HST Spectroscopy of the Ly α Line in A426, A1795 and A2597: Constraints on Absorbing Gas

1. HST Spectroscopy of the Lyα Line in A426, A1795 and A2597: Constraints on Absorbing Gas Stefi Baum, STScI and US State Dept Ari Laor (Technion) Chris O’Dea, Anton Koekemoer, Jennifer Mack (STScI)

2. Outline • Introduction & Motivation • The Current Status of Cold Gas in Clusters • Molecular Gas • Emission Line Gas • 21 cm Absorption • X-ray Absorption • The New Lyα Observations • Absorption towards the Nucleus of NGC1275 • Limits on Absorption towards the Extended Lyα • Constraints on Absorbing Gas • Summary & Implications

3. The Big Questions • How much mass is deposited in cooling flows? • How much primordial matter remains in a cold phase? • To what extent is the ICM a multi-phase medium ? • How important are winds and stripping for removing gas from galaxies and what effect does this have on galaxy evolution? • What are the dominant sources and sinks for the cold gas? • Do cold clouds in the ICM contribute to the Lyα forest (e.g., Crawford et al. 1987) ?

4. Sources and Sinks of Cold Gas in the ICM • Sources of cold gas: • Gas removed from cluster galaxies by ram pressure stripping or by collisions or tidal interactions • Primordial clouds or proto-galaxies • Clouds which condense from thermal instabilities in the ICM • Sinks for cold gas: • Star formation (including brown dwarfs and planets) • Cloud destruction via shredding or heating or evaporation

5. What Do We Already Know About “Cold” Gas in A426, A1795 & A2597 ? • CO Observations • Emission Line Filaments • 21 cm Absorption • X-ray Absorption

6. Molecular Gas in Clusters (Edge 2002) Molecular gas has finally been detected in clusters (other than A426) after a decade of non-detections (Edge 2002). Roughly half of extreme cooling flow clusters exhibit CO emission. NGC1275 N(H2) = 1.7 ± 0.2 x 1010 M⊙ A1795 N(H2) < 2.7 x 109 M⊙ A2597 N(H2) = 8.1 ± 3.3 x 109 M⊙ (marginal) (A new detection of A1795 at 7 x 109 M⊙ is reported by Salome & Combes) CO (1-0) and CO (2-1) spectra of cooling flow clusters from the IRAM 30-m (Edge 2002).

7. Emission Line Filaments in NGC1275 • NGC1275 exhibits a complex network of emission line filaments extending up to 50 kpc from the center. • Both radial and tangential filaments are seen. NGC1275. Hα image before continuum subtraction showing the low velocity system of ionized gas filaments. (Conselice, Gallagher & Wise 2001).

8. Blue Lobes, Dust & Gas in A1795 Blue light associated with the radio lobes was detected in ground based U band images by McNamara & O’Connell (1983). WFPC2 imaging reveals the blue light is composed of knots which lie along the radio source (McNamara et al. 1996). Central 30 kpc of A1795. Sum of V and R WFPC2 images with a galaxy model subtracted. Central insert is smoothed to 0.1” resolution and contours of VLA image are superposed. Lower left insert is unsmoothed. Upper left insert is V-R color map. (McNamara et al. 1996).

9. A2597 • Clumpy and filamentary optical emission line nebula. • Filaments tend to wrap around the radio lobes. • Similar results for A1795 (Pinkney et al. 1996; McNamara et al. 1996).

10. 21 cm Observations of A426 The 21 cm line is detected towards the nucleus of NGC1275 (Crane et al. 1982; Jaffe 1990; Sijbring 1993) N(H) = 1.94 x 1018 T τ Δv cm-2 τ = 0.0024, FWHM = 440 km/s, N(H) =2 x 1020 cm-2 for “warm” clouds (100 K)

