31 research outputs found
Galaxies Probing Galaxies at High Resolution: Co-Rotating Gas Associated with a Milky Way Analog at z=0.4
We present results on gas flows in the halo of a Milky Way-like galaxy at
z=0.413 based on high-resolution spectroscopy of a background galaxy. This is
the first study of circumgalactic gas at high spectral resolution towards an
extended background source (i.e., a galaxy rather than a quasar). Using
longslit spectroscopy of the foreground galaxy, we observe spatially extended H
alpha emission with circular rotation velocity v=270 km/s. Using echelle
spectroscopy of the background galaxy, we detect Mg II and Fe II absorption
lines at impact parameter rho=27 kpc that are blueshifted from systemic in the
sense of the foreground galaxy's rotation. The strongest absorber EW(2796) =
0.90 A has an estimated column density (N_H>10^19 cm-2) and line-of-sight
velocity dispersion (sigma=17 km/s) that are consistent with the observed
properties of extended H I disks in the local universe. Our analysis of the
rotation curve also suggests that this r=30 kpc gaseous disk is warped with
respect to the stellar disk. In addition, we detect two weak Mg II absorbers in
the halo with small velocity dispersions (sigma<10 km/s). While the exact
geometry is unclear, one component is consistent with an extraplanar gas cloud
near the disk-halo interface that is co-rotating with the disk, and the other
is consistent with a tidal feature similar to the Magellanic Stream. We can
place lower limits on the cloud sizes (l>0.4 kpc) for these absorbers given the
extended nature of the background source. We discuss the implications of these
results for models of the geometry and kinematics of gas in the circumgalactic
medium.Comment: 14 pages, 6 figures, submitted to ApJ, comments welcom
Evidence for a Black Hole and Accretion Disk in the LINER NGC 4203
We present spectroscopic observations from the Hubble Space Telescope that
reveal for the first time the presence of a broad pedestal of Balmer-line
emission in the LINER galaxy NGC 4203. The emission-line profile is suggestive
of a relativistic accretion disk, and is reminiscent of double-peaked transient
Balmer emission observed in a handful of other LINERs. The very broad line
emission thus constitutes clear qualitative evidence for a black hole, and
spatially resolved narrow-line emission in NGC 4203 can be used to constrain
its mass, with M_BH less than 6 x 10^6 solar masses at 99.7% confidence. This
value implies a ratio of black-hole mass to bulge mass of less than
approximately 7 x 10^-4 in NGC 4203, which is less by a factor of ~3 - 9 than
the mean ratio obtained for other galaxies. The availability of an independent
constraint on central black-hole mass makes NGC4203 an important testbed for
probing the physics of weak active galactic nuclei. Assuming M_BH near the
detection limit, the ratio of observed luminosity to the Eddington luminosity
is approximately 10^-4. This value is consistent with advection-dominated
accretion, and hence with scenarios in which an ion torus irradiates an outer
accretion disk that produces the observed double-peaked line emission.
Follow-up observations will make it possible to improve the black-hole mass
estimate and study variability in the nuclear emission.Comment: 10 pages (LaTeX, AASTeX v4.0), 2 postscript figures, accepted for
publication in The Astrophysical Journal Letter
Galaxy Size Problem at z=3: Simulated Galaxies Are Too Small
Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic
simulations with a spatial resolution of proper 0.21kpc/h in refined subregions
embedded within a comoving cosmological volume (27.4Mpc/h)^3, we investigate
the sizes of galaxies at z=3 in the standard cold dark matter model where
reionization is assumed to complete at zri~6. Our simulated galaxies are found
to be significantly smaller than the observed ones: while more than one half of
the galaxies observed by HST and VLT ranging from rest-frame UV to optical
bands with stellar masses larger than 2E10 Msun have half-light radii larger
than 2kpc/h, none of the simulated massive galaxies in the same mass range have
half-light radii larger than 2kpc/h, after taking into account dust extinction.
