Bulge Gas Research Articles

Star formation in CALIFA early-type galaxies: a matter of discs

ABSTRACT The star formation main sequence (SFMS) is a tight relation between the galaxy star formation rate (SFR) and its total stellar mass (M⋆). Early-type galaxies (ETGs) are often considered as low-SFR outliers of this relation. We study, for the first time, the separated distribution in the SFR versus M⋆ of bulges and discs of 49 ETGs from the CALIFA survey. This is achieved using c2d, a new code to perform spectrophotometric decompositions of integral field spectroscopy data cubes. Our results reflect that: (i) star formation always occurs in the disc component and not in bulges; (ii) star-forming discs in our ETGs are compatible with the SFMS defined by star-forming galaxies at z ∼ 0; (iii) the star formation is not confined to the outskirts of discs, but it is present at all radii (even where the bulge dominates the light); (iv) for a given mass, bulges exhibit lower sSFR than discs at all radii; and (v) we do not find a deficit of molecular gas in bulges with respect to discs for a given mass in our ETGs. We speculate our results favour a morphological quenching scenario for ETGs.

Supermassive black hole demographics: evading M − σ

We consider black hole - galaxy coevolution using simple analytic arguments. We focus on the fact that several supermassive black holes are known with masses significantly larger than suggested by the $M - {\sigma}$ relation, sometimes also with rather small stellar masses. We show that these are likely to have descended from extremely compact `blue nugget' galaxies born at high redshift, whose very high velocity dispersions allowed the black holes to reach unusually large masses. Subsequent interactions reduce the velocity dispersion, so the black holes lie above the usual $M - {\sigma}$ relation and expel a large fraction of the bulge gas (as in WISE J104222.11+164115.3) that would otherwise make stars, before ending at low redshift as very massive holes in galaxies with relatively low stellar masses, such as NGC 4889 and NGC 1600. We further suggest the possible existence of two new types of galaxy: low-mass dwarfs whose central black holes lie below the $M - {\sigma}$ relation at low redshift, and galaxies consisting of very massive ($\gtrsim 10^{11}$M$_{\odot}$) black holes with extremely small stellar masses. This second group would be very difficult to detect electromagnetically, but potentially offer targets of considerable interest for LISA.

ALMA observations of molecular absorption in four directions toward the Galactic bulge

Context. Alma Cycle 3 observations serendipitously showed strong absorption from diffuse molecular gas in the Galactic bulge at − 200 km s−1 < v < −140 km s−1 toward the compact extragalactic continuum source J1744-3116 at (l, b) = −2.13°, − 1.00°. Aims. We aimed to test whether molecular gas in the bulge could also be detected toward the three other, sufficiently strong mm-wave continuum sources seen toward the bulge at |b| < 3°. Methods. We took absorption profiles of HCO+ (1−0), HCN(1−0), C2H (1−0), CS(2−1) and H13CO+ (1−0) in ALMA Cycle 4 toward J1713-3418, J1717-3341, J1733-3722 and J1744-3116. Results. Strong molecular absorption from disk gas at |v|≲30 km s−1 was detected in all directions, and absorption from the 3 kpc arm was newly detected toward J1717 and J1744. However, only the sightline toward J1744 is dominated by molecular gas overall and no other sightlines showed molecular absorption from gas deep inside the bulge. No molecular absorption was detected toward J1717 where H I emission from the bulge was previously known. As observed in HCO+, HCN, C2H and CS, the bulge gas toward J1744 at v < −135 km s−1 has chemistry and kinematics like that seen near the Sun and in the Milky Way disk generally. We measured isotopologic ratios N(HCO+)/N(H13CO+) > 51 (3σ) for the bulge gas toward J1744 and 58 ± 9 and 64 ± 4 for the disk gas toward J1717 and J1744, respectively, all well above the value of 20−25 typical of the central molecular zone. Conclusions. The kinematics and chemistry of the bulge gas observed toward J1744 more nearly resemble that of gas in the Milky Way disk than in the central molecular zone.