11. 21 cm Observations of A1795 N(H) = 1.94 x 1018 T τ Δv cm-2 τ < 0.11 at 1.5 km/s velocity resolution, N(H) < 3 x 1018 cm-2 for cold clouds (10 K) with narrow velocity width (1.5 km/s) τ < 0.0058 at 47 km/s velocity resolution, N(H) < 1 x 1020 cm-2 for “warm” clouds (100 K) and a population with moderate velocity width (100 km/s) Caveat: column densities could be higher if the covering factor is low. The excess X-ray absorption is ~ 1 x 1020 cm-2 which is inconsistent with cold clouds but could be consistent with a population of warm clouds. O’Dea, Gallimore & Baum ( 1995)

12. Neutral Hydrogen Towards PKS 2322-123 (A2597) • The inner radio source is symmetric. • There is a very broad component (FWHM ~ 735 km/s) towards the nucleus. • A narrow component (FWHM ~ 110 km/s) is seen towards the nucleus and the eastern jet. • Both absorption lines are possibly redshifted by ~ 220 ± 100 km/s. • Extended HI absorption is detected towards the radio lobes of PKS 2322-123 with τ~ 0.006 and FWHM ~ 410 km/s, N(H) ~ 5 x 1020 cm-2 for T=100 K (O’Dea, Baum & Gallimore 1994). • The estimated mass in atomic hydrogen is 7 x 107 M⊙ (integrating the column over the area of the radio source). The mass could be much larger if similar column densities apply to the entire nebula. • The atomic hydrogen is likely to be produced in the cores of the clouds whose ionized skins produce the emission line nebula. PKS 2322-123 (A2597). VLBA 1.3 GHz continuum image (contours) with the spectra for each of the 5 components inlayed. Each channel has width 15.4 km/s and rms noise 0.53 mJy/beam. The arrow at 24,673 km/s indicates the systemic velocity derived from the optical spectrum. (Taylor, O’Dea, Peck & Koekemoer 1999)

13. No Excess X-ray Absorption in A426 • Over most of the inner 4 arcmin the photo-electric absorption is consistent with a constant column of N(H )~ 1.43 x 1021 cm –2 which is close to the measured Galactic value. • The inner 15 arcsec is consistent with a higher value of N(H)~ 1.62 x 1021 cm –2. This is most likely produced by the in-falling high velocity system which contributes a column of N(H) ~ 1021 cm –2. • Thus, there is currently no evidence for additional X-ray absorption due to the cluster. Column densities from single temperature fits to Chandra annular spectra between 2.5 and 121.8 kpc without fixing the absorbing column density. The dashed line indicates the median of N(H)= 0.143 x 1022 cm-2 (Schmidt, Fabian & Sanders 2002). (Top) Adaptively smoothed 0.5-7 keV Chandra image. (Middle) Temperature. (Bottom) Column density of absorbing gas. The dark feature is due to the in-falling high velocity system (Fabian et al. 2000)

14. Evidence for X-ray Absorption in A1795? The estimated absorbing column is consistent with the Galactic value except perhaps in the inner 50 kpc where an excess of N(H) ~1020 cm-2 may be required (Tamura et al. 2001; Ettori et al. 2002). Chandra spectral results with a single absorbed MEKAL model (triangles; solar metallicity from Anders & Grevesse 1989) and the corresponding deprojected results (diamonds). The error bars are at the 1 sigma level. The shaded regions represent the uncertainties at the 90 per cent level of confidence from the ASCA analysis in Allen (2000). The dashed error bars refer to the 90 per cent confidence level (Δχ2 =2.71 on the interesting parameter). The dot-dashed lines indicate the Galactic absorption (NH=1.2x1020 cm-2; Dickey & Lockman 1990) and the optical determination of the redshift at 0.0632 (Girardi et al. 1998), respectively. (Ettori et al. 2002) For the assumed cosmology 1 arcmin = 100 kpc. The lack of agreement with the Tamura et al. results in the inner 50 kpc may be due to a calibration problem in the PN data (Fabian, private communication). A1795. XMM-Newton estimates of the absorbing column density, temperature, metal abundance, and deprojected hydrogen density as a function of radius (from top to bottom). Results are derived from PN (diamonds) and MOS (crosses) spectra. (Tamura et al. 2001).