Corroborative evidence is provided by the rotation curves of the simulated
galaxies with total masses of 1E11-1E12Msun, which display values 300-1000km/s
at small radii (0.5kpc/h) due to high stellar concentration in the central
regions, larger than those of any well observed galaxies. Possible physical
mechanisms to resolve this serious problem include: (1) an early reionization
at zri>>6 to suppress gas condensation hence star formation, (2) a strong,
internal energetic feedback from stars or central black holes to reduce the
overall star formation efficiency, or (3) a substantial small-scale cutoff in
the matter power spectrum.Comment: high resolution pdf file is available at
http://www.astro.princeton.edu/~cen/galaxysize.pdf 15 pages, 3 figures, in
press of ApJ Letter
Nucleosynthetic Layers in the Shocked Ejecta of Cassiopeia A
We present a three-dimensional analysis of the supernova remnant Cassiopeia A using high-resolution spectra from the Spitzer Space Telescope. We observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that the reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km s^(â1). We determine that the velocity width of the nucleosynthetic layers is ~1000 km s^(â1) over 4000 arcsec^2 regions, although the velocity width of a layer along any individual line of sight is <250 km s^(â1). Si and O, which come from different nucleosynthetic layers in the progenitor star, are observed to be coincident in velocity space in some directions, but segregated by up to ~500 km s^(â1) in other directions. We compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale, corrugated velocity structures that are likely caused during the supernova explosion itself, rather than hundreds of years later by dynamical instabilities at the remnant's reverse shock
SuperMassive Black Holes in Bulges
We present spatially extended gas kinematics at parsec-scale resolution for
the nuclear regions of four nearby disk galaxies, and model them as rotation of
a gas disk in the joint potential of the stellar bulge and a putative central
black hole. The targets were selected from a larger set of long-slit spectra
obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei
with STIS (SUNNS). They represents the 4 galaxies (of 24) that display
symmetric gas velocity curves consistent with a rotating disk. We derive the
stellar mass distribution from the STIS acquisition images adopting the stellar
mass-to-light ratio normalized so as to match ground-based velocity dispersion
measurements over a large aperture. Subsequently, we constrain the mass of a
putative black hole by matching the gas rotation curve, following two distinct
approaches. In the most general case we explore all the possible disk
orientations, alternatively we constrain the gas disk orientation from the
dust-lane morphology at similar radii. In the latter case the kinematic data
indicate the presence of a central black hole for three of the four objects,
with masses of 10^7 - 10^8 solar masses, representing up to 0.025 % of the host
bulge mass. For one object (NGC2787) the kinematic data alone provide clear
evidence for the presence of a central black hole even without external
constraints on the disk orientation. These results illustrate directly the need
to determine black-hole masses by differing methods for a large number of
objects, demonstrate that the variance in black hole/bulge mass is much larger
than previously claimed, and reinforce the recent finding that the black-hole
mass is tightly correlated with the bulge stellar velocity dispersion.Comment: 26 pages, 11 Postscript figures, accepted for publication on Ap
Double-Peaked Broad Emission Lines in NGC 4450 and Other LINERs
Spectra taken with HST reveal that NGC 4450 emits Balmer emission lines with
displaced double peaks and extremely high-velocity wings. This characteristic
line profile, previously seen in a few nearby LINERs and in a small fraction of
broad-line radio galaxies, can be interpreted as a kinematic signature of a
relativistic accretion disk. We can reproduce the observed profile with a model
for a disk with a radial range of 1000-2000 gravitational radii and inclined by
27 degrees along the line of sight. The small-aperture HST data also allow us
to detect, for the first time, the featureless continuum at optical wavelengths
in NGC 4450; the nonstellar nucleus is intrinsically very faint, with M_B =
-11.2 mag for D = 16.8 Mpc. We have examined the multiwavelength properties of
NGC 4450 collectively with those of other low-luminosity active nuclei which
possess double-peaked broad lines and find a number of common features. These
objects are all classified spectroscopically as "type 1" LINERs or closely
related objects. The nuclear luminosities are low, both in absolute terms and
relative to the Eddington rates. All of them have compact radio cores, whose
strength relative to the optical nuclear emission places them in the league of
radio-loud active nuclei. The broad-band spectral energy distributions of these
sources are most notable for their deficit of ultraviolet emission compared to
those observed in luminous Seyfert 1 nuclei and quasars. The double-peaked
broad-line radio galaxies Arp 102B and Pictor A have very similar attributes.
We discuss how these characteristics can be understood in the context of
advection-dominated accretion onto massive black holes.Comment: To appear in The Astrophysical Journal. Latex, 15 pages, embedded
figures and tabl
Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse
Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% of
massive galaxies at z~2.3 have evolved stellar populations and little or no
ongoing star formation. Here we determine the sizes of these quiescent galaxies
using deep, high-resolution images obtained with HST/NIC2 and laser guide
star-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11
Solar masses the galaxies are remarkably small, with a median effective radius
of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5
kpc and average stellar densities which are two orders of magnitude lower than
the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm
previous studies at z>2 which lacked spectroscopic redshifts and imaging of
sufficient resolution to resolve the galaxies. Our findings demonstrate that
fully assembled early-type galaxies make up at most ~10% of the population of
K-selected quiescent galaxies at z~2.3, effectively ruling out simple
monolithic models for their formation. The galaxies must evolve significantly
after z~2.3, through dry mergers or other processes, consistent with
predictions from hierarchical models.Comment: Accepted for publication in ApJ Letter