The diffuse molecular component in the nuclear bulge of the Milky Way

Context. The bulk of the molecular gas in the central molecular zone (CMZ) of the Galactic center region shows warm kinetic temperatures, ranging from >20 K in the coldest and densest regions (n ~ 104-5 cm-3) up to more than 100 K for densities of about n ~ 103 cm-3. Recently, a more diffuse, hotter (n ~ 100 cm-3, T ~ 250 K) gas component was discovered through absorption observations of H3+. This component may be widespread in the Galactic center, and low density gas detectable in absorption may be present even outside the CMZ along sightlines crossing the extended bulge of the Galaxy. Aim. We aim to observe and characterize diffuse and low density gas using observations of 3-mm molecular transitions seen in absorption. Methods. Using the Atacama Large (sub)Millimeter Array (ALMA) we observed the absorption against the quasar J1744-312, which is located toward the Galactic bulge region at (l, b) = (−2°.13, −1°.0), but outside the main molecular complexes. Results. ALMA observations in absorption against the J1744-312 quasar reveal a rich and complex chemistry in low density molecular and presumably diffuse clouds. We detected three velocity components at ~0, −153, and −192 km s−1. The component at ~0 km s−1 could represent gas in the Galactic disk while the velocity components at −153, and −192 km s−1 likely originate from the Galactic bulge. We detected 12 molecules in the survey, but only 7 in the Galactic bulge gas.

Powerful Outflows and Feedback from Active Galactic Nuclei

Active galactic nuclei (AGNs) represent the growth phases of the supermassive black holes in the center of almost every galaxy. Powerful, highly ionized winds, with velocities ∼0.1–0.2c, are a common feature in X-ray spectra of luminous AGNs, offering a plausible physical origin for the well-known connections between the hole and properties of its host. Observability constraints suggest that the winds must be episodic and detectable only for a few percent of their lifetimes. The most powerful wind feedback, establishing the M−σ relation, is probably not directly observable at all. The M−σ relation signals a global change in the nature of AGN feedback. At black hole masses below M−σ, feedback is confined to the immediate vicinity of the hole. At the M−σ mass, it becomes much more energetic and widespread and can drive away much of the bulge gas as a fast molecular outflow.

X‐Ray Light Curves and Accretion Disk Structure of EX Hydrae

We present X-ray light curves for the cataclysmic variable EX Hydrae obtained with the Chandra High Energy Transmission Grating Spectrometer and the Extreme Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the shape and amplitude of the binary light curve and discuss a new feature: the phase of the minimum in the binary light curve, associated with absorption by the bulge on the accretion disk, increases with wavelength. We discuss several scenarios that could account for this trend and conclude that, most likely, the ionization state of the bulge gas is not constant, but rather decreases with binary phase. We also conclude that photoionization of the bulge by radiation originating from the white dwarf is not the main source of ionization, but that it is heated by shocks originating from the interaction between the in-flowing material from the companion and the accretion disk. The findings in this paper provide a strong test for accretion disk models in close binary systems.

Near-coeval formation of the Galactic bulge and halo inferred from globular cluster ages

THE morphology of our Galaxy is characterized by a disk of stars moving on circular orbits, surrounding a central spheroidal body of stars on high-velocity, randomly oriented orbits. The spheroid is further differentiated into an inner bulge and an outer halo; the bulge stars are rich in elements heavier than helium ('metals'), whereas the halo stars are metal-poor, suggesting that the latter formed very early in the history of the Galaxy. (They have experienced little chemical enrichment, by previous generations of stars.) It is not known, however, whether the bulge is the inner extension of the halo, having formed as part of the same process1, or whether it formed much later, perhaps by a dynamical distortion of the inner regions of the disk2,3. Here we report observations obtained with the Hubble Space Telescope of two metal-rich globular clusters that form part of the bulge population. Within the uncertainties, these bulge globular clusters appear to be coeval with halo clusters, which suggests that the formation of the bulge was part of the dynamical process that formed the halo, and that the bulge gas underwent rapid chemical enrichment, in less than a few billion years.

A wind in the Galaxy

The presence of a wind in the galactic bulge is investigated as the cause of the observed minimum of neutral gas density from 0.5 to 4 kpc. Estimates of gas temperature in the bulge are shown to predict the existence of an outward flow of gas throughout a large portion of the bulge, given current estimates of supernova heating rates and stellar mass-loss rates. A hydrodynamic model of a bulge wind consisting of gas recently shed from evolving stars is then used to investigate the interaction of disk and bulge gas, and it is found that the presence of a disk of gas at any temperature and density does not stop the flow of a wind from the bulge. Cold molecular clouds in the bulge region are argued to be dynamically isolated from the wind; however, if they interact thermally through electron conduction, the wind will be limited to outside of a critical radius from 0.3 to 1 kpc. Calculations of the mass of the molecular gas accumulated within this radius over the Hubble time are found to be in agreement with the observed amount. It is pointed out that although the proposed wind prevents new gas from accumulating in the bulge, mechanisms for the depletion of the original gas from the area and the future evolution of the wind remain unclear.