15. No X-ray Absorption in A2597? • Sarazin & McNamara (1997) using ROSAT data place an upper limit on intrinsic absorption of N(H) < 1.7 x 1020 cm-2. But Allen et al. (2001) using ROSAT and ASCA data find a value of N(H) ~ 5 x 1021 cm-2. • The Chandra data of McNamara et al. (2001) are consistent with no excess absorption. Radial variation of (a) surface brightness, (b) electron density, c) temperature, and (d) temperature. (McNamara et al. 2001) Smoothed Chandra 0.3-8.0 keV X-ray image of A2597. McNamara et al. (2001).

16. Status of Cold Gas in Cooling Flow Clusters • The observed mass increases as the temperature decreases. • Emission line gas M~106-7 M⊙ • Atomic hydrogen M~107-8 M⊙ • Molecular hydrogen M~109-10 M⊙ • The nebulae are ionization bounded. • The total amount of cold gas is still much less than the hot X-ray emitting component in the cluster core M~1012 M⊙ within 100 kpc. • Excess X-ray absorbing columns are not required in A426 and A2597, but may be required in the inner 50 kpc of A1795. • High column density clouds are present in the ICM, N(H) ~ 1020 cm –2 , but may have a low covering factor.

17. Lyα Absorption is a Powerful Probe of the ICM. • Lyα is > 107 times more sensitive to optically thin HI absorption than the 21 cm line (Laor 1997). • Limits on column density of ~ 1012 cm-2 are possible. • Lyα absorption is sensitive to a wide range of conditions in the absorbing media. (Top). Lyα and 21 cm curves of growth for various velocity dispersions (b parameters). The horizontal dashed lines indicate absorption equivalent widths from Jaffe (1990) and Johnstone & Fabian (1995), and the vertical dashed lines indicate the derived limits on N(H). (Laor 1997). (Middle) Lyα curves of growth for various velocity dispersions (b values are marked). The horizontal dashed line indicates a Lyα absorption equivalent width of 0.5Å. The three black squares mark the values corresponding to the three profiles given below. (Bottom) Voigt profile calculations for three different absorbing media (column density and velocity dispersions are marked), which produce an absorption equivalent width of 0.5Å.

18. HST STIS Lyα Spectroscopy of A426, A1795, and A2597 • We used the STIS FUV MAMA in long slit mode with the G140M grating centered on the redshifted Lyα line. • Spatial resolution is 0.029” per pixel; Velocity resolution is 20 km/s. Slit width is 0.1”. • We observed in TIME-TAG mode to allow us to reject data with high background. • We first obtained Lyα images to allow us to optimize the slit location. • In each galaxy we observed at a single slit position chosen to maximize the amount of Lyα and FUV continuum flux. • Exposures are ~2 orbits (~4500 s)

19. Roadmap to the Results • Detection of absorption towards the nucleus of NGC1275 • Effects of the v=0 and v=8000 km/s damped Lyα systems • Where are the 21 cm absorbers in NGC1275? • Comparison with the previous HST/FOS spectrum • What are the detected absorbers? • Intrinsic nuclear systems? • Candidate galaxy identifications. • Lyα forest systems? • Limits on absorption towards the extended Lyα in A426, A1795 & A2597 • Constraints on the covering factor • Summary & Implications

20. Slit Position Angles A426. PA=100o FUV. A1795. PA= –158o Lyα. A2597. PA=19o Lyα.

21. Extended Lyα Emission in the 2-D Spectra A426 • We detect the extended Lyα emission in all three clusters. • We also detect bright nuclear (line and continuum) emission in NGC1275. A1795 A2597 2-D spectra through G140M grism. Note the geocoronal lines in A426 and A2597. The observations of A1795 have a very high and non-uniform background.

