Extragalactic Astrophysics



Figure 1: Light curves for supernova SN2013ej, obtained with the 12-inch RIT Observatory telescope.

For the second time in three years, a bright supernova exploded in a nearby galaxy. This time, it was the type IIP supernova 2013ej in the face-on spiral galaxy M74. Just as we (Richmond et al.) had done in the case of SN 2011fe two years ago, we followed the optical evolution of this titanic explosion with the 12-inch telescope at the RIT Observatory. Six months of measurements yield a light curves with a clear plateau (hence the ’P’ in ’type IIP’), followed by a sharp drop in light (see Fig 1). A paper describing these observations has been submitted to the Journal of the American Association of Variable Star Observers.

Supermassive black holes and their environments.

Figure 2: Upper left: NGC 1386 imaged with the Hubble Space Telescope. The green box marks the region observed with the GEMINI IFU. Upper right: Flux map derived from our observation. Although emission seems strongly localized in a central blob and two elongations approximately aligned along the north-south direction, low-level emission is present over the whole field of view. Lower left: Velocity map (in km/s) derived from the modeling of the spectra. It shows the typical pattern of a rotating disk with additional distortions. Lower right: Cartoon showing the inferred structure of the nuclear region of NGC 1386.

Infall vs. outflow in active galactic nuclei:

For decades it has been known that the nuclei of certain galaxies are characterized by unusually large energy outputs, the presence of radio-emitting jets, outflows and a number of highly energetic phenomena varying on short time-scales (days). Such galaxies are known as active galactic nuclei (AGN). The key mechanism that keeps them active is believed to be well understood: gravitational potential energy is released in the form of radiation during the accretion of gas on a super-massive black hole (SMBH) residing at the galactic center. Nevertheless it is not yet clear how the gas is funneled from the inner kiloparsec down to the sub-parsec scale accretion disk that fuels the SMBH. Is there a relation between the accretion rate of the SMBH and the presence of outflows? An international (US/Brazilian/Chilean) team led by Robinson and AST Ph.D. student Davide Lena have been using the the Integral Field Unit (IFU) on the Gemini Multi Object Spectrograph (GMOS) on the 8-m GEMINI telescopes to observe the ionized gas in the nuclear regions of a sample of AGN. We completed the analysis and modeling of the active galaxy NGC 1386 (see Fig. 2), proposing that the complex spectral features observed within 250 pc from its nucleus are due to (i) the presence of a rotating nuclear outflow and (ii) the illumination of the large-scale galactic disk by the energetic photons generated in vicinity of the SMBH.

Black hole “kicks” from galaxy mergers:

The current standard model of cosmology predicts that galaxies grow mainly via mergers with other galaxies. The hypothesis that super-massive black holes (SMBHs) reside in the center of all galaxies above a given a mass threshold implies that many of such galactic mergers result in the formation of SMBH-binaries which will eventually coalesce. Asymmetries in the binaries (different masses and spins) result in the anisotropic emission of gravitational waves which impart a recoil to the newly formed SMBH. The recoil velocity can be as large as a few ×1000 km s-1, removing the SMBH from the nucleus or ejecting it from the host galaxy. Whilst large recoils are rare (they require special configurations of the binary), small recoils are expected to be more common. LAMA’s Robinson and Lena, in collaboration with colleagues in Italy and at Florida Inst. of Technology, studied a set of nearby, active, giant elliptical galaxies imaged with the Hubble Space Telescope to look for spatial offsets between the active nucleus (supposed to pinpoint the SMBH position) and the photometric center of the galaxy (supposed to mark the minimum of the large-scale potential well, the location where the SMBH is expected to reside). We found evidence for small (<10 pc) displacements in 6 out of 14 galaxies.In individual objects these displacements can reasonably be attributed to residual gravitational recoil oscillations, following a merger within the last few Gyr. For plausible merger rates, however, there is a high probability of larger displacements than those actually observed, if SMBH coalescence events took place in these galaxies. When relatively powerful and well-defined kpc-scale jets are present, the SMBH-photocenter displacements are approximately aligned with the radio jets. Although such alignments could also result from gravitational recoil, this suggests intrinsic asymmetries in radio jet power as a possible displacement mechanism.

OH megamaser galaxies:

Collisions between galaxies leading to the merger of the two original galaxies into a single, larger galaxy are believe to play an important role in galaxy evolution. One important consequence of these events is channeling of interstellar gas into the center of the merging system, where it fuels rapid star formation and may ”ignite” a hitherto dormant supermassive black hole, creating an active galactic nucleus (AGN). Lena and Robinson and an international team (that includes former LAMA postdoc Aires de Sales) are conducting a multiwavelength study of a particular class of merging galaxy, known as OH megamaser galaxies (OHMGs), which are characterized by intense maser emission (the microwave equivalent of a laser) from the hydroxyl (OH) molecule (see Fig. 6). The maser emission may be a signature of a nascent AGN, or massive star formation that is deeply embedded in interstellar dust and therefore difficult discern at shorter wavelengths. The figure shows images in optical, infrared and radio wavelengths of one particular OHMG, IRAS 16399-931. This system is an advanced galaxy merger - the nuclei of the two original galaxies can be seen in the images, surrounded by an envelope of stars, dust and ionized gas. Using these and other data we have shown that the northern nucleus harbors a luminous AGN, which is almost completely concealed by surrounding dust, whereas the southern nucleus is dominated by star formation. However, the luminosities of both nuclei imply relatively modest gas inflow rates.

Figure 3: A four-color composite HST/WFC3 image of the strong lensing cluster SDSS J1531+3414 and its two central brightest cluster galaxies, which are likely undergoing a major merger. The F160W and F814W images are shown in yellow/orange, the F606W image is shown in green, and the F390W image containing rest-frame near-ultraviolet emission from young stars is assigned to the blue channel. Left: a wide (~200×200 kpc2) view of the galaxy cluster. Tangential gravitational arcs from strongly lensed background galaxies are clearly seen. Right: a zoom-in on the left-hand panel, showing the two merging central cluster galaxies. Bright NUV emission associated with ongoing star formation is observed in blue. (From Tremblay et al., ApJ, in press.)

Interacting galaxies and star formation:

LAMA faculty members Baum and O’Dea, RIT AST graduate student Kevin Cooke, and former graduate student Grant Tremblay, in collaboration with the cluster lensing group led by Mike Gladders, are studying a spectacular 30 kpc long filament system undergoing “beads-on-a-string” star formation. New Hubble Space Telescope (HST) ultraviolet and optical imaging of the strong-lensing galaxy cluster SDSS J1531+3414 (z = 0.335) reveals two centrally dominant elliptical galaxies participating in an ongoing major merger (see Fig. 3). The interaction is at least somewhat rich in cool gas, as the merger is associated with a complex network of nineteen massive superclusters of young stars (or small tidal dwarf galaxies) separated by ~1 kpc in projection from one another, combining to an estimated total star formation rate of ~5 M yr-1. The resolved young stellar superclusters are threaded by narrow H, [O ii], and blue excess filaments arranged in a network spanning ~27 kpc across the two merging galaxies. This morphology is strongly reminiscent of the well-known “beads on a string” mode of star formation observed on kpc-scales in the arms of spiral galaxies, resonance rings, and in tidal tails between interacting galaxies. Nevertheless, the arrangement of this star formation relative to the nuclei of the two galaxies is difficult to interpret in a dynamical sense, as no known “beads on a string” systems associated with kpc-scale tidal interactions exhibit such lopsided morphology relative to the merger participants.