gradients (~ 30 $\mathrm{km \ s^{-1} \ kpc^{-1}}$) appear to emanate from the oldest stellar association. We identify the velocity gradient as an outflow viewed edge-on based on the increased line width and skew in a biconical region. We discuss how this outflow and the gas inflow necessary to trigger the starburst affect the chemical evolution of J1044+0353. We conclude that the stellar associations driving the galactic outflow are spatially offset from the youngest association, and a chemical evolution model with a metal-enriched wind requires a more realistic inflow rate than a homogeneous chemical evolution model.

Abstract: Fully analytical dynamical models usually have an infinite extent, while real star clusters, galaxies, and dark matter haloes have a finite extent. The standard method for generating dynamical models with a finite extent consists of taking a model with an infinite extent and applying a truncation in binding energy. This method, however, cannot be used to generate models with a pre-set analytical mass density profile. We investigate the self-consistency and dynamical properties of a family of power-law spheres with a general tangential Cuddeford (TC) orbital structure. By varying the density power-law slope $\gamma$ and the central anisotropy $\beta_0$, these models cover a wide parameter space in density and anisotropy profiles. We explicitly calculate the phase-space distribution function for various parameter combinations, and interpret our results in terms of the energy distribution of bound orbits. We find that truncated power-law spheres can be supported by a TC orbital structure if and only if $\gamma \geqslant 2\beta_0$, which means that the central density slope-anisotropy inequality is both a sufficient and a necessary condition for this family. We provide closed expressions for structural and dynamical properties such as the radial and tangential velocity dispersion profiles, which can be compared against more complex numerical modelling results. This work significantly adds to the available suite of self-consistent dynamical models with a finite extent and an analytical description.

Abstract: Investigating the extinction properties in dense molecular clouds is of significant importance for understanding the behavior of interstellar dust and its impact on observations. In this study, we comprehensively examined the extinction law in the Ophiuchus cloud across a wavelength range from 0.8$\,\mu\rm m$ to 8$\,\mu\rm m$. To achieve this, we analyzed NIR and MIR data obtained from the UKIDSS GCS and the Spitzer c2d survey, respectively. By fitting a series of color-color diagrams, we determined color-excess ratios $E_{J-\lambda}/E_{J-K}$ for seven passbands. These ratios were then directly converted to derive the relative extinction law $A_\lambda/A_K$. Our findings demonstrate that the Ophiuchus cloud exhibits a characteristic of flat MIR extinction, consistent with previous studies. Additionally, our results reveal variations in the extinction law with extinction depth, indicating a flatter trend from the NIR to MIR bands as extinction increases. Notably, our analysis reveals no significant difference in the MIR extinction law among the four dark clouds: L1712, L1689, L1709, and L1688. However, distinct variations were observed in the extinction law for regions outside the dark clouds, specifically L1688N and L1688W. These regions displayed lower color-excess ratios $E_{J-\lambda}/E_{J-K}$ in the Spitzer/IRAC bands. This observation lends support to the dust growth occurring in the dense regions of the Ophiuchus cloud.

Abstract: We propose a new method for finding galaxy protoclusters that is motivated by structure formation theory, and is also directly applicable to observations. Protoclusters are defined as the galaxy groups whose virial mass $M_{\rm vir} < 10^{14}\,M_{\odot}$ at their epochs but would exceed that limit by $z=0$. They are distinguished from clusters, groups of galaxies whose virial mass currently exceeds $10^{14}\,M_{\odot}$. According to these definitions there can be a mixture of clusters and protoclusters at a given epoch. The future mass that a protocluster would acquire at $z=0$ is estimated using the spherical collapse model. The centers of protoclusters are identified using the critical overdensity for collapse by $z=0$ that is predicted by the spherical collapse model, and the physical size of protoclusters is defined by the overdensity corresponding to the turnaround radius. We use the cosmological hydrodynamical simulation Horizon Run 5 (HR5) to calibrate this prescription and demonstrate its performance. We find that the protocluster identification method suggested in this study is quite successful. Its application to the high redshift HR5 galaxies shows a tight correlation between the mass within the protocluster regions identified according to the spherical collapse model and the final mass to be found within the cluster at $z=0$, meaning that the regions can be regarded as the bona-fide protoclusters with high reliability.