22. Lyα Absorption towards NGC1275 • Ten Lyα absorption systems are detected towards the nucleus with column densities ~ 1012 to 1014 cm –2. • Feature 8 occurs at the systemic velocity, where 21 cm H I absorption is also detected (Jaffe 1990). However, the 21 cm absorption indicates a column >1019, i.e. seven orders of magnitude larger than the Lyα detection N(H) ~ 3 x 1012 cm-2. Upper panel: The observed and absorption corrected Lyα profile of NGC 1275. The spectrum is integrated in the cross slit direction since the light distribution is consistent with a point source emission. Lower panel: The fraction of transmitted flux vs. observer frame velocity. The table on the right lists the parameters of the 10 absorption systems detected, including the central velocity, total H I column, and b parameter. Our 3σ limit on column density is 1 x 1012 cm-2. (Baum, Laor, O’Dea, Koekemoer, & Mack 2003)

23. The Damped Lyα Absorption Systems towards NGC1275 • These results include the corrections for the two damped Lyα systems along the line of sight - the Galaxy (at v=0), and the foreground in-falling galaxy (at v=8200 km/s). • We find that the v=0 and v=8200 km/s systems exhibit columns consistent with those measured in the 21 cm line N(H) ~ 1021 cm-2. This provides a consistency check on our results. Upper panel: The observed and absorption corrected Lyα profile of NGC 1275 including corrections for damped Lyman alpha absorption by the Galaxy (at v=0), and by the foreground in-falling galaxy (at v=8200 km/s). The Lyman alpha absorption profile of both systems suggests a column of about 1x1021 cm-2, consistent with the 21 cm absorption by these systems. Lower panel: The fraction of transmitted flux vs. observer frame velocity. (Baum, Laor, O’Dea, Koekemoer, & Mack 2003)

24. Where are the 21 cm Absorbers in NGC1275? • The high HI columns seen in the 21 cm line are not detected towards the nucleus via Lyα. This implies that the line of sight to the nucleus is relatively “clean” and that the 21 cm line is detecting HI against the “extended” parsec scale radio components. • The HI might be in a disk or torus as suggested for the free-free absorption (Levinson, Laor, Vermeulen 1995; Walker et al. 2000). • Alternately, if the HI is distributed on a larger scale, the clouds along the line of sight must have been ionized by the AGN. Two-dimensional distribution of the free-free absorption in 3C 84 over the region of the counter-feature in the 1995 January images. There is no detected absorption over the brighter southern feature. The strong radial gradient away from the core is apparent. (Walker et al. 2000)

25. Comparison of the STIS and FOS Spectra at Similar Resolution • The FOS observations were too low resolution and too low S/N to have detected the 10 Lyα absorption features that we detect. • However, we should have seen the feature in the FOS spectrum at 1237Å. Could this be a variable absorption feature? • If this is due to motion of clouds near the nucleus, at a velocity of 0.01c (at most) they could move only 0.07 light years in ~7 years, so they should be less than 0.025 pc across. Our STIS long slit G140M data rebinned to 0.85Å per pixel resolution in black and the FOS spectrum from Johnstone & Fabian (1995) in blue. FOS spectrum is scaled by a factor of 1.55.

26. Are the Lyα Systems “Intrinsic” Associated Absorbers? • Intrinsic absorbers in AGN tend to be blue shifted, variable, with a range of line widths ~ 20-400 km/s. • They also tend to exhibit high ionization metal lines. • They are found in ~half of type 1 AGN (e.g., Crenshaw et al. 1999). • We will reobserve NGC1275 to search for variability and metal lines. (Top). 3C351. Spectrum of Lyα emission (dashed line) with the best fit (solid line) to the associated absorption lines. The data are not binned, and 1 σ errors are shown with dotted lines. The broad absorption components (FWHM > 60 km s-1) are marked with their designations above the spectrum, and the narrow components (FWHM < 60 km s-1) are marked below the spectrum. (Yuan et al. 2002). (Bottom) NGC5548. Portions of the STIS echelle spectra showing the intrinsic absorption lines in different ions. Fluxes are plotted as a function of the radial velocity (of the strongest member, for the doublets), relative to an emission-line redshift of z = 0.01676. The kinematic components are identified for the strong members of the doublets, and vertical dotted lines are plotted at their approximate positions. Strong Galactic absorption lines are labeled with ``G''. Fits to the NLR profiles are plotted as dashed lines. (Crenshaw et al. 2003).

27. Are the Lyα absorbers associated with galaxies in the Perseus Cluster? 4 Absorption lines have potential associations with cluster galaxies (30-80 kpc from the line of sight and 27-106 km/s offset in velocity). Candidate Identifications. Results from a NED search using V= 1000-6000 km/s and a box 8 arcmin across centered on NGC1275.

28. What are the Lyα Absorbers towards NGC1275? • Nuclear Clouds ? Foreground Galaxies? Emission Line Filaments? Intergalactic Clouds? • The column densities and Doppler parameters are similar to those of Lyα forest clouds (e.g., Rauch 1998) and intrinsic nuclear systems (e.g., Crenshaw et al. 1999). • Most of the absorption features are beyond the velocity range of the emission line filaments. • Feature 8 is at the systemic velocity and could be associated with emission line filaments or other gas in NGC1275. • Features 4,5,9,10 may be associated with galaxies in the Perseus cluster. • Note that extended Lyα line is narrower than the nuclear line. Thus, we cannot place useful limits on the features detected in the blue and red wings of the nuclear line. Only absorption feature 8 falls in the same wavelength range in both spectra.

29. No Absorption Towards the Extended Lyα Emission in A426, A1795 & A2597 • The upper limit to the equivalent width of any absorption is ~0.25Å. • This corresponds to an average N(H) < 3 x 1013 cm-2 for optically thin gas. Alternately, the covering factor of any strongly absorbing gas must be less than ~25%. (Top). NGC1275. STIS G140M spectrum summed 5” along the slit excluding the nucleus. The extended Lyα emission is detected with a flux 7 x 10-13 ergs/s/cm2/Å/arcsec2 and FWHM 3.2Å (~780 km/s). (Middle). A1795. STIS G140M spectrum summed 11” along the slit. The extended Lyα emission is detected with a flux 1.7 x 10-12 ergs/s/cm2/Å/arcsec2 and FWHM 2.7Å (~630 km/s). The slope is due to varying background in the detector. (Bottom). A2597. STIS G140M spectrum summed 16” along the slit. The extended Lyα emission is detected with a flux 1.7 x 10-12 ergs/s/cm2/Å/arcsec2 and FWHM 3.7Å (~844 km/s). (Baum, Laor, O’Dea, Koekemoer, Mack 2003).

30. Low Covering Factor ? • If the absorbing clouds are in pressure balance with the external medium with nT ~ 106, the expected column density is N(H) ~ 3 x 1020 (n/104) (L/.01 pc) cm-2 for T=100 K. • Why don’t we see absorption? • The nebula is composed of clouds which individually on average have low columns N(H) < 1013 cm-2 because of extremely small sizes, e.g., 10-9 pc. Would this low column and small size permit the clouds to retain atomic cores ? • The absorbing clouds are high column, but the covering factor is low. The Lyα observations would require a covering factor < 0.25. In PKS2322-123, the extended 21 cm absorption is consistent with a covering factor as low as 6 x 10-3 (O’Dea, Baum & Gallimore 1994).

31. Summary & Implications • We detect 10 Lyα absorption systems towards the nucleus of NGC1275 with columns of N(H) ~ 1012 to 1014 cm –2 . Most features are located in the broad wings of the line. The detected absorption features are consistent with Lyα forest systems or associated nuclear absorption systems. • There is very little absorption at the systemic velocity in NGC1275 (Feature 8 contains N(H) ~ 3x 1012 cm –2). This implies that the large columns detected in the 21 cm line avoid the line of sight to the nucleus. This gas may be located in a circumnuclear disk or torus. • We do not detect Lyα absorption towards the extended diffuse Lyα emission in A426, A1795, and A2597 with upper limits N(H) ~ 3 x 1013 cm –2 for optically thin absorbers. Our data constrain the covering factor of any high column density gas (N(H) > 1015 cm –2) in the ICM to be less than 25%.

32. The End